Module: Protein Chemistry

Protein Architecture

Working with Proteins

Protein Folding

Tutorials

Working with proteins

Proteins are the most abundant and functionally diverse macromolecules in living cells. They are the principal effectors of genetic information, carrying out nearly every biological process required for cellular organization, growth, metabolism, communication, and reproduction. Unlike nucleic acids, which primarily store and transmit genetic information, proteins execute the instructions encoded by genes, making them the direct determinants of cellular phenotype.

The study of proteins encompasses their isolation, purification, characterization, structural analysis, and functional investigation. Collectively, these approaches provide insights into how proteins contribute to normal cellular physiology and disease.

From Genome to Proteome

The flow of biological information follows the Central Dogma:

DNA → RNA → Protein

While the genome of an organism remains relatively constant throughout its lifetime (except for mutations), gene expression is tightly regulated. Consequently, the protein complement of a cell changes continuously in response to developmental stage, cell type, environmental conditions, physiological state, and external stimuli.

This dynamic nature of protein expression makes the study of proteins essential for understanding biological function.

Proteome

The entire complement of proteins, expressed by an organism, cell, or tissue at a particular time is called proteome. It's much more complex and dynamic than the genome.

The proteome is not static; it is highly dynamic and varies significantly because it represents the actual functional expression of genetic information. The specific set of proteins expressed is constantly changing based on factors like:

·         Cell Type (e.g., muscle cell vs. nerve cell)

·         Developmental Stage (e.g., embryo vs. adult)

·         Environmental Conditions (e.g., pH, temperature, or the presence of hormones or nutrients).

Why is the Proteome Dynamic?

Several biological processes contribute to the dynamic nature of the proteome.

• Differential Gene Expression: Different genes are expressed in different cell types and developmental stages.

• Alternative RNA Processing: Alternative splicing and RNA editing generate multiple protein isoforms from a single gene.

• Translational Regulation: The efficiency with which mRNAs are translated into proteins varies depending on cellular conditions.

• Post-translational Modifications (PTMs): Proteins undergo numerous covalent modifications after synthesis, including phosphorylation, glycosylation, acetylation, methylation, ubiquitination, lipidation, and proteolytic cleavage. These modifications regulate protein activity, localization, interactions, and stability.

• Protein Turnover: Proteins differ widely in their half-lives and are continuously synthesized and degraded through pathways such as the ubiquitin–proteasome system and lysosomal degradation.

• Environmental and Physiological Stimuli: Changes in nutrient availability, stress, hormones, developmental cues, and disease states alter protein abundance and activity.

Protein Isolation and Purification

Extraction from Cells

Protein extraction is the first and one of the most critical steps in protein analysis. The objective is to release proteins from cells while preserving their native structure, biological activity, and interactions. The extraction method should be tailored to the biological source and the intended downstream applications, such as enzyme assays, electrophoresis, immunoblotting, or proteomics.

General Considerations

• All extraction steps are to be performed at 0–4°C to minimize protein degradation.

• Sample handling should be completed as rapidly as possible to reduce proteolysis and protein denaturation.

• An extraction buffer should be selected according to the biological source and the intended downstream application.

• Appropriate protease inhibitors and stabilizing agents should be included in the extraction buffer whenever required.

• Cell debris should be removed by centrifugation to obtain a clear protein extract suitable for further analysis.

Selection of Biological Material

The composition of the sample determines the extraction strategy. Different organisms and tissues contain distinct contaminants that must be considered during extraction.

A. Plant Tissues

Plant cells are difficult to disrupt because of the rigid cell wall and often contain interfering compounds.

Challenges

• Cellulose-rich cell wall
• Vacuolar proteases
• Phenolic compounds
• Polysaccharides
• Photosynthetic pigments
• High levels of secondary metabolites

Precautions

• Plant tissues are to be ground in liquid nitrogen to prevent protein degradation during homogenization.
• Polyphenol oxidase inhibitors like PVPP or PVP should be included in the extraction buffer to bind phenolic compounds.
• Reducing agents such as β-mercaptoethanol may be added to minimize oxidation of proteins and phenolic compounds.

