Carbohydrate Chemistry and Metabolic Pathways

A set of vocabulary flashcards covering carbohydrates, their structures, glycosidic linkages, digestion, and metabolic pathways including glycolysis, gluconeogenesis, and the fates of pyruvate.

Ribose

An aldopentose monosaccharide sugar containing five carbon atoms and an aldehyde functional group, found in ribonucleic acid (RNA), with all hydroxyl ($OH$) groups positioned to the right in its straight-chain representation.

Deoxyribose

An aldopentose sugar found in deoxyribonucleic acid (DNA) that structurally resembles ribose but lacks an oxygen atom at carbon atom number two ($C2$), carrying a hydrogen atom instead of a hydroxyl group.

Anomeric carbon

The carbon atom that was part of the carbonyl functional group (aldehyde or ketone) in the open-chain form, which reacts with an alcohol group to produce a cyclic ring sugar; it is located at $C1$ for aldoses and at $C2$ for ketoses such as fructose.

Alpha ($\alpha$) anomer

The cyclic stereoisomeric form of a sugar in which the hydroxyl group (or glycosidic bond) at the anomeric carbon is oriented downwards (remembered as looking like a fish swimming down).

Beta ($\beta$) anomer

The cyclic stereoisomeric form of a sugar in which the hydroxyl group (or glycosidic bond) at the anomeric carbon is oriented upwards (remembered as a bird flying up in the air).

Starch

A plant storage polysaccharide made of glucose units that acts as an energy source, consisting of two structural components: amylose and amylopectin.

Amylose

An unbranched component of starch found in plants, consisting of glucose units linked by $\alpha(1 \rightarrow 4)$ glycosidic bonds arranged in a spiral structure.

Amylopectin

A branched polysaccharide component of starch made of glucose units joined by $\alpha(1 \rightarrow 4)$ glycosidic bonds with branch points occurring via $\alpha(1 \rightarrow 6)$ glycosidic bonds every $25$ to $30$ units.

Debranching enzyme

A specific digestive enzyme that hydrolyzes the $\alpha(1 \rightarrow 6)$ glycosidic branch points found in branched polysaccharides like amylopectin.

Cellulose

A structural plant polysaccharide of glucose units joined by $\beta(1 \rightarrow 4)$ glycosidic bonds forming a sheet structure that provides strength, serves as a water-repellent barrier, and acts as indigestible dietary fiber in humans.

Cellulase

The enzyme capable of hydrolyzing $\beta(1 \rightarrow 4)$ glycosidic bonds in cellulose; it is absent in humans but produced by symbiotic bacteria in the digestive systems of herbivores (such as cows and horses) and termites.

Glycogen

A highly branched energy-storage polysaccharide found in animals, composed of glucose units connected by $\alpha(1 \rightarrow 4)$ glycosidic bonds with $\alpha(1 \rightarrow 6)$ branch points occurring every $8$ to $12$ units.

Cellobiose

A disaccharide composed of two glucose units connected by a $\beta(1 \rightarrow 4)$ glycosidic bond that cannot be digested by humans as an energy source due to the lack of appropriate enzymes.

Lactose

A disaccharide known as milk sugar composed of galactose and glucose connected by a $\beta(1 \rightarrow 4)$ glycosidic bond, broken down during digestion by the enzyme lactase.

Sucrose

A disaccharide composed of glucose and fructose joined by an $\alpha,\beta(1 \rightarrow 2)$ glycosidic bond, which is hydrolyzed by the enzyme sucrase.

Stachyose

An oligosaccharide found in vegetables like peas and beans, composed of two galactose units, one glucose unit, and one fructose unit linked by $\alpha(1 \rightarrow 6)$ and $\alpha,\beta(1 \rightarrow 2)$ glycosidic bonds.

Pyranose

A six-membered cyclic ring structure formed by monosaccharides such as glucose and galactose.

Furanose

A five-membered cyclic ring structure formed by monosaccharides such as fructose, ribose, and deoxyribose.

Glycolysis

A catabolic pathway taking place in $10$ linear steps that breaks down one molecule of glucose into two pyruvate molecules, yielding a net harvest of $2\,\text{ATP}$ and $2\,\text{NADH}$.

Gluconeogenesis

An anabolic metabolic pathway that synthesizes glucose from non-carbohydrate sources such as lactate, glycerol/fats, certain amino acids/proteins, and oxaloacetate, consuming $6$ high-energy phosphates ($\text{ATP}/\text{GTP}$) and $2\,\text{NADH}$.

Oxaloacetate

A four-carbon metabolic intermediate common to both the citric acid cycle and the gluconeogenesis pathway.

Lactate

The product formed when pyruvate undergoes reduction by $\text{NADH}$ under anaerobic conditions in muscle tissue, which can accumulate to cause muscle soreness or be used to regenerate glucose via gluconeogenesis.

Acetyl coenzyme A

A two-carbon molecule formed in the mitochondria by the oxidative decarboxylation of pyruvate under aerobic conditions, which subsequently enters the citric acid cycle.

Ketone bodies

Three specific chemical compounds produced from excess acetyl coenzyme A when oxaloacetate is diverted into gluconeogenesis and the citric acid cycle slows down, which can accumulate to cause ketoacidosis.

Acetone

A volatile ketone body (commonly used in nail polish remover) that gives off a sickly sweet odor on the breath of individuals undergoing ketosis resulting from starvation or low-carbohydrate diets.

Sucralose

A calorie-free, heat-stable artificial sweetener $600$ times sweeter than sucrose, structurally derived from galactose and fructose residues with chlorine substitutions replacing specific hydroxyl groups.