Lipid Biochemistry (Part I)

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Lipids

Lipids are water-insoluble biomolecules that are highly soluble in organic solvents like chloroform.

Lipids are a heterogeneous group of compounds related to fatty acids and include fats, oils, waxes, and other related substances.

Lipids are hydrophobic in nature.

The term ‘lipid’ was first coined by a German biochemist, Bioor, in 1943.

Chemically fats are defined as the esters of glycerol and fatty acids or triglycerides of fatty acids.

Functions of lipids

A diverse range of functions in biological systems:

Important constituents of diet because of their high energy value

Serve as a source of fat-soluble vitamins and essential fatty acids in natural foodstuffs.

Serve as a stored form of energy in adipose tissues.

Serve as an insulating material in subcutaneous tissues and around certain organs.

Lipoproteins are important constituents of biological membranes.

Plays crucial roles as:

enzyme cofactors, 

electron carriers, 

light absorbing pigments, 

emulsifying agent in digestive tracts, 

hormones and intracellular messengers

May act as chaperones to help membrane proteins fold.

Advantage of energy storage as fatty acids

The carbons in the fatty acids (mostly –CH₂ group) is almost completely reduced compared to the carbon in other simple biomolecules (sugars, amino acids, etc.). Therefore, oxidation of fatty acids will yield more energy (in form of ATP) than any other form of carbon.

Fatty acids are not hydrated as mono- and polysaccharides and thus can be packed more closely in storage tissues.

Fatty acids

Fatty acids are hydrocarbon chains of various lengths and degrees of unsaturation that terminate with the carboxyl group.

Fatty acids that occur in natural fats are usually monocarboxylic.

They contain an even number of carbon atoms as they are synthesized from condensation of 2 carbon units. 

These range from 4 to 36 C-atoms.

The chain may be saturated (no double bonds) or unsaturated (contain one or more double bonds).

Some fatty acids may have hydroxyl groups in the chain (hydroxy-fatty acid) and still, others may possess ring structures (cyclic-fatty acids).

Alcohols in lipid molecules

Alcohols present in lipid molecules commonly include glycerol, cholesterol, and higher alcohols like cetyl alcohol and mericyl alcohol.

In the structural formula of glycerol, the C-atoms are numbered 1, 2, 3 from any end. Since C1 and C3 are identical, they are also denoted as α, β, and α’.

Nomenclature of fatty acids

The systematic nomenclature of fatty acids is based on the Genevan System under the instruction of IUPAC.

The fatty acid is named after the hydrocarbon with the same number of carbon atoms, the suffix –oic is written in place of the final letter ‘-e’ in the name of the hydrocarbon.

The names of the saturated fatty acids end with the suffix –anoic and those of unsaturated fatty acids with the suffix –enoic.

The positions of the carbon atoms are denoted by numbering (carboxyl carbon atom is C1, the adjacent carbon atom is C2 and so on) or by using Greek letters (C2 is denoted as α-carbon, C3 is denoted as β-carbon and so on).

A widely used convention is to indicate the carbon atom number followed by the number and position of double bonds in the case of unsaturated fatty acids (position of a double bond is indicated by a lower number of the two carbon atoms involved in double bonding).

In nearly all naturally occurring unsaturated fatty acids, the double bonds are in the cis configuration.

Trans fatty acids are produced by fermentation in the rumen of dairy animals and are obtained from dairy products and meat.

They are also produced during hydrogenation of fish or vegetable oils. 

Diets high in trans fatty acids correlate with increased blood levels of LDL (bad cholesterol) and decreased HDL (good cholesterol).

French fries, doughnuts, fast foods, and cookies tend to be high in trans fatty acids.

Lipid classification

Bioor (1943) classified lipids on the basis of their chemical composition:

Simple lipids or homolipids

Compound lipids or heterolipids

Derived lipids

Conn and Stumpf (1976) traditionally classified lipids into the following categories:

Acyl glycerols

Waxes

Phospholipids

Sphingolipids

Glycolipids

Terpenoid lipids including carotenoids and steroids

Lipid types (on the basis of function)

Physical properties of lipids

State: Saturated fatty acids are solid at room temperature while unsaturated fatty acids are liquid, in general.

Colour, odour and taste: Pure fats are colourless, odourless and bland taste.

Solubility: Soluble in organic solvent. Longer the fatty acyl chain and fewer the double bonds, the lower is its solubility in water.