B. Animal Tissues

Animal cells lack a cell wall but contain abundant proteases that can rapidly degrade proteins.

Challenges

• High protease activity
• Lipid-rich tissues (brain, adipose tissue)
• Blood contamination in tissue samples

Precautions

• Tissue samples should be maintained at low temperature (0–4°C) throughout the extraction procedure.
• A suitable protease inhibitor cocktail should be included to prevent proteolytic degradation.
• Homogenization should be carried out gently to preserve protein structure and activity.

C. Microorganisms

Extraction depends on the type of microorganism:

Bacteria

• Gram-positive bacteria possess thick peptidoglycan walls and require vigorous disruption.
• Gram-negative bacteria are comparatively easier to lyse.

Yeast and fungi

• Possess rigid cell walls containing chitin and glucans.
• Often require enzymatic digestion or mechanical disruption.

Cell Disruption Methods

Cell disruption (cell lysis) releases intracellular proteins into the extraction buffer. The choice of method depends on the sample type and whether native protein activity must be preserved. Different cell disruption methods are as follows:

Note: Mechanical and pressure-based methods are preferred for large-scale protein extraction, whereas chemical and enzymatic methods are useful when gentle lysis or membrane protein solubilization is required.

Extraction Buffers

Extraction buffers maintain protein stability during isolation and prevent degradation. It includes the following components:

A. Buffer Systems

A buffer system maintains a constant pH during extraction. Selection of buffer depends on:

• Protein stability
• Desired pH
• Downstream application

Examples:

• Tris-HCl
• Phosphate buffer
• HEPES
• PBS

B. Protease Inhibitors

Protease inhibitors prevent degradation of proteins by endogenous proteases released during cell lysis.

Examples:

• PMSF (serine proteases)
• Leupeptin
• Pepstatin A
• Aprotinin
• Commercial protease inhibitor cocktails

Note: PMSF is unstable in aqueous solution and should be added immediately before use.

C. Reducing Agents

They maintain proteins in the reduced state by preventing oxidation of cysteine residues and disulfide bond formation which improves protein stability. These are essential for SDS-PAGE sample preparation.

Examples:

• DTT (Dithiothreitol)
• β-Mercaptoethanol (β-ME)
• TCEP

D. Chelating Agents

Bind divalent metal ions that are required by many metalloproteases.

Examples

• EDTA
• EGTA

Functions

• Inhibit metalloproteases
• Reduce metal-catalyzed oxidation

Caution: Avoid EDTA when purifying metalloproteins or using metal-affinity chromatography (e.g., Ni-NTA).

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E. Detergents

Disrupt lipid membranes and solubilize membrane proteins.

Type	Examples	Features
Non-ionic	Triton X-100, Tween-20	Mild; preserves protein activity
Zwitterionic	CHAPS	Suitable for membrane proteins and proteomics
Ionic	SDS	Strong denaturant; used mainly for SDS-PAGE

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F. Plant-Specific Additives

Plant extracts often require additional reagents to remove interfering compounds.

PVPP (Polyvinylpolypyrrolidone)

• Binds phenolic compounds.
• Prevents oxidation and protein precipitation.

Antioxidants

• Ascorbic acid
• Sodium metabisulfite
• β-Mercaptoethanol

Functions

• Prevent oxidation of phenolics.
• Preserve protein integrity and enzyme activity.

Key Points to Remember

• The extraction protocol should be selected according to the biological source and the intended downstream application.
• Samples should be maintained at low temperature, and appropriate protease inhibitors should be included to minimize protein degradation.
• The cell disruption method should be chosen based on the nature of the biological material and the requirement to preserve protein integrity.
• Extraction buffers should contain appropriate buffering agents, inhibitors, reducing agents, chelators, and detergents to maximize protein yield and stability.
• Plant tissues generally require the addition of PVPP and antioxidants to remove phenolic compounds and protect proteins from oxidative damage.

