chapter7 .. .

Chapter 7: Carbohydrates

Section 7.1: Monosaccharides

Definition and Classification of Carbohydrates

  • Carbohydrates are classified as the most abundant biomolecules in nature with various cellular functions including energy provision, structural components, communication, and serving as precursors for other biomolecules.

  • Carbohydrates establish a direct link between solar energy and the chemical bond energy.

  • The term 'carbohydrate' refers to compounds containing Carbon (C), Hydrogen (H), and Oxygen (O) typically represented by the formula (CH₂O)n, indicating compounds where n carbon atoms appear to be associated with n water molecules.

  • Monosaccharides are characterized by their reducing properties due to the presence of a carbonyl group, either as an aldehyde or a ketone, in addition to multiple hydroxyl (-OH) groups.

Monosaccharide Classification

  • Current Definition of Carbohydrates: Monosaccharides, or simple sugars, are defined as polyhydroxy aldehydes or ketones.   - Aldoses: Sugars containing an aldehyde functional group.   - Ketoses: Sugars containing a ketone functional group.

  • Monosaccharides are further categorized based on the number of carbon atoms:   - Trioses (3 Carbon), Tetroses (4 Carbon), Pentoses (5 Carbon), Hexoses (6 Carbon).

  • The most prevalent sugars in living cells are hexoses and pentoses.

  • The class names often succinctly denote both the number of carbons and the functional group.

Common Monosaccharides

  • Common non-systematic names include glucose, mannose, ribose, and fructose, which do not necessarily reflect the structural aspects of the ketoses.

  • Monosaccharides predominantly exist as colorless, crystalline solids that are water-soluble.

Stereochemistry of Monosaccharides

  • Stereoisomers: The number of stereoisomers can be determined by the formula 2n2^n, where n represents the number of chiral carbons.

  • Most naturally occurring monosaccharides are of the D-form.

  • All these sugars can be derived from:   - (A) D-glyceraldehyde (a chiral molecule).   - (B) Dihydroxyacetone (a non-chiral molecule).

  • In optical isomers, the reference carbon is the asymmetric carbon farthest from the carbonyl carbon.

  • Diastereomers: Stereoisomers that are not enantiomers (mirror-image isomers), exemplified by D-ribose and D-arabinose.

  • Epimers: A subset of diastereomers that differ at a single chiral carbon (e.g., D-glucose and D-galactose).

Structural Forms of Monosaccharides

  • The open-chain structure of D-glucose is present solely in solution.

  • Cyclic Structure of Monosaccharides: Sugars with four or more carbons primarily exist in cyclic forms forming through reactions between aldehyde/ketone groups and hydroxyl groups in aqueous solutions to produce hemiacetals and hemiketals.

  • Two types of diastereomers resulting from cyclization are called anomers, characterized by the position of the hydroxyl group on the hemiacetal at carbon 1, which may be oriented either up (above the ring) or down (below the ring).

  • The cyclic forms can exhibit two anomeric forms, the α-anomer and the β-anomer based on the orientation of the hydroxyl group.

  • Haworth Structures: These representations more accurately reflect bond angles and lengths in ring structures compared to classical Fischer projections.

Ring Structures

  • Furanoses: Five-membered rings, such as fructose in its cyclic form, termed fructofuranose.

  • Pyranoses: Six-membered rings, such as glucose in pyranose form, called glucopyranose.

  • Conformational Structures: Chair and boat conformations describe the puckered nature of sugar rings; chair is more stable.

Mutarotation and Reactions of Monosaccharides

  • Mutarotation: The phenomenon of interconversion between α- and β-forms in aqueous environments leading to an equilibrium mixture.

  • Monosaccharides' carbonyl and hydroxyl groups can engage in various chemical reactions, notably oxidation, reduction, isomerization, esterification, glycoside formation, and glycosylation reactions.

  • Reducing Sugars: Sugars capable of oxidation by weak oxidizing agents (e.g., Benedict’s reagent) and necessitating an open-chain form; all aldoses are reducing sugars while specific ketoses such as fructose are also reducing sugars due to isomerization.

Glycoside Formation

  • Hemiacetals and hemiketals can react with alcohols to form glycosides through glycosidic linkages, which specifies the sugar component.

  • When an acetal linkage forms between the hemiacetal hydroxyl of one monosaccharide and the hydroxyl of another, this results in the formation of a disaccharide.

