Enzyme Inhibition

Types of Enzyme Inhibition

Enzyme inhibition is a critical concept in biochemistry, affecting metabolic pathways and drug action. There are several types of inhibition, each with distinct mechanisms and effects on enzyme activity.

• Competitive Inhibition: The inhibitor resembles the substrate and competes for the active site. This increases the apparent Km (substrate concentration at half-maximal velocity) but does not affect Vmax (maximum velocity).

• Noncompetitive Inhibition: The inhibitor binds to a site other than the active site, causing a conformational change that prevents substrate binding. Vmax is decreased, but Km remains unchanged.

• Uncompetitive Inhibition: The inhibitor binds only to the enzyme-substrate complex, not to the free enzyme. Both Km and Vmax are decreased by the same ratio.

• Irreversible Inhibition: The inhibitor covalently modifies the enzyme, rendering it permanently inactive.

Example: Sarin irreversibly inhibits acetylcholinesterase by covalent modification.

Enzyme Regulation

Mechanisms of Regulation

Enzymes are regulated to maintain homeostasis and respond to cellular needs. Regulation can be achieved through several mechanisms:

• Feedback Control: The end product of a pathway acts as an allosteric regulator of the first enzyme, either inhibiting (negative feedback) or enhancing (positive feedback) its activity.

• Allosteric Regulation: Molecules bind to sites other than the active site, causing conformational changes that affect enzyme activity. Positive regulators increase activity; negative regulators decrease it.

• Covalent Modification: Addition or removal of functional groups (e.g., phosphorylation) alters enzyme activity.

• Zymogen Activation: Enzymes are synthesized in inactive forms (proenzymes) and activated by removal of a protein segment.

• Genetic Control: Regulation of enzyme synthesis at the DNA, RNA, or protein processing level.

Example: Phosphorylation of serine residues can activate or deactivate enzymes.

Vitamins and Coenzymes

Classification of Vitamins

Vitamins are essential organic molecules required for various biological functions. They are classified based on their solubility:

• Water-Soluble Vitamins: Hydrophilic, contain OH and COOH groups, not stored in the body, often form coenzymes. Examples: Vitamin C, B-complex vitamins.

• Fat-Soluble Vitamins: Hydrophobic, mainly composed of carbon and hydrogen, stored in the liver. Examples: Vitamins A, D, E, K.

Reason for Classification: Solubility affects storage, toxicity, and function. Water-soluble vitamins are excreted easily, while fat-soluble vitamins can accumulate and cause toxicity.

Names and Functions of Vitamins

• Vitamin A: Retinol (active form), β-carotene (provitamin). Function: Vision, cell growth, antioxidant for lipids.

• Vitamin D: Calciferol (active form), 7-dehydrocholesterol (provitamin). Function: Bone growth, calcium and phosphate absorption.

• Vitamin E: α-tocopherol. Function: Antioxidant, prevents oxidative damage.

• Vitamin K: Phylloquinone (K1), menaquinone (K2). Function: Blood clotting, bone metabolism.

• Vitamin C: Ascorbic acid. Function: Antioxidant, collagen synthesis.

• B Vitamins: Thiamine (B1), Riboflavin (B2), Niacin (B3), Pantothenic Acid (B5), Pyridoxine (B6), Biotin (B7), Folic Acid (B9), Cobalamin (B12). Functions: Metabolism, redox reactions, nucleic acid synthesis.

Coenzymes Derived from Vitamins

• Coenzyme: An organic compound required for enzyme catalysis, often derived from water-soluble vitamins.

• Examples:

  • Thiamine (B1): Thiamine pyrophosphate (TPP)

  • Riboflavin (B2): Flavin mononucleotide (FMN), Flavin adenine dinucleotide (FAD)

  • Niacin (B3): NAD+, NADP+

  • Pantothenic Acid (B5): Coenzyme A

  • Pyridoxine (B6): Pyridoxal phosphate (PLP)

  • Biotin (B7): Biocytin

  • Folic Acid (B9): Tetrahydrofolate (THF)

  • Cobalamin (B12): Methylcobalamin

  • Ascorbic Acid (C): Vitamin C (self)

Diagnostic Enzymes and Serological Testing

Enzyme Markers for Disease

Enzyme activity in blood can be used to diagnose organ damage or disease.

• Liver and Heart: AST, ALT, LDH, CK/CPK

• Pancreas: α-amylase

• Prostate: PSA, Acid Phosphatase

• Bone and Liver: Alkaline Phosphatase (ALP)

Example: Elevated CK and troponin indicate heart attack.

Lateral Flow Assay for COVID-19

Lateral flow assays use antibodies (a class of protective proteins) to detect antigens such as the SARS-CoV-2 spike protein.

• Procedure: Sample is applied, flows through a membrane, and binds to antibodies, producing a visible result.

• Proteins Used: Protective proteins (antibodies)

Summary Tables

Water-Soluble Vitamins

Fat-Soluble Vitamins

Additional info: These notes expand on the lecture content with definitions, examples, and summary tables for clarity and completeness.