Biochemistry: Carbohydrates, Lipids, Amino Acids, and Proteins

Biochemistry: The Chemistry of Life

Introduction to Biochemistry

Biochemistry explores the chemical processes and substances that occur within living organisms. It is a vital part of general chemistry, especially in understanding the molecular basis of life, including the structure and function of carbohydrates, lipids, amino acids, and proteins.

• Carbohydrates: Provide energy and structural support.

• Lipids: Store energy and form cell membranes.

• Amino acids: Building blocks of proteins.

• Proteins: Catalyze reactions, provide structure, and regulate processes.

Carbohydrates

Classification and Structure

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with the general formula $C_n(H_2O)_n$. They are classified based on the number of sugar units:

• Monosaccharides: Single sugar units (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides joined together (e.g., sucrose, lactose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, cellulose, glycogen).

Monosaccharides

Monosaccharides are the simplest carbohydrates. They can be classified by the number of carbon atoms and the type of carbonyl group (aldose or ketose).

• Aldoses: Contain an aldehyde group (e.g., glucose).

• Ketoses: Contain a ketone group (e.g., fructose).

General formula: $C_n(H_2O)_n$

Structural Representations

• Fischer projections: Two-dimensional representations showing the stereochemistry of carbohydrates.

• Haworth projections: Represent cyclic forms of monosaccharides.

Physical Properties

• Monosaccharides are crystalline solids, soluble in water due to hydrogen bonding.

• They exhibit optical activity due to chiral centers.

Disaccharides and Oligosaccharides

Disaccharides are formed by a glycosidic bond between two monosaccharides. Examples include:

• Sucrose: Glucose + Fructose

• Lactose: Glucose + Galactose

• Maltose: Glucose + Glucose

Polysaccharides

• Starch: Storage polysaccharide in plants, composed of amylose and amylopectin.

• Glycogen: Storage polysaccharide in animals, highly branched.

• Cellulose: Structural polysaccharide in plants, composed of β-glucose units.

Lipids and Triglycerides

Structure and Function

Lipids are hydrophobic molecules, including fats, oils, and steroids. Triglycerides are the main form of stored energy in animals, consisting of glycerol and three fatty acids.

General structure of a triglyceride:

$\text{Triglyceride:} \quad \text{Glycerol} + 3 \text{Fatty Acids} \rightarrow \text{Triglyceride} + 3 \text{H}_2\text{O}$

Fatty Acids

• Saturated fatty acids: No double bonds, higher melting points.

• Unsaturated fatty acids: One or more double bonds, lower melting points.

Physical Properties of Triglycerides

• Melting point depends on the degree of saturation and length of fatty acid chains.

• Triglycerides are insoluble in water but soluble in organic solvents.

Soaps and Detergents

Soaps are produced by the saponification of triglycerides with a strong base, forming fatty acid salts. Detergents are synthetic cleaning agents with similar properties but improved solubility in hard water.

Saponification reaction:

$\text{Fat} + \text{NaOH} \rightarrow \text{Soap} + \text{Glycerol}$

Amino Acids

Structure and Properties

Amino acids are organic compounds containing both an amino group ($-NH_2$) and a carboxyl group ($-COOH$). They are the building blocks of proteins.

General structure:

$\text{H}_2\text{N}-\text{CHR}-\text{COOH}$

• Chirality: Most amino acids (except glycine) are chiral and exist as L- and D- isomers.

• Classification: Based on side chain properties (non-polar, polar, acidic, basic).

Acid-Base Properties

• Amino acids can act as both acids and bases (amphoteric).

• At physiological pH, amino acids exist as zwitterions (both positive and negative charges).

Peptides, Polypeptides, and Proteins

Peptide Bond Formation

Peptides are formed by condensation reactions between amino acids, creating peptide bonds:

$\text{Amino Acid}_1 + \text{Amino Acid}_2 \rightarrow \text{Dipeptide} + \text{H}_2\text{O}$

Protein Structure

• Primary structure: Sequence of amino acids in a polypeptide chain.

• Secondary structure: Local folding patterns (α-helix, β-pleated sheet) stabilized by hydrogen bonds.

• Tertiary structure: Overall 3D shape of a single polypeptide chain, stabilized by various interactions.

• Quaternary structure: Arrangement of multiple polypeptide chains in a protein complex.

Enzymes

Function and Mechanism

Enzymes are biological catalysts that speed up chemical reactions in living organisms. They lower the activation energy required for reactions and are highly specific for their substrates.

• Active site: Region of the enzyme where substrate binding and catalysis occur.

• Induced fit model: Enzyme changes shape to accommodate the substrate.

Example reaction:

$\text{Enzyme} + \text{Substrate} \rightarrow \text{Enzyme-Substrate Complex} \rightarrow \text{Product} + \text{Enzyme}$

Denaturing Proteins

Denaturation involves the loss of protein structure due to changes in temperature, pH, or chemical exposure, resulting in loss of biological function.

Tables

Table: Common Fatty Acids

Name	Structure	Number of Carbons	Melting Point (°C)
Palmitic acid	CH3(CH2)14COOH	16	63
Stearic acid	CH3(CH2)16COOH	18	70
Oleic acid	CH3(CH2)7CH=CH(CH2)7COOH	18	13

Table: 20 Common Amino Acids

Name	Structure	Side Chain Type
Glycine	NH2CH2COOH	Non-polar
Alanine	NH2CH(CH3)COOH	Non-polar
Serine	NH2CH(CH2OH)COOH	Polar
Glutamic acid	NH2CH(CH2CH2COOH)COOH	Acidic
Lysine	NH2CH(CH2CH2CH2CH2NH2)COOH	Basic

Summary

• Carbohydrates, lipids, amino acids, and proteins are essential biomolecules in living organisms.

• Understanding their structure, classification, and function is fundamental to biochemistry and general chemistry.

• Enzymes catalyze biochemical reactions, and protein structure determines biological activity.