  • Large polysaccharides consist of numerous monosaccharide units bound via acetal linkages.

Overview of Important Monosaccharides

  • Glucose (D-Glucose): Initially termed dextrose; extensively present in nature and serves as the primary energy source, particularly vital for brain cells and cells devoid of mitochondria (such as erythrocytes).

  • Fructose (D-Fructose): Commonly referred to as fruit sugar; noted for its sweetness (double that of sucrose), utilized as a sweetening agent in processed foods, particularly high-fructose corn syrup.

  • Galactose: Essential for synthesizing various biomolecules including lactose, glycolipids, and glycoproteins. The genetic disorder galactosemia arises from the deficiency of enzymes in galactose metabolism.

  • Deoxy Sugars: These monomers have an -OH group substituted with -H or -CH₃. Notable examples are 2-deoxy-D-ribose (the pentose sugar present in DNA) and fucose (related to ABO blood group determinants).

Section 7.2: Disaccharides

Classification of Disaccharides

  • Disaccharides comprise two monosaccharides linked via glycosidic bonds, with the linkages named based on the anomeric configuration (α or β) and the carbon atoms involved (e.g., α(1,4) or β(1,4)).

  • Lactose (milk sugar) is a specific disaccharide formed from one molecule of galactose and one molecule of glucose via a β(1,4) linkage; the enzyme lactase is essential for its breakdown, and it qualifies as a reducing sugar.

  • Maltose (malt sugar) originates from starch hydrolysis and has an α(1,4) linkage between two glucose molecules; it is not typically found freely in nature.

  • Maltose can exist in α or β forms in equilibrium with a “open chain” structure.

  • Sucrose (table sugar) represents a common disaccharide, comprised of one glucose and one fructose molecule linked through an α, β(1,2) glycosidic bond; the glycosidic bond forms between both anomeric carbons, rendering sucrose a non-reducing sugar.

Section 7.3: Polysaccharides

Composition and Classification of Polysaccharides

  • Polysaccharides (glycans) consist of numerous monosaccharides connected by glycosidic linkages, with smaller oligosaccharides (10-15 monomers) often associated with polypeptides as glycoproteins.

  • Polysaccharides can be classified based on their structures into two main types: homoglycans (single type of monosaccharide) and heteroglycans (multiple types of monosaccharides).

Structural Functions of Homoglycans

  • Starch: The primary energy reservoir for plant cells and a significant carbohydrate source in human diets; consists of amylose (long, unbranched chains of D-glucose with α(1,4) linkages) and amylopectin (a branched polymer with both α(1,6) and α(1,4) linkages).

  • Glycogen: The primary carbohydrate storage molecule in vertebrates, concentrated notably in liver and muscle cells, comprising 8-10% of liver cell wet weight and 2-3% in muscle cells; exhibits a structure similar to amylopectin, but with an increased branching frequency, rendering it more compact than other polysaccharides.

  • Cellulose: Represents a polymer of D-glucopyranosides linked by β(1,4) glycosidic bonds, recognized as the most abundant organic polymer on Earth, forming the primary structural component of plant cell walls.

Differences Between Polysaccharides

  • A question posed: Why is cellulose insoluble in water whereas starch, having a seemingly similar structure, is water-soluble? This query leads to a discussion on the structural variances and hydrogen-bonding capabilities between glucose chains.

Section 7.4: Glycoconjugates

Definition and Role of Glycoconjugates

  • Glycoconjugates emerge from the linkage of carbohydrates to proteins and lipids; they play crucial roles in biological processes.

  • Proteoglycans: Distinguishing features include high carbohydrate content (approximately 95%); they reside on cell surfaces or are secreted to the extracellular matrix, contributing to cellular metabolism, organization, and signal transduction. Genetic disorders such as Hurler’s syndrome can affect proteoglycan metabolism.

Glycoproteins

  • Defined as proteins covalently linked to carbohydrates through N- and O-glycosidic bonds, glycoproteins constitute 1%-85% of total weight in certain contexts. They serve critical functions in cells, manifesting both in soluble and membrane-bound forms, prevalent across numerous living organisms, especially in vertebrates, where they are particularly abundant.

  • The diverse functions of glycoproteins encompass enzymes, blood clotting factors, hormone receptors, transport proteins, and cell adhesion molecules.