Salting In and Salting Out

• Salting In: Proteins have surface charges that, in the absence of salt, can lead to unfavorable protein-protein aggregation and precipitation. At low salt concentrations (e.g., NaCl, KCl), the added ions shield these charges, reducing inter-protein attraction, increasing protein-solvent interaction, and thereby increasing solubility. 
• Salting Out: At high ionic strength (high salt concentration, typically with ammonium sulfate), salt ions compete with proteins for water molecules (hydration shell). This effectively reduces the water available to solvate the proteins, causing increased hydrophobic-hydrophobic interactions between proteins, leading to aggregation and precipitation (fractionation). Different proteins precipitate at different salt concentrations.

Dialysis

• A technique to remove small molecules (like salts or detergents) from a protein solution based on size.

• Mechanism:

o    A protein solution is placed inside a semi-permeable membrane (dialysis bag) with a defined Molecular Weight Cut-Off (MWCO).

o    The bag is immersed in a large volume of dialysis buffer (dialysate).

o    Small molecules diffuse freely across the membrane down their concentration gradient until equilibrium is reached, while large proteins are retained inside the bag. Repeated changes of the dialysate buffer efficiently remove the small contaminants or exchange the buffer system.

Protein Chromatography Techniques

Gel Filtration / Size Exclusion Chromatography (SEC)

·         Principle: Separation based on hydrodynamic radius (molecular size and shape).

·      Stationary Phase: Inert, porous beads (e.g., cross-linked dextran or agarose, like Sephadex or Sepharose) with a defined range of pore sizes.

·         Mechanism:

o    The total volume of the column is summation of the void volume (volume outside the beads, V_o), the inner volume (volume inside the pores, V_i), and the gel matrix volume (V_g).

o    Large proteins are completely excluded from the pores, travel only through the void volume (V_o), and elute first (they have the smallest elution volume, V_e ≈V_o).

o    Small proteins can fully enter the pores, travel the longest path (V_e ≈ V_o + V_i), and elute last.

o    Proteins of intermediate size are partially excluded.

·    Key Application: Determining the molecular weight of a native protein (by comparing V_e to known standards) and separating proteins from small molecules (like salts/dyes).

·   Elution Volume (V_e): The volume of mobile phase required to elute a specific protein. The relationship between log(MW) and V_e is linear within the fractionation range of the column.

Ion Exchange Chromatography (IEX)

·      Principle: Separation based on the net electrical charge of the protein, which is determined by the buffer pH relative to the protein's isoelectric point (pI).

·    Stationary Phase: An insoluble polymer matrix with covalently attached charged functional groups (the ion exchanger).

·   Mechanism: The binding of the protein to the column is an electrostatic interaction (ionic bond).

o   Anion Exchanger (e.g., DEAE-cellulose): Has a positive charge; binds negatively charged proteins. Used when the pH of the buffer is > pI (protein is anionic).

o    Cation Exchanger (e.g., CM-cellulose): Has a negative charge; binds positively charged proteins. Used when the pH of the buffer is < pI (protein is cationic).

 

·        Elution: Proteins are released (eluted) by disrupting the electrostatic bond, typically by:

o    Increasing the salt concentration (NaCl or KCl): Salt ions compete with the protein for binding to the resin. Proteins with the lowest net charge elute first.

o    Changing the pH of the buffer to alter the net charge of the protein or the resin.

Affinity Chromatography

·         Principle: Highly specific separation based on biological specificity (a specific, reversible non-covalent binding) between the protein of interest and a specialized ligand.

·     Stationary Phase: An insoluble matrix to which the ligand (e.g., substrate analog, inhibitor, antibody, metal ion) is covalently attached.