Specific gravity: Less than 1

Geometric isomerism: Present

Emulsification: Found

Melting point: Depends on the chain length of the constituent fatty acyl chain and the degree of unsaturation.

Chemical properties of lipids

Hydrolysis: Fats are hydrolyzed by enzyme lipase to yield fatty acids and glycerol.

Saponification: Hydrolysis of fats by alkali is called saponification which results in the formation of glycerol and salts of fatty acids (called soaps).

Hydrogenation: Unsaturated fatty acids react with gaseous hydrogen to yield saturated fatty acids.

Halogenation: Unsaturated fatty acids and their esters can take up halogens (Br2, I2, etc.) at their double bonds at room temperature in acetic acid or methanol solution. This reaction is the basis of ‘iodine number determination’.

Oxidation: Unsaturated fatty acids are susceptible to oxidation at their double bonds.

Acid number: It is the number of milligrams of KOH required to neutralize the free fatty acids present in 1 gram of fat. The acid number indicates the quantity of free fatty acid present in fat.

Saponification number: It is the number of milligrams of KOH required to saponify 1 gram of fat. The saponification number provides information on the average chain length of the fatty acids in fats.

Iodine number: It is the number of grams of iodine absorbed by 100 grams of fat. The iodine number indicates the degree of unsaturation of the fatty acids present in fat.

Acetyl number: It is the number of milligrams of KOH required to neutralize the acetic acid obtained by saponification of 1 gram of fat after it has been acetylated. The acetyl number indicates the number of –OH groups present in fat.

Rancidity: When lipid-rich foods are exposed for too long in the air (oxygen), they may spoil and become rancid. The unpleasant taste and smell associated with rancidity results from oxidative cleavage of the double bonds in unsaturated fatty acids, producing aldehydes and carboxylic acids of shorter chain length which are volatile.

Essential fatty acids

Our body can synthesize most of the fatty acids except a few. These fatty acids must be supplied through diet and are termed essential fatty acids.

Example-α-Linolenic acid (ω-3 fatty acid), Linoleic acid (ω-6 fatty acid)

It is recommended that essential fatty acids make up 3% to 6% of your daily caloric intake. Of this percentage, you should consume 2 to 4 times more omega-6 fatty acids than omega-3 fatty acids. 

Omega-3 sources include:

Nuts

Soybeans

Walnut oil

Canola oil

Flaxseed oil

Cold water fatty fish such as salmon, herring, cod, flounder, tuna, bluefish, and shrimp

Omega-6 sources include:

Leafy vegetables

Seeds

Nuts

Grains

Vegetable oils (corn, safflower, soybean, cottonseed, sesame, sunflower)

Roles of Essential fatty acids

• Help with cellular development and the formation of healthy cell membranes.
• Blocks tumor formation and growth of cancer cells
• Assist in the development and function of the brain and nervous system.
• Regulates proper thyroid and adrenal activity. 
• Plays a role in thinning your blood, which prevents blood clots, heart attacks and stroke.
• Regulates blood pressure, immune responses and liver function.
• Deficiency causes skin problems, including eczema, dandruff, split nails and brittle hair.
• Forms Lipid rafts which affects cellular signalling.
• Acts on DNA, activates or inhibits transcription factors.

Carbohydrate Biochemistry (Part II)

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Disaccharides

Disaccharides consist of two monosaccharides joined covalently by an O-glycosidic bond, which is formed when a hydroxyl group of one sugar reacts with the anomeric carbon of the other.

Example: maltose, lactose, and sucrose

Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base. Thus disaccharides can be hydrolyzed to yield their free monosaccharide components by boiling with dilute acid.

N-glycosyl bonds join the anomeric carbon of a sugar to a nitrogen atom in glycoproteins and nucleotides.

General formula: Cₙ(H2O)ₙ₋₁

The oxidation of a sugar’s anomeric carbon by cupric or ferric ion (the reaction that defines a reducing sugar) occurs only with the linear form, which exists in equilibrium with the cyclic form(s).

When the anomeric carbon is involved in a glycosidic bond, that sugar residue cannot take the linear form and therefore becomes a non-reducing sugar.

The end of a chain with a free anomeric carbon (one not involved in a glycosidic bond) is commonly called the reducing end.

The disaccharide maltose contains two D-glucose residues joined by a glycosidic linkage between C-1 (the anomeric carbon) of one glucose residue and C-4 of the other.

Because the disaccharide retains a free anomeric carbon (C-1 of the glucose residue on the right), maltose is a reducing sugar.

