Comprehensive Notes on Carbohydrates and Disaccharides

General Introduction to Carbohydrates

Abundance: Carbohydrates represent the most abundant molecules found in the biological world.
Structural Range: The category of carbohydrates includes a vast spectrum of structures, ranging from simple molecules containing as few as $3$ carbon atoms to exceptionally large molecules comprised of thousands of rings.
Chemical Definition: Carbohydrates are defined as polyhydroxy aldehydes or ketones, or compounds that can be hydrolyzed to form such polyhydroxy aldehydes or ketones.
Physiological Roles: * Metabolic Energy: They serve as a primary source of metabolic energy. * Genetic Material: They are essential components of DNA and RNA. * Protein Modification: Many proteins exist with attached carbohydrates (glycoproteins). * Structural Support: They act as critical structural components in many cell types. * Biological Identity: Carbohydrates bound to cell surfaces function as antigenic determinants that uniquely define an individual's identity.

Classification of Carbohydrates

Monosaccharides (Simple Sugars): * These are the simplest forms of carbohydrates. * They contain between $3$ and $6$ carbon atoms. * They cannot be further hydrolyzed into smaller molecules. * Examples: Glucose and fructose.
Oligosaccharides: * These contain a small number of linked monosaccharide units. * They can be hydrolyzed back into their constituent monosaccharides. * Subtypes: Disaccharides, trisaccharides, etc. * Examples: Lactose, maltose, and sucrose.
Polysaccharides: * These contain thousands of covalently linked monosaccharide units. * Classification: Divided into homopolysaccharides (identical units) and heteropolysaccharides (different units). * Examples: Starch, cellulose, glycogen, heparin, and hyaluronic acid.

High Potential for Structural Diversity

Comparative Complexity: Carbohydrates possess a significantly higher potential for structural diversity than either proteins or nucleic acids.
Comparison of Amino Acid vs. Monosaccharide Combinations: * Two Amino Acids: Based on ( $\text{Ala, Ala}$ ), only $1$ possible structure exists. * Two Monosaccharides: Based on ( $\text{Glc, Glc}$ ), there are $11$ possible structures. * Four Amino Acids: Based on ( $\text{Ala, Gly, Ser, Thr}$ ), there are $256$ possible structures. * Four Monosaccharides: Based on ( $\text{Glc, Man, Gal, GlcNAc}$ ), there are more than $35,000$ possible structures.

General Chemistry of Disaccharides

Formation: Disaccharides are formed by the combination of $2$ monosaccharide units.
Dehydration Synthesis: During the formation process, water ( $H_2O$ ) is removed.
Glycosidic Bond: The bond linking the two monosaccharide units is specifically known as a glycosidic bond.
Chemical Components: * A disaccharide is essentially a glycoside formed from two monosaccharides. * One monosaccharide unit acts as a hemiacetal or hemiketal. * This unit is bonded through its anomeric carbon to the second monosaccharide, which is referred to as the aglycone.
Bond Nomenclature: * Linkages often occur between $C-1$ of the hemiacetal of an aldose and $C-4$ of the second monosaccharide, termed a $(1,4')$ or $(1 ightarrow 4)$ glycosidic bond. * The " $1$ " refers to the anomeric carbon of the first sugar; the " $4'$ " refers to carbon- $4$ of the aglycone. * The configuration at the anomeric carbon ( $C-1$ ) is designated as either $\alpha$ or $\beta$ . * While $(1,4')$ is common, in principle, any hydroxyl group of the aglycone can provide the linkage, and every possible linkage variant has been found in nature.