·         Mechanism:

o   Loading and Washing: Only the target protein binds specifically to the immobilized ligand. All other non-binding proteins are washed away.

o   Elution: The target protein is released by methods that disrupt the specific protein-ligand interaction:

§  Competitive Elution: Adding a high concentration of the free ligand in the mobile phase, which competes for the protein's binding site.

§  Non-Specific Elution: Changing the pH or ionic strength (e.g., high salt) to destabilize the binding.

·      Key Example: IMAC (Immobilized Metal Affinity Chromatography): Used for His-tagged proteins. The tag binds to immobilized metal ions (Ni²⁺ or Co²⁺). Elution is done with high concentrations of imidazole, which competitively binds to the metal ions.

High-Performance Liquid Chromatography (HPLC)

·   Description: An advanced, highly precise form of column chromatography utilizing high pressure to pump the mobile phase through densely packed columns.

·         Key Characteristics:

o   Finer Stationary Phase: Uses very small, uniform particles (typically 3–5 µm), which significantly increases the surface area and efficiency.

o  High Pressure: Requires high-pressure pumps (≥ 5,000 psi) to overcome the flow resistance caused by the tightly packed column.

o    High Resolution: Provides superior separation quality and narrower peaks.

o    Fast Separation: Enables quick analysis due to rapid flow and high efficiency.

·    Application: Often used for analytical protein and peptide separation, particularly in Reverse-Phase HPLC (RP-HPLC), where peptides are separated based on their hydrophobicity using a non-polar stationary phase and a polar-to-non-polar solvent gradient.

 

 

 

 

Module: Protein Chemistry

Protein Architecture

Working with Proteins

Protein Folding

Tutorials and slides

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Protein Architecture

Proteins are central to nearly all biological processes and possess key properties that enable their wide range of functions.

Crucial Functions of Proteins

Proteins perform diverse, essential roles in living systems:

• Catalysts: Function as enzymes to speed up specific chemical reactions.

• Transport and Storage: Carry and store molecules, such as oxygen.

• Structural Support: Provide mechanical support (e.g., in connective tissue and the cytoskeleton).

• Immunity: Offer immune protection.

• Movement: Generate movement (e.g., in muscle cells).

• Communication: Transmit nerve impulses and signals.

• Regulation: Control growth and differentiation.

Key Properties of Proteins

1. Linear Polymers with Defined Structures

• Building Blocks: Proteins are linear polymers made of monomer units called amino acids, linked end-to-end.

          

Amino acid structure

• Peptide Bond: Amino acids are linked by a peptide bond (or amide bond), formed by linking the α-carboxyl group of one amino acid to the α-amino group of another, with the loss of a water molecule. This bond is kinetically stable.

Peptide bond formation

• Polypeptide Chain: A series of amino acids joined by peptide bonds forms a polypeptide chain; each amino acid in the chain is called a residue.
• Directionality: A polypeptide chain has directionality:
• N-terminal (α-amino) end: The beginning of the chain.
• C-terminal (α-carboxyl) end: The end of the chain.
• Protein Structure Levels: The amino acid sequence dictates the three-dimensional structure:
• Primary Structure: The specific sequence of linked amino acids
• Secondary Structure: Local three-dimensional structure formed by hydrogen bonds between amino acids near one another (e.g., α-helices, β-sheets).
• Tertiary Structure: Overall long-range three-dimensional structure formed by long-range interactions between amino acids. Protein function depends directly on this structure.
• Quaternary Structure:  Exists in proteins composed of several distinct polypeptide chains (subunits) that assemble into a functional complex.

Protein structure levels

• Cross-links: Polypeptide chains can be cross-linked; the most common are disulfide bonds, formed by the oxidation of a pair of cysteine residues, resulting in a unit called cystine.

2. Wide Range of Functional Groups

• Proteins contain diverse functional groups including alcohols, thiols, carboxylic acids, and basic groups.
• The array and sequence of these chemically reactive groups account for the broad spectrum of protein function, especially the catalytic activity of enzymes.