Nomenclature of disaccharides (or oligosaccharides)

By convention, the name describes the compound with its nonreducing end to the left.

Give the configuration (α or β) at the anomeric carbon joining the first monosaccharide unit (on the left) to the second.

Name the nonreducing residue; to distinguish five- and six-membered ring structures, insert “furano” or “pyrano” into the name.

Add suffix '-syl' to the name of the first residue. For example, 'glucopyranosyl'.

Indicate in parentheses the two carbon atoms joined by the glycosidic bond, with an arrow connecting the two numbers; for example, (1🠊4) shows that C-1 of the first-named sugar residue is joined to C-4 of the second. 

If the anomeric carbons from both residues are involved in bond formation, a double headed arrow is given.

Name the second residue similarly. Add suffix '-ose' to the name of the second residue in case of reducing sugars and '-oside' in case of non-reducing sugars.

If there is a third residue, describe the second glycosidic bond by the same conventions.

Short name:Glc(α1🠊4)Glc

Sucrose

Sucrose (cane sugar) is made up of α-D-glucose and β-D-fructose linked by a glycosidic bond (α1 ⟺ β2). 

The reducing  groups (anomeric carbon) of both glucose and fructose are involved in glycosidic bond formation. Hence, sucrose is non-reducing sugar and it cannot form osazones.

The systematic name of sucrose is α-D-glucopyranosyl-(1 ⟺ 2)- β-D-fructofuranoside. 

This indicates:

It is composed of two monosaccharides: glucose and fructose

Ring type: Glucose is pyranose and fructose is furanose

Linkage: oxygen on C1 of α-D-glucose  is linked to C2 of β-D-fructose 

Suffix '–oside' and '⟺' indicates that the anomeric carbons of both the monosaccharides participate in glycosidic bond formation

Sucrose is a major carbohydrate produced in photosynthesis. It has the advantage as storage and transport as its functional groups are held together and are protected from oxidative attacks.

Intestinal enzyme, sucrase hydrolyze sucrose to glucose and fructose.

Inversion of sucrose:

Sucrose is dextrorotatory (+66.5°). But when hydrolyzed, it becomes levorotatory (-28.2°). The process of change in optical rotation from dextrorotatory(+) to levorotatory (-) is referred to as inversion. The hydrolyzed mixture of sucrose, containing glucose and fructose, is known as invert sugar.

Sucrose first splits into α-D-glucopyranose (+) and β-D-fructofuranose (+). But β-D-fructofuranose is less stable and gets converted into β-D-fructopyranose (-). The overall effect in the mixture becomes levorotatory (-).

Lactose

Lactose (milk sugar) is composed of β-D-galactose and β-D-glucose held together by   β(1🠊4) glycosidic bond.

The anomeric carbon of C1 of glucose is free. Hence lactose exhibits reducing properties and forms osazones (powder-puff or hedgehog shape).

The systematic name is β-D-galactopyranosyl-(1🠊4)-β-D-glucopyranose.

It is hydrolyzed by intestinal enzyme lactase into glucose and galactose.

Maltose

Maltose (malt sugar) is produced during digestion of starch by enzyme amylase.

Maltose is composed of two α-D-glucose  units held together by α(1🠊4) glycosidic bond. 

A free aldehyde group is present on C1 of the second glucose unit and hence maltose exhibits reducing properties and forms osazones (sunflower shaped).

It can be hydrolyzed by dilute acid or enzyme maltase.

 

In isomaltose, the glucose units are held together by α(1🠊6) glycosidic bond.

Cellobiose

It is identical to maltose, except that in it the linkage is  β(1🠊4) glycosidic bond.

It is formed during hydrolysis of cellulose.

Trehalose

It is identical to maltose, except that in it the linkage is  (α1⇔α1) glycosidic bond.

It is formed during hydrolysis of cellulose.

It is a non-reducing sugar.

Examples of other oligosaccharides

Raffinose (trisaccharides): Fructose+Galactose+Glucose

Stachyose (Tetrasaccharide):  Galactose+Galactose+Glucose+Fructose

Verbascose (Pentasaccharide): Galactose+Galactose+Galactose+Glucose+Fructose

Polysaccharides

Carbohydrates containing repeating units (more than 10 units) of the monosaccharides or their derivatives linked by glycosidic linkages are called polysaccharides.

