MCAT Biochemistry Review

Chapter 4: Carbohydrate Structure and Function

Conclusion

This chapter examined, in depth, the unique characteristics of carbohydrates. Monosaccharides are the most basic form of carbohydrates, and in terms of human biochemistry, they typically range from three to seven carbon atoms. Classifying these monomers depends on the number of chiral centers present, which tells us the number of potential stereoisomers. Open-chain structures are most easily represented through Fischer projection diagrams; however, sugars tend to exist in ring form in biological systems, so Haworth projections are also used to depict three-dimensional structure. The most important reactions monosaccharides undergo are redox reactions, esterification, and glycoside formation—particularly when glycoside formation results in the formation of disaccharides. Polysaccharides are formed by glycosidic bonding of carbohydrates, and the polymers cellulose, starch, and glycogen are most commonly found in nature. Glycogen is the primary storage form of glucose in humans and other animals, and its unique branching structure allows for rapid access to these glucose stores.

The body has two primary energy-storage molecules: glycogen and triacylglycerols. Each has its own pros and cons—glycogen is more rapidly mobilized, but requires water of hydration, which increases its weight. Triacylglycerols serve as a long-term repository of energy, but take time to utilize. In the next chapter, we turn our attention to triacylglycerols, as well as the lipids used for structure and cell signaling.

Concept Summary

Carbohydrate Classification

·        Carbohydrates are organized by their number of carbon atoms and functional groups.

o   Common names are also frequently used when referring to sugars, such as glucose, fructose, and galactose.

o   Three-carbon sugars are trioses, four-carbon sugars are tetroses, and so on.

o   Sugars with aldehydes as their most oxidized group are aldoses; sugars with ketones as their most oxidized group are ketoses.

·        The nomenclature of all sugars is based on the D- and L-forms of glyceraldehyde. Sugars with the highest-numbered chiral carbon with the –OH group on the right (in a Fischer projection) are D-sugars; those with the –OH on the left are L-sugars. D- and L-forms of the same sugar areenantiomers.

·        Diastereomers are nonsuperimposable configurations of molecules with similar connectivity. They differ at at least one–but not all–chiral carbons. These also include epimers and anomers.

o   Epimers are a subtype of diastereomers that differ at exactly one chiral carbon.

o   Anomers are a subtype of epimers that differ at the anomeric carbon.

Cyclic Sugar Molecules

·        Cyclization describes the ring formation of carbohydrates from their straight-chain forms.

·        When rings form, the anomeric carbon can take on either an α- or β-conformation.

o   The anomeric carbon is the new chiral center formed in ring closure; it was the carbon containing the carbonyl in the straight-chain form.

o   α-anomers have the –OH on the anomeric carbon trans to the free –CH2OH group.

o   β-anomers have the –OH on the anomeric carbon cis to the free –CH2OH group.

·        Haworth projections provide a good way to represent three-dimensional structure.

·        Cyclic compounds can undergo mutarotation, in which they shift from one anomeric form to another with the straight-chain form as an intermediate.

Monosaccharides

·        Monosaccharides are single carbohydrate units, with glucose as the most commonly observed monomer. They can undergo three main reactions: oxidation-reduction, esterification, and glycoside formation.

o   Aldoses can be oxidized to aldonic acids and reduced to alditols.

o   Sugars that can be oxidized are reducing agents (reducing sugars) themselves, and can be detected by reacting with Tollen's or Benedict's reagents.

o   Sugars with a –H replacing an –OH group are termed deoxy sugars.

o   Sugars can react with carboxylic acids and their derivatives, forming esters (esterification). Phosphorylation is a similar reaction in which a phosphate ester is formed by transferring a phosphate group from ATP onto a sugar.

o   Glycoside formation is the basis for building complex carbohydrates and requires the anomeric carbon to link to another sugar.

Complex Carbohydrates

·        Disaccharides form as result of glycosidic bonding between two monosaccharide subunits; polysaccharides form by repeated monosaccharide or polysaccharide glycosidic bonding.

·        Common disaccharides include sucrose (glucose-α-1,2-fructose), lactose (galactose-β-1,4-glucose), and maltose (glucose-α-1,4-glucose).

·        Polysaccharides play various roles:

o   Cellulose is the main structural component for plant cell walls and is a main source of fiber in the human diet.

o   Starches (amylose and amylopectin) function as a main energy storage form for plants.

o   Glycogen functions a main energy storage form for animals.

Answers to Concept Checks

·        4.1

1.   

2.    D-glucose's epimers are D-mannose (C-2), D-allose (C-3), and D-galactose (C-4). None of the D-stereoisomers is an enantiomer for glucose; L-glucose is the enantiomer of D-glucose.

·        4.2

1.   

2.   

·        4.3

1.    Esterification is the reaction by which a hydroxyl group reacts with either a carboxylic acid or a carboxylic acid derivative to form an ester. Glycoside formation refers to the reaction between an alcohol and a hemiacetal (or hemiketal) group on a sugar to yield an alkoxy group.

2.    It makes sense for carbohydrates to become oxidized while reducing other groups. This is the case because aerobic metabolism requires reduced forms of electron carriers to facilitate processes such as oxidative phosphorylation. Because carbohydrates are a primary energy source, they are oxidized.

·        4.4

1.    Amylopectin is more soluble in solution than amylose because of its branched structure. The highly branched structure of amylopectin decreases intermolecular bonding between polysaccharide polymers and increases interaction with the surrounding solution.

2.    Glycogen has a higher rate of enzymatic branch cleavage because it contains significantly more branching than amylopectin.

Equations to Remember

(4.1) Number of stereoisomers with common backbone = 2n

Shared Concepts

·        Biochemistry Chapter 9

o   Carbohydrate Metabolism I

·        Biochemistry Chapter 10

o   Carbohydrate Metabolism II

·        Organic Chemistry Chapter 1

o   Nomenclature

·        Organic Chemistry Chapter 2

o   Isomers

·        Organic Chemistry Chapter 5

o   Alcohols

·        Organic Chemistry Chapter 6

o   Aldehydes and Ketones I