3. Ability to Form Complex Assemblies

• Proteins can interact with each other and other macromolecules (e.g., DNA, lipids) to form complex, synergistic assemblies or "macromolecular machines."
• Examples include machinery for DNA replication, signal transmission, and muscle contraction.

4. Varying Rigidity and Flexibility

• Some proteins are rigid (e.g., structural elements in the cytoskeleton or connective tissue).
• Others display flexibility and can act as hinges, springs, or levers.
• Conformational changes in flexible proteins enable the transmission of information and the regulated assembly of larger complexes.

Amino Acids: The Building Blocks

• Structure: An α-amino acid consists of a central α-carbon atom linked to:
• An amino group (NH₂)
• A carboxylic acid group (COOH)
• A hydrogen atom (H)
• A distinctive R group (or side chain)

L and D isomers of amino acids

• Chirality: With four different groups attached to the α-carbon, α-amino acids are chiral (except glycine, whose R group is a single H atom, making it achiral).
• Isomers: Chiral amino acids exist as L and D isomers (mirror images). Only L amino acids are constituents of proteins.
• Ionization (Zwitterions): In solution at neutral pH, amino acids exist predominantly as dipolar ions (zwitterions), where the amino group is protonated (-NH₃⁺) and the carboxyl group is deprotonated (-COO^-). The ionization state varies with pH.

The ionization state of amino acids under different pH conditions

• The Set of 20: All proteins are constructed from the same set of 20 common amino acids, with side chains varying in size, shape, charge, hydrogen-bonding capacity, hydrophobic character, and chemical reactivity.

Classification of Amino Acids (Based on R Group)

The 20 common amino acids are categorized into four groups:

1. Hydrophobic Amino Acids (Nonpolar R groups)

• Tend to cluster together in the interior of water-soluble proteins (hydrophobic effect).
• Aliphatic: Glycine (achiral, H side chain), Alanine (CH₃), Valine, Leucine, Isoleucine (includes an additional chiral center), Methionine (includes a thioether -S- group).

Structures of hydrophobic amino acids with aliphatic R group

• Cyclic/Restricted: Proline (side chain bonded to both the N and α-carbon atoms, forming a pyrrolidine ring, which restricts conformation).

                                Structures of hydrophobic amino acids with cyclic R group

• Aromatic: Phenylalanine (purely hydrophobic), Tryptophan (less so due to its NH group in the indole ring).

Structures of hydrophobic amino acids with aromatic R group

2. Polar Amino Acids (Neutral R groups, Uneven Charge Distribution)

• More hydrophilic (water-loving) than hydrophobic amino acids.
• Hydroxyl Groups (-OH): Serine, Threonine (has an additional asymmetric center), Tyrosine (aromatic ring). The -OH groups make them reactive.

Structures of polar amino acids with hydroxyl groups in R group

• Carboxamide Groups (-CONH₂): Asparagine, Glutamine.

Structures of polar amino acids with carboxamide groups in R group

• Sulfhydryl Group (-SH): Cysteine (structurally similar to serine but with a more reactive thiol group; pairs can form disulfide bonds).

Structure of polar amino acid with sulfhydryl group in R group

3. Positively Charged Amino Acids (Positive Charge at Physiological pH)

• Highly hydrophilic.
• Lysine (primary amino group at terminus).
• Arginine (guanidinium group at terminus).
• Histidine (imidazole group; pK_α near 6, can be uncharged or positively charged near neutral pH, often found in enzyme active sites).

Structures of positively charged amino acids

4. Negatively Charged Amino Acids (Negative Charge at Physiological pH)

• Also highly hydrophilic.
• Aspartic acid and Glutamic acid.
• Often called Aspartate and Glutamate to emphasize their negatively charged state (COO^-) at physiological pH.

