Comprehensive Biochemistry Lecture Notes: Carbohydrates Structure and Functions

Introduction to Biochemistry and Carbohydrates

Biochemistry is defined as the scientific study of the chemical substances naturally found in living organisms and the various chemical interactions these substances exhibit with one another. This discipline focuses on the structure and function of essential cellular components, including proteins, carbohydrates, lipids, nucleic acids, and various other biomolecules. Within this field, carbohydrates arise as a critical focus. The term carbohydrate is derived from the general chemical formula $[C(H_2O)]_n$, which suggests that these compounds are essentially hydrates of water. Closely related to the term carbohydrate is the word saccharide, which originates from the word used for table sugar in several languages. Among these substances, glucose is recognized as the most significant carbohydrate in the human body.

Distribution and Synthesis of Carbohydrates

Carbohydrates are widely distributed across both the plant and animal kingdoms, serving vital structural and metabolic purposes. In the plant world, glucose is synthesized via the process of photosynthesis, where carbon dioxide and water are combined using light energy. Plants then store this glucose as starch or convert it into cellulose to form the plant's structural framework. Conversely, animals possess the ability to synthesize carbohydrates from lipid glycerol and specific amino acids; however, most animal carbohydrates are ultimately derived from plant sources. Chemically, carbohydrates may be defined as polyhydroxy aldehydes or polyhydroxy ketones, or as compounds that yield these substances upon hydrolysis.

Primary Functions of Carbohydrates

The utility of carbohydrates in biological systems is multifaceted. Their primary role is the production of energy through oxidation during the process of cellular respiration. For energy management, the body stores carbohydrates in the form of glycogen, which acts as a short-term energy reserve to facilitate various bodily functions. Beyond energy, carbohydrates supply the necessary carbon atoms required for the synthesis of other essential biochemical substances, including proteins, lipids, and nucleic acids. They are also integral to the genetic framework, forming part of the structure of DNA and RNA molecules.

Furthermore, carbohydrates play a structural role when linked to lipids as components of cell membranes. When linked to proteins, they serve in a variety of cell-to-cell and cell-molecule recognition processes. In this capacity, they function as useful markers for antibodies, aiding in the body's immune response and cellular identification.

Classification of Carbohydrates Based on Structure

Sugars are categorized based on their chemical structures, specifically the number of carbon atoms present and the number of sugar units per molecule within a polymer held together by glycosidic bonds. Based on the number of carbon atoms, they are classified as follows: trioses contain three carbon atoms, tetroses contain four carbon atoms, pentoses contain five carbon atoms, and hexoses contain six carbon atoms.

Classification also occurs based on the functional group present in the molecule. Sugars containing an aldehyde group are referred to as aldoses, while those containing a ketone group are known as ketoses. For example, a triose with an aldehyde group is an aldotriose, such as glyceraldehyde. A triose with a ketone group is a ketotriose, such as dihydroxyacetone. Similarly, hexoses are categorized as aldohexoses (e.g., glucose) or ketohexoses (e.g., fructose).

Categorization by Monosaccharide Units

Carbohydrates are also classified by the number of individual sugar units they contain. Monosaccharides are the simplest units of carbohydrates and cannot be hydrolyzed into smaller sugars; examples include glucose and fructose. Disaccharides are the condensation products of two monosaccharide units, such as maltose and sucrose. Oligosaccharides consist of three to ten monosaccharide units joined together, such as maltotriose or raffinose. Polysaccharides are large polymers containing more than ten monosaccharide units, which may be linear or branched; common examples include starch, glycogen, cellulose, and dextrin.

Specific examples of carbohydrate size include trisaccharides (3 units like raffinose, which is glucose + fructose + galactose), tetrasaccharides (4 units like stachyose, which is glucose + fructose + 2 galactose), and pentasaccharides (5 units like verbascose, which is glucose + fructose + 3 galactose).

Biological Importance and Specific Examples

Different types of monosaccharides serve specific biological roles. Trioses such as glyceraldehyde and dihydroxyacetone are essential intermediates in glycolysis and serve as precursors for glycerol in lipid synthesis. The tetrose D-erythrose is an intermediate in carbohydrate metabolism. Pentoses like D-ribose are structural elements of nucleic acids like RNA and serve as components of various co-enzymes.

Hexoses are particularly vital. D-glucose is the main sugar in the body, used as primary metabolic fuel. D-fructose is converted into glucose and subsequently utilized by the body. D-galactose is synthesized in the mammary glands to produce the lactose found in milk. D-mannose is a critical constituent of glycoproteins and glycolipids. In the context of disaccharides, maltose is composed of glucose and glucose, lactose is composed of glucose and galactose, and sucrose is a combination of glucose and fructose.

