Nitrogen Metabolism

 Click here for slides (PG)

Click here for slides (UG)

Nitrogen Cycle

Nitrogen metabolism is the complex process of how organisms convert and utilize nitrogen. The nitrogen cycle describes the exchange of nitrogen between three main pools: the atmosphere, soil and groundwater, and biomass.

The nitrogen cycle involves several key processes:

Nitrogen Fixation: This is the conversion of atmospheric nitrogen gas (N2​) into a usable form like ammonia (NH3​). It can occur through biological, industrial, or electrical means.

Ammonification: In this decomposition process, microorganisms convert organic nitrogen from decaying biomass into ammonia (NH3​) and ammonium ions (NH4+​). Example: Bacillus vulgaris and Bacillus ramosus

Nitrification: This two-step process converts ammonia to nitrate (NO3−​). First, bacteria like Nitrosomonas or Nitrococcus oxidize ammonia to nitrite (NO2−​). Next, Nitrobacter oxidizes the nitrite to nitrate. These bacteria are chemoautotrophs and are known as nitrifying bacteria.

Denitrification: This process converts nitrates and nitrites back into dinitrogen gas (N2​) and is carried out by microorganisms called denitrifiers, such as Thiobacillus denitrificans.

Nitrate Assimilation: Plants actively absorb nitrate from the soil and convert it into organic nitrogen compounds.

Nitrate Assimilation in Plants

Nitrate assimilation is a two-step process in plants:

Step 1: Reduction of Nitrate to Nitrite 

This reaction occurs in the cytosol and is catalyzed by the enzyme nitrate reductase (NR). It involves the transfer of two electrons from an electron donor, typically NADH in most cases or NADPH in nongreen tissues.

Nitrate reductases in higher plants are composed of two identical subunits, each containing three prosthetic groups:

• flavin adenine dinucleotide (FAD)
• heme
• a molybdenum ion complexed to a pterin molecule

FAD accepts two electrons from NAD(P)H, which are then passed to the heme domain and finally to the molybdenum complex to be transferred to nitrate. Molybdenum is crucial for this enzyme's function, and a deficiency in it can lead to nitrate accumulation.

Regulation of Nitrate Reductase

NR synthesis is regulated at transcriptional and translational levels by several factors, including nitrate, light, and carbohydrate accumulation. The enzyme is also subject to posttranslational modification through reversible phosphorylation.

• Activation: Light, carbohydrates, and other environmental factors stimulate a protein phosphatase that dephosphorylates a key serine residue on the hinge 1 region of NR enzyme, activating it.

• Deactivation: In darkness, a protein kinase stimulated by Mg2+ phosphorylates the same serine residue. This phosphorylation allows the enzyme to interact with a 14-3-3 inhibitor protein, rendering it inactive.

This phosphorylation-based regulation provides a rapid way to control NR activity, happening in minutes compared to the hours it takes for enzyme synthesis or degradation.

Step 2: Reduction of Nitrite to Ammonium 

Nitrite is a highly reactive and potentially toxic ion, so it's immediately transported from the cytosol to chloroplasts or plastids where it is reduced to ammonium (NH4+​) by the enzyme nitrite reductase (NiR). This process requires the transfer of six electrons. The electrons are donated by reduced ferredoxin (Fred​). 

NiR consists of a single polypeptide with two prosthetic groups: 

• an iron-sulfur cluster (Fe4​S4​) 
• a specialized heme

Electrons flow from ferredoxin, through the iron-sulfur cluster and heme, to nitrite. Reduced ferredoxin is derived from the photosynthetic electron transport chain or from NADPH generated in oxidative pentose phosphate pathway in nongreen tissue.

Regulation of Nitrite Reductase

• Activation: High nitrate levels or exposure to light induce the transcription of NiR mRNA.

• Deactivation: The accumulation of end products, such as asparagine and glutamine, represses this induction.

Ammonia Assimilation

Plant cells quickly convert the ammonium generated from nitrate assimilation into amino acids to prevent toxicity. The main pathway for this conversion is the GS-GOGAT pathway, which involves two key enzymes: 

• glutamine synthetase (GS) 
• glutamate synthase (GOGAT)

Step 1: Glutamine Synthetase (GS) 

GS combines ammonium with glutamate to form glutamine. This reaction requires ATP and a divalent cation like Mg2+, Mn2+,or Co2+ as a cofactor.

Plants have three classes of GS with different functions and locations:

• Cytosolic GS: Found in germinating seeds and vascular bundles, producing glutamine for intercellular nitrogen transport. Its expression isn't regulated by light or carbohydrates.

• Plastidal GS: Located in roots, generating amide nitrogen for local consumption. Its expression is regulated by light and carbohydrate levels.

• Chloroplastic GS: Found in shoots, where it reassimilates photorespiratory ammonium (NH4+​). Its expression is also regulated by light and carbohydrate levels.

Step 2: Glutamate Synthase (GOGAT) 

GOGAT transfers the amide group from glutamine to 2-oxoglutarate, producing two molecules of glutamate. There are two types of GOGAT in plants:

• NADH-GOGAT: Accepts electrons from NADH and is found in the plastids of non-photosynthetic tissues like roots or vascular bundles. It assimilates ammonium absorbed from the soil or glutamine translocated from roots or senescing leaves.

• Fd-GOGAT: Accepts electrons from ferredoxin and is found in chloroplasts, where it functions in photorespiratory nitrogen metabolism.

Overall reaction in GS-GOGAT pathway

Alternate Ammonia Assimilation Pathways

Plants can also assimilate ammonia through several alternative pathways.

Glutamate Dehydrogenase (GDH)

This enzyme catalyzes a reversible reaction that can either synthesize or deaminate glutamate. An NADH-dependent form is found in mitochondria, while an NADPH-dependent form is in the chloroplasts of photosynthetic organs.

Transamination

This process incorporates nitrogen from glutamine and glutamate into other amino acids. It involves the transfer of an amino group from one amino acid to a keto acid, producing a new amino acid and a new keto acid. This is a reversible process requiring pyridoxal phosphate (vitamin B6) as a cofactor. 

Examples:

Asparagine Synthetase (AS)

AS transfers the amide nitrogen from glutamine to aspartate, producing asparagine and glutamate. It is found in the cytosol of leaves, roots, and nitrogen-fixing nodules.

Regulation of Ammonium Metabolism

• The regulation of ammonium metabolism is vital for maintaining the nitrogen-to-carbon (N:C) ratio in plants.

• High light and carbohydrate levels: Under these conditions, AS is inhibited, and plastid GS and Fd-GOGAT are activated. This favors the production of carbon-rich compounds like glutamine and glutamate, which are used for synthesizing new plant materials.

• Energy-limited conditions: In these situations, the expression of plastid GS and Fd-GOGAT is reduced, and AS is activated. This leads to more asparagine, an amide that helps in long-distance transport or long-term nitrogen storage.

Amino acid biosynthesis

Humans and most animals cannot synthesize certain essential amino acids and must obtain them from their diet. In contrast, plants can synthesize all 20 standard amino acids. The nitrogen-containing amino group is derived from transamination reactions with glutamine or glutamate. The carbon skeletons for amino acids are derived from intermediates of glycolysis (3-phosphoglycerate, phosphoenolpyruvate, or pyruvate) or the citric acid cycle (2-oxoglutarate or oxaloacetate).

Carbohydrate Biochemistry (Part II)

To access and download PowerPoint presentation on 'Carbohydrate Biochemistry' click on the link below:

Download slides

 

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)

To access and download PowerPoint presentation on 'Carbohydrate Biochemistry' click on the link below:

Download slides

 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)