They are primarily concerned with 2 important functions:

Structural role

Storage of energy

Polysaccharides can be linear or branched. The occurrence of branched polysaccharides is due to the fact that glycosidic linkages can be formed at any one of the –OH groups of a monosaccharide.

Polysaccharides are of high molecular weight. They are usually tasteless (non-sugars) and form colloids with water.

Polysaccharides are of two types:

Homopolysaccharides (Homoglycans): They, on hydrolysis, yield only one type of monosaccharide. They are named based on the nature of the monosaccharide unit. 

Example: Glucan (polymer of glucose), Fructosan (polymer of fructose)

Heteropolysaccharides (heteroglycans): They, on hydrolysis, yield a mixture of a few types of monosaccharide units or their derivatives. 

Example: Peptidoglycan (polymer of N-acetylglucosamine and N-acetylmuramic acid residues)

Starch

Starch is the carbohydrate reserve of plants which is the most important dietary source for higher animals.

Starch is a homopolysaccharide composed of D-glucose units held by glycosidic bonds. 

It is known as glucosan or glucan.

Starch consists of two polysaccharide components:

Water soluble amylose (15-20%)

Water insoluble amylopectin (80-85%)

Chemically amylose is a long unbranched chain with 200-1000 D-glucose units held by (α1🠊4) glycosidic linkage.

Amylopectin is a branched chain with (α1🠊6) glycosidic bonds at the branching points and (α1🠊4) glycosidic bonds everywhere else.

Starches are hydrolyzed by amylases (pancreatic or salivary) to liberate dextrins and finally maltose and glucose units. Amylase acts specifically on the (α1🠊4) glycosidic bonds.

(b) amylopectin

Dextrins

These are the breakdown products of starch by the enzyme amylase or dilute acids.

Starch is hydrolyzed through different dextrins and finally to maltose and glucose. 

The various intermediates (identified by iodine coloration) are soluble starch (blue), amylodextrin (violet), erythrodextrin (red) and achrodextrin (no colour).

Inulin

Inulin is a polymer of fructose.

It occurs in dahlia bulbs, garlic, onion, etc.

It is a low molecular weight (~ 5000) polysaccharide easily soluble in water.

Inulin is not utilized by the body. 

It is used for assessing kidney function through measurement of glomerular filtration rate (CFR).

Cellulose

Cellulose occurs extensively in plants and is totally absent in animals.

Cellulose is composed of β-D-glucose units linked by β(1🠊4) glycosidic bonds.

Cellulose can not be digested by mammals due to lack of the enzyme that cleaves β-glycosidic bonds. Hydrolysis of cellulose yields a disaccharide, cellobiose, which is further broken down to β-D-glucose units.

It is a major constituent of fibers, the non-digestible carbohydrate.

Glycogen

Glycogen is the main storage polysaccharide of animal cells.

Like amylopectin, glycogen is a polymer of (α1🠊4)-linked subunits of glucose, with (α1🠊6)-linked branches, but glycogen is more extensively branched (on average, every 8 to 12 residues) and more compact than starch.

Glycogen is especially abundant in the liver, it is also present in skeletal muscle.

Chitin

Chitin is a linear homopolysaccharide composed of N-acetylglucosamine residues in β linkage.

The only chemical difference from cellulose is the replacement of the hydroxyl group at C-2 with an acetylated amino group. 

Peptidoglycan

The rigid component of bacterial cell walls is a  heteropolymer  of  alternating (β1🠊4)-linked N-acetylglucosamine and N-acetylmuramic acid residues.

The enzyme lysozyme kills bacteria by hydrolyzing the   (β1🠊4) glycosidic bond between N-acetylglucosamine and Nacetylmuramic acid.

Carbohydrate biochemistry (Part I)

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 Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis.

Many, but not all, carbohydrates have the empirical formula (CH₂O)ₙ, [n≥3];  some also contain nitrogen, phosphorus, or sulfur.

Carbohydrate literally means ‘hydrates of carbon’.

Carbohydrates are the most abundant biomolecules on Earth.

Occurrence & Function

Certain carbohydrates (sugar and starch) are a dietary staple and abundant dietary source of energy (4 cal/g) 

Insoluble carbohydrate polymers serve as structural and protective elements:

       in the cell walls of bacteria and plants

       in the connective tissues of animals

       lubricate skeletal joints

       participate in recognition and adhesion between cells

Complex carbohydrate polymers that are covalently attached to proteins or lipids are called glyco-conjugates.

act as signals that determine the intracellular location or metabolic fate of these hybrid molecules

Carbohydrates are precursors of many organic molecules (fats, amino acids, etc.)