Structures of negatively charged amino acids

Significance of Amino Acid Sequence

The amino acid sequence (primary structure) is profoundly important:

• Determines Function: Essential for elucidating a protein's function (e.g., enzyme catalytic mechanism).
• Determines 3D Structure: The sequence is the link between the genetic message (DNA/RNA) and the resulting functional 3D structure.
• Disease: Alterations in a single amino acid can cause severe diseases (e.g., sickle-cell anemia).
• Evolutionary History: Similar sequences indicate a common ancestor, allowing molecular events in evolution to be traced.

Primary Structure: Geometry and Conformation of the Protein Backbone

The specific geometry of the polypeptide backbone is crucial because it limits the number of possible protein folds, allowing proteins to adopt a unique, functional three-dimensional structure.

Key Features of the Peptide Bond

The chemical nature of the peptide bond (linking the carbonyl C and amino N) imposes significant constraints on the backbone:

·     Planar Structure: The peptide bond is essentially planar. Six atoms lie in the same plane: α-carbon and CO (from the first amino acid) and NH, α-carbon (from the second amino acid).

Planar structure of peptide bond

·       Partial Double-Bond Character: The bond resonates between a single bond and a double bond, giving it partial double-bond character.

o    This prevents rotation about the C-N peptide bond, constraining the conformation of the peptide backbone.

Peptide bond resonance

o    The C-N bond length (typically 1.32 Å) is intermediate between a single bond (1.49 Å) and a double bond (1.27 Å).

·   Uncharged: The peptide bond is uncharged, which permits polymers to form tightly packed globular structures.

·      Trans Configuration Preference: Two configurations are possible for the planar peptide bond:

o    Trans: The two α-carbon atoms are on opposite sides of the peptide bond. Almost all peptide bonds in proteins are trans due to fewer steric clashes.

o    Cis: The two α-carbon atoms are on the same side of the peptide bond.

o    Exception: The most common cis peptide bonds are X-Pro linkages, where the cyclic side chain of proline reduces the steric difference between the trans and cis forms.

Configurations of peptide bond

 Freedom of Rotation and Torsion Angles

In contrast to the rigid peptide bond, the adjacent single bonds allow rotation:

·       Single Bonds: The bonds between the N and the C_α atom, and between the C_α and the CO atom, are pure single bonds.

·     Conformational Freedom: The rotation around these two bonds per residue allows proteins to fold in many different ways.

·       Torsion Angles: These rotations are specified by torsion angles:

o    Phi (Φ): Angle of rotation about the bond between the nitrogen (N) and the α-carbon (C_α) atoms.

o    Psi (ψ): Angle of rotation about the bond between the α-carbon (C_α) and the carbonyl carbon (CO) atoms.

o    The Φ and ψ angles determine the path of the polypeptide chain.

Rotation about bonds in a polypeptide chain

Ramachandran Plot and Steric Exclusion

Not all combinations of Φ and ψ angles are possible due to physical limitations:

·      Steric Clashes: Many combinations of Φ and ψ angles are forbidden because of steric collisions (atoms physically occupying the same space).

·    Ramachandran Plot: The allowed combinations of Φ and ψ angles are visualized on a two-dimensional plot called a Ramachandran plot.

·      Limitation: Approximately three-quarters of the possible Φ and ψ combinations are excluded simply by local steric clashes.

·     Thermodynamic Role: The rigidity of the peptide unit and the restricted set of allowed Φ and ψ angles limit the number of possible conformations for the unfolded protein. This reduction in conformational flexibility is essential to overcome the unfavorable entropy of folding, allowing proteins to fold into a unique, defined structure.

Ramachandran plot

Protein Secondary Structure: Helices, Sheets, and Turns

Secondary structure refers to the regular, local folded segments of a polypeptide chain, stabilized by a regular pattern of hydrogen bonds between the peptide N-H and C=O groups of amino acids that are near one another in the linear sequence.

1. The Alpha (α) Helix

The α-helix is a common, rod-like, coiled structure proposed by Linus Pauling and Robert Corey.

