Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that hydrolyse to give them. The name comes from “carbon hydrate” — the empirical formula of many is (CH₂O)ₙ. They are classified by size: monosaccharides are the simplest units (glucose, fructose, galactose); disaccharides are two monosaccharides joined by a glycosidic bond (sucrose, lactose, maltose); oligosaccharides have a few units; and polysaccharides are long chains of thousands of monosaccharide units (starch, glycogen, cellulose).

Monosaccharides are further divided by where the carbonyl group sits. An aldose has the carbonyl at C-1 (an aldehyde). A ketose has the carbonyl at C-2 (a ketone). Glucose is an aldose; fructose is a ketose. The prefix tells you the number of carbons: triose (3C), tetrose (4C), pentose (5C), hexose (6C). Combine them: glucose is an aldohexose; fructose is a ketohexose; ribose is an aldopentose.

Sugars are drawn in Fischer projections — a cross-like 2D representation where the carbon chain runs vertically (C-1 at top by convention), horizontal bonds point toward the viewer, and vertical bonds point away. The D/L designation is determined by the configuration of the highest-numbered chiral carbon (the one furthest from the carbonyl) in the Fischer projection. If the –OH on this carbon is on the right, the sugar is a D-sugar; if on the left, it is L. Almost all naturally occurring sugars are D-sugars.

In aqueous solution, glucose doesn't exist primarily as the open-chain aldehyde — it cyclises. The C-5 hydroxyl attacks the C-1 aldehyde (an intramolecular hemiacetal formation), forming a six-membered ring called a pyranose ring (named after pyran). Five-membered rings (from C-4 attacking C-2 ketone, as in fructose) are furanose rings.

The ring is drawn as a Haworth projection — a flat hexagon (or pentagon) viewed from the side. The ring oxygen is at the back-right. Groups that were on the right in the Fischer projection are drawn below the Haworth ring; groups on the left go above. The critical new centre formed by ring closure is C-1 for aldoses (the anomeric carbon) — more on this in §9.5.

When glucose cyclises, a new chiral centre forms at C-1 — the anomeric carbon. The two possible configurations of the C-1 OH are called anomers: in α-D-glucose, the C-1 OH is axial/below the ring; in β-D-glucose, the C-1 OH is equatorial/above the ring. These are not enantiomers (mirror images) or epimers (differ at a non-anomeric carbon) — they are specifically anomers (differ only at the anomeric carbon).

Mutarotation is the spontaneous interconversion of α and β anomers in solution, going through the open-chain aldehyde as an intermediate. If you dissolve pure α-D-glucose in water, its optical rotation gradually changes until equilibrium is reached (about 36% α and 64% β at equilibrium for glucose). This is mutarotation, and it is clinically relevant because the enzyme lactase, maltase, and other glucosidases are anomer-specific.

A reducing sugar is one that can reduce an oxidising agent (like Cu²⁺ in Fehling's or Tollens' reagent). For a sugar to do this, it needs a free aldehyde or a free hemiacetal at C-1 that can open to the aldehyde form. Any monosaccharide is a reducing sugar because it has a free anomeric OH (hemiacetal). In disaccharides, it depends on whether the glycosidic bond uses C-1 of both sugars.

In maltose (glucose + glucose, α-1,4), one glucose has its C-1 OH free — it can open to the aldehyde form, so maltose is a reducing sugar. In lactose (galactose + glucose, β-1,4), similarly, the glucose C-1 is free. But in sucrose (glucose + fructose, α,β-1,2), the glycosidic bond uses C-1 of glucose AND C-2 of fructose — both anomeric carbons are locked in the bond, leaving no free hemiacetal. Sucrose is therefore a non-reducing sugar.

Three disaccharides appear repeatedly in both exams and clinical medicine, each with a specific bond and specific significance. You need to know the monomers, the glycosidic bond type, whether it is reducing or not, and the enzyme that cleaves it.

Polysaccharides are long chains of monosaccharide units joined by glycosidic bonds. The three most important are starch, glycogen, and cellulose — all polymers of glucose, yet with completely different properties due to the type of glycosidic bond (α vs β) and degree of branching.