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MCAT TOPIC REVIEWS & RESOURCES

Biological and Biochemical Foundations of Living Systems

BC OC
Carbohydrates

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Description

Carbohydrates are carbon-containing molecules that incorporate hydrogen and oxygen in the same ratio as water with molecular formula CnH2nOn. These energy-rich molecules can undergo catabolic oxidation into carbon dioxide and water to produce energy for a cell, making them very important energy sources for living organisms.

Nomenclature and classification, common names

The basic structure of a carbohydrate is a chain of carbon atoms, numbered from the most oxidized end, which will contain a carbonyl carbon in the form of an aldehyde or ketone. Thus carbohydrates can be referred to by whether they are "aldo" or "keto" (an aldose, a ketose), the number of carbons in the chain (a pentose has five carbons, a hexose has six carbons), or by putting the concepts together (an aldopentose is a five carbon carbohydrate with an aldehyde functional group on its most oxidized end).

The presence of chiral carbons in carbohydrates also necessitates specifying a molecule's stereochemistry as either the D or L configuration. The D configuration is naturally occurring in metabolism (similar to L-amino acids).

Carbohydrates are generally referred to by their common names. Examples are glucose, sucrose, galactose, and ribose.

Carbohydrate molecules can exist as single monomers (monosaccharides), or chained together by glycosidic linkage in pairs of two (disaccharides), several (oligosaccharides), or many (polysaccharides).

Absolute configuration

The absolute configuration for a carbohydrate is assigned based on the last chiral carbon in the chain as compared to the configurations of glyceraldehyde. In the Fischer notation of L-glyceraldehyde, the hydroxyl group on its chiral carbon is on the left; in the Fischer notation of D-glyceraldehyde, the hydroxyl is on the right.

Cyclic structure and conformations of hexoses

Pentoses and hexoses commonly switch between their chain structure and cyclic formations, forming five-member rings (furanoses) and six-member rings (puranoses). This ring structure is formed by nucleophilic attack by one of the hydroxyls on the carbonyl carbon, which can occur on either face of the carbonyl, producing two possible outcomes (anomers) for the newly chiral carbon (the anomeric carbon, formerly of the carbonyl). The hydroxyl in an axial or equatorial position is labelled based on its position being down (α) or up (β) compared to the hydrogen [axial position can be above or below, so it does not specify α or β in itself].

Hexoses form six-member ring structures that adopt a more conformationally favorable chair structure that reduces steric hindrance. In this formation, axial groups project straight up or down, and equatorial groups project out from the ring.

Moving from Fischer notation to chair representations or Haworth projection, the substituents on the left of the chain appear on the top of the ring.

Epimers and anomers

The difference in the position (on the left or right in Fischer notation) of the hydroxyl (OH) at one chiral carbon along a carbohydrate creates a pair of epimers, stereoisomers that differ at a single chiral center. Glucose and galactose are epimers that differ at the C4 carbon (on the right/down in glucose, on the left/up in galactose).

Anomers are cyclic epimers that differ in position at the anomeric carbon, which will be the C1 in aldoses and C2 in ketoses. The two forms, α and β, interconvert in solution through mutarotation.

Hydrolysis of the glycoside linkage

Carbohydrates bond with each other through glycosidic linkage, a covalent bond formed through dehydration. These linkages differ in terms of which carbons are bonded and the anomeric form. For example, sucrose is a disaccharide made of glucose and fructose monosaccharides with an α-1,2 linkage. Different linkages require specific enzymes for hydrolysis to break the bond, allowing monosaccharides to be released for metabolism. The presence of the appropriate enzymes will determine whether an organism is able to metabolize certain carbohydrates (e.g. lactase is required to digest lactose into galactose and glucose).

Monosaccharides

Two key monosaccharides are glucose and fructose, each a six-carbon carbohydrate (hexose) with glucose being an aldose that creates a six-member ring and fructose being a ketose that creates a five-member ring. Ribose is an aldopentose that forms a five-member ring and is important for the synthesis of nucleic acids.

Disaccharides

Important disaccharides include sucrose (glu-α-1,2-fru), lactose (gal-β-1,4-glu), maltose (glu-α-1,4-glu), and cellobiose (glu-β-1,4-glu).

Polysaccharides

Important polysaccharides include starch, glycogen, and cellulose, all of which are polymers of glucose. Starch has α-1,4 linkages and may be branched or unbranched. Glycogen has α-1,4 linkages with branches formed by α-1,6 linkages. Cellulose forms an unbranched chain (useful for rigid plant cell walls) with β-1,4 linkage.

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