·    Structure: A tightly coiled backbone forms the inner part of the rod, with side chains extending outward in a helical array.

·   Stabilization: Stabilized by intrachain hydrogen bonds (within the same polypeptide strand).

·   Hydrogen Bonding Pattern: The C=O group of each amino acid forms a hydrogen bond with the N-H group of the amino acid situated four residues ahead in the sequence (i to i+4). This pattern ensures that almost all main-chain C=O and N-H groups are hydrogen bonded (except near the ends).

H-bonding pattern in a helix

·   Geometry:

o    Residues per turn: 3.6 amino acid residues per complete turn.

o    Rise (Translation) per residue: 1.5 Å along the helix axis.

o    Pitch (Length of one turn): 3.6 residues x 1.5 Å / residue = 5.4 Å.

·   Screw Sense:

o    The α-helices found in proteins are right-handed (clockwise) because this orientation is energetically more favorable due to less steric clash between the side chains and the backbone.

·   Helix Destabilizers ("Helix Breakers"):

o    Proline: Lacks an N—H group for hydrogen bonding and its rigid ring structure prevents the necessary Φ torsion angle.

o  Amino Acids with β-carbon branching (Valine, Threonine, Isoleucine): Their bulkiness causes steric clashes.

o    Serine, Aspartate, Asparagine: Their side chains can compete for the main-chain N—H and C═O groups, disrupting the regular pattern.

· Occurrence: α-helices are abundant; for example, they make up about 75% of the residues in ferritin. Many proteins that span biological membranes also contain α-helices.

Alpha helix and beta sheet structures

2. The Beta (β) Pleated Sheet

The β-pleated sheet (or β-sheet) is another common periodic structure, markedly different from the rod-like α-helix.

·  Components: Formed by two or more polypeptide chains or segments called β-strands lying next to one another.

·    β-Strand Structure: Each β-strand is almost fully extended (not coiled).

o    Distance between adjacent amino acids: Approximately 3.5 Å (much longer than 1.5 Å in an α-helix).

o    Side chains of adjacent amino acids point in opposite directions (up and down).

· Stabilization: Stabilized by interchain hydrogen bonds (between the N—H and C═O groups of adjacent strands).

·   Arrangements: Adjacent β-strands can run in two ways:

o    Antiparallel β-Sheet: Adjacent strands run in opposite directions. The N—H and C═O of each amino acid are directly hydrogen bonded to the C═O and N—H groups of a partner on the adjacent strand.

o    Parallel β-Sheet: Adjacent strands run in the same direction. The hydrogen bonding is slightly more complex, connecting each residue's N—H to the C═O of one residue on the adjacent strand, and its C═O to the N—H of an amino acid two residues farther along.

·   Shape: β-sheets are structurally diverse; while they can be flat, most adopt a somewhat twisted shape.

·   Occurrence: They are important structural elements, forming the basis of proteins like fatty acid-binding proteins. β-sheets typically contain 4 or 5 strands, but can have 10 or more.

Anti-parallel beta sheet

Parallel beta sheet

3. Turns and Loops

These elements facilitate the compact, globular shape of proteins by reversing the polypeptide chain's direction.

·  Reverse Turn (or β-turn/Hairpin Turn): A common structural element that causes an abrupt reversal in the polypeptide chain's direction.

o    Often stabilized by a hydrogen bond between the C═O groups of residue i and the N—H group of residue i+3.

·   Loops (or Ω-loops): More elaborate structures responsible for chain reversals.

o    Lack regular, periodic structures (unlike α-helices and β-strands).

o    Are often rigid and well defined despite their lack of periodicity.

· Location and Function: Turns and loops invariably lie on the surfaces of proteins and frequently participate in interactions between proteins and other molecules.

