Carbohydrates

Structure of carbohydrates

Carbohydrates are an important class of biomolecules comprising of aldehydes or ketones with multiple hydroxyl groups, which is why they are poly hydroxy aldehydes or ketone

Function of carbohydrates

• They store energy and act as fuels and important metabolic intermediates. Carbohydrates serve as crucial sources of energy. For example, the starch found in rice consists of individual glucose molecules that undergo breakdown to ultimately contribute to the generation of ATP.

• Carbohydrates like ribose and deoxyribose form the structural framework of nucleic acids like DNA and RNA.

• They are structural elements in the cell wall of plants and bacteria eg. cellulose, the most abundant organic compound in nature.

• They play key roles in mediating cell to cell interactions through their linkage with proteins and lipids.

Properties of carbohydrates

Monosaccharides

• Empirical formula : (C-H20)n

• Trioses are the smallest monosaccharides (n=3), classified as dihydroxyacetone (with a ketone group) and glyceraldehyde (with an aldehyde group)
  

Enantiomers, diasterioisomers and epimers

• Glyceraldehyde, due to its chiral carbon exists in 2 enantiomers: l and d

• Tetroses, pentoses, hexoses and heptoses are monosaccharides with n= 4, 5, 6 or 7 and they exist as diastereoisomers (not mirror images of each other) as they have many asymmetric or chiral carbons

• Epimers are sugars differing in configuration at a single asymmetric carbon. For eg. d- glucose and d- mannose differ at C2, d- galactose and d- glucose differ at C4
  

Cyclic structures

• Most sugars in the solution are not in open chains, but rather cyclise into closed structures or rings. While aldehyde reacts with alcohol to form hemiacetal and ketone reacts with alcohol to form hemiketal

Sugars with aldehydes form a pyranose ring ( 6 membered ) by the reaction of C1 of aldehyde and C5 of hydroxyl group

Sugars with ketones form a furanose ring (5 membered) by the reaction of C2 of keto group and C6 of hydroxyl group

• Anomers: the formation of a cyclic hemiacetal generates an additional asymmetric center. For instance, C1 forms the asymmetric centre in glucose giving rise to alpha-d-glucopyranose and beta-d-glucopyranose. In this case, the anomeric carbon is C1 and the two forms of glucose are anomers.

• Chair and boat form: the pyranose and furanose rings are not planar due to its tetrahedral geometry, thereby allowing them to take two forms- chair and boat and propensity of the carbohydrate towards a particular form is due to stearic hindrance. An envelope is a puckered conformation taken up by Furanose rings

• Modifications of monosaccharides: upon reactions with alcohols and amines, they can form adducts. Eg:
  • anomeric carbon of glucose + hydroxyl group of methanol= alpha d glucopyranoside and beta d glucopyranoside (both anomers)
• Glycosidic bonds are formed between the anomeric carbon and hydroxyl group is called , specifically O glycosidic bond and it is N glycosidic bonds if anomeric carbon is linked to nitrogen of an amine group

Another classic example is seen in adducts formed between sugars (like ribose or deoxyribose) and amines (like adenine) to give nucleosides

Complex carbohydrates

• The presence of many hydroxyl groups can help link monosaccharides together through glycosidic linkage, thereby forming disaccharides, oligosaccharides or polysaccharides

• For instance, maltose is a disaccharide made of 2 molecules of glucose through alpha 1,4- glycosidic bond ( C1 of one sugar and hydroxyl oxygen atom of C4)

Disaccharides

• They are made of 2 monosaccharides connected through O- glycosidic linkage 

• Common disaccharides- sucrose, lactose, maltose
  

Glucose + Fructose= Sucrose. Commercially synthesised from beets or canes, enzyme sucrase cleaves sucrose to glucose and fructose

Glucose + Galactose= Lactose. Linked by beta 1,4-glycosidic bond, disaccharide of milk, enzyme lactase hydrolyzes lactose to glucose and galactose

Glucose + Glucose= maltose. Linked by alpha 1,4 glycosidic bond, Enzyme maltase cleaves maltose to glucose and galactose

• The outer surface of epithelial cells in the small intestine contains these three enzymes

Polysaccharides 

• Formed by the linkage of multiple monosaccharides

• They play crucial role in energy storage and maintaining the structural integrity of an organism

• They are Homopolysaccharides if all monosaccharides are the same and two or more types of monosaccharides make up Heteropolysaccharides

• Most common homopolymer in animals- glycogen, which stores glucose. The glucose units are linked by alpha 1,4-glycosidic bonds and the branches are formed by alpha 1,6-glycosidic bonds (once every 10 units)

• Counterpart of glycogen (nutritional reservoir) in plants- starch, which exists in 2 forms: unbranched amylose (alpha 1,4 linkage) and branched amylopectin (alpha 1,6 linkage).
• Amylase, secreted by salivary glands and pancreas cleave both amylose and amylopectin

• Another important polysaccharide in plants serves a structural role rather than nutritional i.e. cellulose. Glucose units, joined by beta 1,4-linkages, form an unbranched structure, which allows it to form long straight chains, optimal for constructing fibres having high tensile strength

Important carbohydrates

Peptidoglycan: component of the bacterial cell wall

• It is a heteropolymer containing N acetylglucosamine and N acetylmuramic acid in an alternating manner and beta 1-4 glycosidic bond links the monomers.

• Short peptides connect the linear polymers lying side-by-side and due to their heavy cross linking make it a strong sheath like structure that surrounds the entire cell and prevents cellular swelling and lysis. 

• Lysozyme is an enzyme which kills bacteria by hydrolysing the beta 1,4 glycosidic bond and is even present in tears of eyes providing defence against bacterial infection

Mucopolysaccharides

• They are also known as glycosaminoglycans and are negatively charged polysaccharides consisting of repeating disaccharide units. It involves hexosamines and some non nitrogenous sugars linked by glycosidic bonds

• They are often found in the fluid lubricating our joints

Glycoproteins

Carbohydrate + Protein= Glycoprotein

This linking could be N linked or O linked

• N liked glycoproteins: sugar and an amide nitrogen of asparagine are joined together. The nitrogen can accept an oligosaccharide only if it has an Asn-X-Ser or Asn-X-Thr sequence, thereby helping us identify potential glycosylation sites. All N linked oligosaccharides have a core made of three mannose and two N acetylglucosamine residues. A donor called dolichol phosphate, a lipid containing 20 isoprene units, attaches an oligosaccharide destined to be attached to asparagine. The entire process of glycosylation occurs in the lumen of Endoplasmic Reticulum (ER) and golgi complex
• O linked glycoproteins: sugar attaches to the oxygen of serine or threonione. It can accept an oligosaccharide only if it is rich in Glycine, Valine and Proline residues. O linked glycoproteins are unique as glycosylated positions in the protein carry only single residues of N acetylglucosamine. This modification is important in the regulation of protein activity