Properties and Chemistry of Monosaccharides

Monosaccharides are typically colorless, crystalline solids that exhibit high solubility in water but remain insoluble in nonpolar solvents. Most monosaccharides possess a sweet taste. Their structural backbones consist of unbranched carbon chains linked by single bonds. In their open-chain form, one carbon atom is double-bonded to oxygen to form a carbonyl group, while the remaining carbons each bear a hydroxyl ($-OH$) group. This results in the classification into aldoses or ketoses depending on the position of the carbonyl group. Because many of the carbon atoms bearing hydroxyl groups are chiral centers, monosaccharides exhibit various stereoisomers.

Polysaccharides: Complex Carbohydrates

Polysaccharides are carbohydrates that yield more than ten molecules of monosaccharides upon hydrolysis. Notable examples include cellulose, starch, and glycogen. Unlike simpler sugars, these are often referred to as "non-sugars" because they do not have a sweet taste. They are widely distributed in nature and generally correspond to the formula $(C_6H_{10}O_5)_n$. Chemically, they are divided into homopolysaccharides, which yield only a single type of monosaccharide upon hydrolysis, and heteropolysaccharides, which yield a mixture of different monosaccharides.

The Significance and Structure of Glucose

Glucose is the most important physiological and biochemical monosaccharide. It serves as the major metabolic fuel for mammals and is a universal fuel for the fetus. It also serves as the precursor for synthesizing other carbohydrates, including glycogen for storage, ribose and deoxyribose for nucleic acids, and galactose for milk lactose. Glucose is also found in glycolipids and glycoproteins. Structurally, glucose can be represented in three ways: a straight-chain structure, a ring or cyclic structure, and the boot and chair forms.

Isomerism and Asymmetry

The phenomenon of existence of isomers is known as isomerism. In glucose, the position of the $-OH$ group on the four asymmetric carbon atoms (carbons 2, 3, 4, and 5) can be changed to produce different stereoisomers. Theoretically, there are $2^4 = 16$ possible stereoisomers for glucose, though not all exist in nature. Fructose, possessing three asymmetric carbons, has $2^3 = 8$ possible stereoisomers.

Stereoisomers may differ only in their 3-D arrangement in space despite having the same bonding patterns. Sugars often exist as pairs of mirror-image stereoisomers. These differences typically do not change physical properties but significantly impact biochemical properties by altering the molecule's shape. In an L-isomer, the $-OH$ group is on the left of the center carbon, while in a D-isomer, the $-OH$ group is on the right.

Optical Isomerism and Mutarotation

Molecules with asymmetric carbon atoms exhibit optical activity, meaning they rotate the direction of vibration of plane-polarized light passing through them. A compound that rotates light to the right is dextrorotatory, designated as the (+)-isomer or using the letter 'd'. A compound that rotates light to the left is levorotatory, designated as the (-)-isomer or using the letter 'l'. For instance, the naturally occurring form of fructose is the D(-)-isomer.

Structural Projections and Cyclic Forms

Carbohydrates are represented using different projection systems. Fischer projections show the hydrogen and hydroxyl groups attached to carbon atoms in a straight chain. Haworth projections represent the molecule as a ring viewed from the side and slightly above the plane; by convention, the bonds nearest the viewer are drawn thicker. The chair conformation highlights the six-membered ring containing one oxygen atom in a non-planar chair shape.

Monosaccaharides often form furanose (five-membered) or pyranose (six-membered) rings. In these rings, the remaining hydroxyl groups point up or down depending on the sugar's identity. For D-saccharides, the $CH_2OH$ group is always positioned above the ring.

Anomers and Equilibrium in Solution

When a sugar ring forms, a new chiral center is created at carbon-1, known as the anomeric carbon. In the pyranose form of glucose, this results in two stereoisomers called anomers: the $\alpha$-anomer (where the $-OH$ group points down) and the $\beta$-anomer (where the $-OH$ group points up). Specifically, in the $\alpha$-form, the $-OH$ at C1 and the $CH_2OH$ at C5 are on opposite sides, while in the $\beta$-form, they are on the same side.

In an aqueous solution, glucose exists in equilibrium. The pyranose form dominates heavily, accounting for over 99% of the molecules ($36\%$ as $\alpha$-D-glucose and $64\%$ as $\beta$-D-glucose). Furanose forms account for less than $1\%$ (approximately $0.25\%$), and the open-chain linear form represents only about $0.02\%$ of the total. References for these study materials include "Chemical Basis of Life" by George H. Schmidt and "Principle of Biochemistry" by David L. Nelson and Michael M. Cox.