Structure of a reverse turn

                                            

Tertiary and Quaternary Protein Structure

1. Tertiary Structure: The Overall Fold

Tertiary structure refers to the overall course of the polypeptide chain; specifically, the spatial arrangement of amino acid residues that are far apart in the linear sequence, and the pattern of any disulfide bonds.

Features of Tertiary Structure

·         It represents the final, stable, three-dimensional shape of a single polypeptide chain.

·         Example (Myoglobin): The oxygen-storage protein in muscle is a single, compact polypeptide chain (153 amino acids) with approximately 70% of the chain folded into eight α-helices connected by turns and loops.

3-D structure of myoglobin

·         Driving Force: The Hydrophobic Effect

o    In an aqueous environment, protein folding is primarily driven by the tendency of hydrophobic residues to be excluded from water (the Hydrophobic Effect).

o    Interior Core: Consists almost entirely of nonpolar residues (e.g., leucine, valine, methionine). Charged residues are excluded from the interior.

o    Exterior Surface: Consists of a mixture of polar and nonpolar residues.

·         Stabilizing Interactions:

o    Hydrogen Bonding: To bury a segment of the main chain in the hydrophobic interior, all peptide N—H and C═O groups must be paired by hydrogen bonding (achieved by forming α-helices or β-sheets).

o    Van der Waals Interactions: These maximize stability when the nonpolar side chains in the core are tightly packed to minimize empty space.

·       Amphipathic Secondary Structures: Many α-helices and β-strands are amphipathic, meaning they have a hydrophobic face (pointing into the protein interior) and a polar face (pointing into the surrounding aqueous solution).

·         Membrane Proteins (The "Exception"): Proteins that span biological membranes, like porins in bacterial outer membranes, are "inside out" because they function in a hydrophobic environment. Their outside surface is covered largely with hydrophobic residues to interact with the membrane's alkane chains, while the interior often contains polar/charged residues surrounding a water-filled channel.

Motifs and Domains

·         Motifs (Super-secondary Structures): Certain combinations of secondary structure that are present in many proteins and often exhibit similar functions.

o    Example: A helix-turn-helix unit, found in many DNA-binding proteins.

Structure and DNA-binding nature of Helix-turn-helix domain

·         Domains: Compact globular units into which a polypeptide chain can fold, ranging from 30 to 400 amino acid residues.

o    Domains may be connected by flexible segments and may be shared among proteins with different overall tertiary structures (e.g., the CD4 immune protein has four similar domains).

Structure of pyruvate kinase tetramer showing different subunits and domains

2. Quaternary Structure: Subunit Arrangement

Quaternary structure refers to the spatial arrangement of multiple polypeptide chains (subunits) and the nature of their interactions. It is the fourth level of protein organization.

·    Subunit: Each individual polypeptide chain in a protein with quaternary structure is called a subunit.

·         Simple Structures:

o    Dimer: Two identical subunits (e.g., the Cro DNA-binding protein).

o    Tetramer: Four subunits.

Formation of homo- and heterodimers

·     Complex Structures: Proteins can contain more than one type of subunit, often in variable numbers.

·         Example (Hemoglobin): The oxygen-carrying protein in blood is a α₂β₂ tetramer, consisting of two α subunits and two β subunits. Subtle changes in the arrangement of these subunits are critical for its function.

·    Viral Coats: Viruses often utilize quaternary structure to make the most of limited genetic information, forming symmetric capsids using many copies of a few types of subunit (e.g., rhinovirus coat uses 60 copies of each of four subunits).

 

The α2β2 tetramer of hemoglobin

References

Berg, J. M., Tymoczko, J. L., & Stryer, L. (2007). Biochemistry (6th ed.). W. H. Freeman.

Nelson, D. L., Cox, M. M., & Hoskins, A. A. (2021). Lehninger principles of biochemistry (8th ed.). W. H. Freeman.

Voet, D., Voet, J. G., & Pratt, C. W. (2016). Fundamentals of biochemistry: Life at the molecular level (5th ed.). John Wiley & Sons.

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