They serve as storage form of energy (Ex- Glycogen, Starch)

Classification

The word “saccharide” is derived from the Greek ‘sakcharon’, meaning “sugar”.

Monosaccharides (simple sugars): Consist of a single polyhydroxy aldehyde or ketone unit. Ex- Glucose, Fructose

Glucose

Oligosaccharides: Consist of short chains of monosaccharide units, or residues (2-10), joined by characteristic linkages called glycosidic bonds. Ex- Raffinose

Raffinose

Disaccharides: Consists of  two monosaccharide units joined by glycosidic bond. Ex- Sucrose (Glucose + Fructose)

Sucrose

Polysaccharides: Sugar polymers containing more than 20 or so monosaccharide units, and some have hundreds or thousands of units. Ex- Cellulose, Glycogen

Monosaccharides

Simplest carbohydrates that cannot be hydrolyzed to smaller carbohydrates.

General chemical formula of unmodified monosaccharide is (C.H₂O)ₙ  where n≥3

Consist of a single polyhydroxy aldehyde or ketone unit. 

The most abundant monosaccharide in nature is the six-carbon sugar D-glucose. 

Monosaccharides of more than four carbons tend to have cyclic structures.

Ex- Glyceraldehyde, Glucose, Fructose, etc. 

Classification of monosaccharides

Classified according to 3 different characteristics:

Placement of its carbonyl group

Number of carbon atoms present

Chiral handedness

Classification based on placement of carbonyl group

ALDOSE: Functional group is an aldehyde group (-CHO)

Ex- Glyceraldehyde, Glucose, etc

KETOSE: Functional group is a keto group (>C=O)

Ex- Dihydroxyacetone, Fructose, etc.

Classification based on number of carbon atoms

On the basis of the number of carbon atoms present, monosaccharides can be classified as:

Triose (3 C)

Tetrose (4 C)

Pentose (5 C)

Hexose (6 C)

Heptose (7 C)

Stereoisomers

Stereoisomers: Compounds that have same structural formulae but differ in their spatial configuration. 

A carbon is said to be asymmetric (chiral) when it is attached to four different atoms or groups. 

The number of asymmetric carbon atoms (n) determines the possible number of isomers of a given compound which is equal to 2ⁿ. 

Stereoisomerism is a characteristic feature of all sugars except Dihydroxyacetone. 

Example- 

    Glucose has 4 asymmetric carbon atoms. No. of isomers = 2⁴ = 16

   Glyceraldehyde has 1 asymmetric carbon atom. No. of isomers = 2¹ = 2

   Dihydroxyacetone has no asymmetric carbon atoms. Hence, no isomer is possible.

Classification based on chiral handedness

D and L isomers: Assignment of D or L isomer is made according to the orientation of the asymmetric carbon atom furthest from the carbonyl group.

In a standard Fischer projection if the hydroxyl group is on the right, the molecule is D sugar, and if the hydroxyl group is on the left, the molecule is L sugar. 

D-sugars are biologically more common.

Optical activity of sugars

It is the characteristic feature of compounds with asymmetric carbon atoms. 

When a beam of polarized light is passed through a solution of an optical isomer, it will be rotated to either the right or left. 

The terms dextrorotatory (+) and levorotarory (-) are used to compounds that respectively rotate the plane of polarized light to the right or to the left.

It may be noted that the D and L configurations of sugars are primarily based on the structure, optical activities may be different.

Racemic mixture: If dextrorotatory and levorotatory isomers are present in equal concentration, it is known as racemic mixture or DL mixture. Racemic mixture does not exhibit any optical activity, since the dextro- and levorotatory activities cancel each other.

Epimers

If two monosaccharides differ from each other in their configuration around a single specific carbon (other than anomeric carbon), they are referred to as epimers to each other.

D-glucose and D-mannose differ only in the stereochemistry at C-2, are epimers.

D-glucose and D-galactose which differ at C-4, are epimers.

Inter-conversions of epimers (eg.- glucose to galactose and vice versa) is known as epimerization and is catalyzed by a group of enzymes called epimerases.

Common Monosaccharides Have Cyclic Structures

In aqueous solution, aldotetroses and all monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl group along the chain. 

The formation of these ring structures is the result of a general reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals. 

These structures contain an additional asymmetric carbon atom and thus can exist in two stereoisomeric forms.