Maltose

Composition: Consists of two molecules of glucose linked by an $\alpha-(1,4')$ glycosidic bond.
Production: * Results from the enzymatic hydrolysis of the homopolysaccharide amylose by the enzyme amylase. * Commercial maltose is produced from starch.
Hydrolysis: The enzyme maltase hydrolyzes the glycosidic bond, converting maltose into two molecules of glucose.
Chemical Properties: * Reducing Sugar: Because the aglycone is a hemiacetal, maltose can undergo mutarotation. * Reactivity: The free aldehyde formed during ring opening reacts with Benedict's, Fehling's, and Tollens' solutions. * Structural Ends: The acetal part is called the "nonreducing end," while the hemiacetal part is the "reducing end."
IUPAC Nomenclature: $4-O-(\alpha-D-glucopyranosyl)-D-glucopyranose$ . * Specifically, the $\beta$ -anomer form is named $4-O-(\alpha-D-glucopyranosyl)-\beta-D-glucopyranose$ . * Terminology Breakdown: * Parentheses: Refers to the glucose unit on the left (the acetal portion). * -pyrano-: Indicates a six-membered ring. * -osyl: Indicates the ring is linked via a glycosidic bond. * 4-O-: Refers to the position of the oxygen atom on the right-hand ring (the aglycone). * \beta-D-glucopyranose: Describes the aglycone unit.
Equilibrium: Maltose exists in $\alpha$ and/or $\beta$ configurations in equilibrium with its open-chain form.

Lactose ("Milk Sugar")

Occurrence: Found in the milk of many mammals, including humans.
Composition: Composed of galactose and glucose.
Galactose Relationship: Galactose is the $C-4$ epimer of glucose.
Linkage: The pyranosyl group of galactose is linked by a $\beta-(1,4')$ glycosidic bond to $D$ -glucose.
IUPAC Name: $4-O-(\beta-D-galactopyranosyl)-D-glucopyranose$ .
Digestion: The enzyme $\beta$ -galactosidase (also called lactase) hydrolyzes the $\beta-(1,4')$ glycosidic bond.
Equilibrium: Similar to maltose, the reducing end ( $D$ -glucose) exists in $\alpha$ and $\beta$ forms in equilibrium with the open-chain form.

Cellobiose

Composition: Consists of two molecules of glucose linked by a $\beta-(1,4')$ glycosidic bond.
Comparison to Maltose: It differs from maltose only in the configuration of the glycosidic bond ( $\beta$ vs $\alpha$ ).
Chemistry: * Reducing Sugar: The aglycone is a hemiacetal, allowing for mutarotation and reactivity with Benedict's, Fehling's, and Tollens' solutions. * IUPAC Name: $4-O-(\beta-D-glucopyranosyl)-D-glucopyranose$ .
Biological Limitation: Humans do not possess the enzyme required to hydrolyze the $\beta-1,4'$ glycosidic bond in cellobiose.
Enzyme Specificity: Glycosidases are highly specific; an enzyme for $\alpha-1,4'$ bonds will not hydrolyze $\beta-1,4'$ bonds and vice-versa.

Sucrose ("Table Sugar")

Composition: A disaccharide formed from $\alpha-D-glucopyranose$ and $\beta-D-fructofuranose$ .
Linkage: The anomeric centers of both sugars are linked together, designated as an $\alpha, \beta-(1,2')$ or $\alpha(1 \rightarrow 2)\beta$ linkage.
Functional Groups: Contains both an acetal and a ketal functional group.
Unique Properties: * Non-reducing Sugar: Because both anomeric carbons are involved in the bond, neither ring can open to exist in equilibrium with an aldehyde or ketone. * No Mutarotation: Sucrose cannot undergo mutarotation. * Suffix: The suffix "-oside" in the IUPAC name ( $\alpha-D-glucopyranosyl-\beta-D-fructofuranoside$ ) indicates it is a non-reducing sugar.
Hydrolysis: The bond can be broken by acid, heat, or enzymes such as sucrase or $\alpha$ -glucosidase.

Lactulose

Composition: Consists of galactose and fructose linked via a $\beta$ linkage between $C1$ of galactose and $C4$ of fructose.
Properties: * Lactulose is a synthetic, non-absorbable disaccharide. * It is neither broken down nor absorbed in the small intestine.
Clinical Applications: * Constipation: Used in management strategies. * Liver Disease: Used for chronic conditions like hepatic encephalopathy. * Hyperammonemia: Oral administration relieves high ammonia levels. In the colon, microflora convert lactulose into organic acids (e.g., lactic acid) that acidify the colonic contents.
Physiological Note: While sucrose, maltose, and lactose are nutritionally and physiologically significant, lactulose is strictly synthetic.