Hemiacetal formation

Hemiketal formation

D-glucose exists in solution as an intramolecular hemiacetal in which the free hydroxyl group at C-5 has reacted with the aldehydic C-1, rendering the latter carbon asymmetric and producing two stereoisomers, designated  as α and β.

These six-membered ring compounds are called pyranoses because they resemble the six membered ring compound pyran.

The systematic names for the two ring forms of D-glucose are α-D-glucopyranose  and β-D-glucopyranose.

Only aldoses having five or more carbon atoms can form pyranose rings.

Aldohexoses and ketohexoses also exist in cyclic forms having five membered rings, which, because they resemble the five membered ring compound furan, are called furanoses.

The six-membered aldopyranose ring is much more stable than the aldofuranose ring and predominates in aldohexose solutions.

Anomers

Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom are called anomers.

The hemiacetal (or carbonyl) carbon atom is called the anomeric carbon.

In case of α-anomer, the –OH group held by anomeric carbon is on the opposite side of the –CH2OH group of the sugar ring. The opposite is true for β-anomers.

The α- and β-anomers of D-glucose interconvert in aqueous solution by a process called mutarotation.

Thus, a solution of α-D-glucose and a solution of β-D-glucose eventually form identical equilibrium mixtures having identical optical properties. This mixture consists of about one-third α-D-glucose (36%), two-thirds β-D-glucose (63%), and very small amounts of the linear and five-membered ring (glucofuranose) forms (1%).

             

 D-glucose      ⃡       Equilibrium mixture        ⃡         β-D-glucose 

   +112.2°                             +52.7°                                  +18.7°

Ketohexoses also occur in α and β anomeric forms.

In these compounds the hydroxyl group at C-5 (or C-6) reacts with the keto group at C-2, forming a furanose (or pyranose) ring containing a hemiketal linkage.

D-Fructose readily forms the furanose ring, the more common anomer of this sugar in combined forms or in derivatives is D-fructofuranose.

The specific optical rotation of fructose is -92° at equilibrium. 

Monosaccharides Are Reducing Agents

Monosaccharides can be oxidized by relatively mild oxidizing agents such as ferric (Fe3+) or cupric (Cu2+).

The carbonyl carbon is oxidized to a carboxyl group.

Sugars capable of reducing ferric or cupric ion are called reducing sugars. They have free aldehyde or ketone group present in their structure.

Ex- Glucose

Sugars not capable of reducing ferric or cupric ion are called non-reducing sugars. They do not have free aldehyde or ketone group present in their structure.

Ex- Sucrose

This property is the basis of Fehling’s reaction, a qualitative test for the presence of reducing sugar.

Monosaccharide derivatives

There are a number of sugar derivatives in which a hydroxyl group in the parent compound is replaced with another substituent, or a carbon atom is oxidized to a carboxyl group.

• In amino sugars, an –NH2 group replaces one of the -OH groups in the parent hexose. 
• Substitution of –H for –OH produces a deoxy sugar.
• The acidic sugars contain a carboxyl group, which confers a negative charge at neutral pH.

Sugar acids: Oxidation of aldehyde or primary alcohol groups in the monosaccharide results in sugar acids.

The acidic sugars contain a carboxyl group, which confers a negative charge at neutral pH.

Examples:

Gluconic acid is produced from glucose by oxidation of aldehyde group.

Glucuronic acid is formed from glucose by oxidation of primary alcohol group (C6).

Amino sugars: When one or more hydroxyl groups of the monosaccharide are replaced by amino groups, the products formed are called amino sugars.

They are present as constituents of heteropolysaccharides.

Examples:

D-glucosamine

D-galactosamine

They are sometimes acetylated.

Example:

N-acetyl-D-glucosamine

Deoxysugars: They contain one oxygen less than that of their parent molecule. 

The groups –CHOH and –CH2OH become –CH2 and –CH3 due to absence of one oxygen atom.

Examples: 

D-2-Deoxyribose

L-Rhamnose

L-Fucose

Sugar alcohols: Sugar alcohols (polyols) are produced by reduction of aldoses or ketoses.

Examples: 

Sorbitol from glucose

Mannitol from mannose

Alditols: The monosaccharides on reduction yield polyhydroxy alcohols known as alditols.

Examples:

Ribitol (constituent of flavin coenzymes)

Glycerol (Component of lipid)

Xylitol (Sweetener used in sugarless gums and candies)