Regulation, Vitamins, and Their Significance to Enzyme Function; Serological Testing

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Enzyme Inhibition

Types of Enzyme Inhibition

Enzyme inhibition is a critical concept in biochemistry, describing how molecules can decrease or halt enzyme activity. There are several types of inhibition, each affecting enzyme function differently.

Competitive Inhibition: The inhibitor resembles the substrate and competes for the same binding site on the enzyme. This type of inhibition 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 change in enzyme configuration that prevents substrate binding. Vmax is lowered, but Km remains unchanged.
Uncompetitive Inhibition: The inhibitor binds only to the enzyme-substrate complex, not to the free enzyme. Both Vmax and Km are lowered by the same ratio.
Irreversible Inhibition: The inhibitor covalently modifies the enzyme, rendering it permanently inactive.

Example: Sarin, a nerve agent, irreversibly inhibits acetylcholinesterase by covalent modification.

Diagram of competitive, noncompetitive, and uncompetitive inhibition Irreversible inhibition mechanism Reaction rate curves for different types of inhibition

Enzyme Regulation

Mechanisms of Enzyme Regulation

Enzymes in metabolic pathways are tightly regulated to ensure proper cellular function. Regulation can be achieved through several mechanisms:

Feedback Control: A product of a metabolic pathway acts as an allosteric regulator of an enzyme earlier in the pathway. Negative feedback inhibits the enzyme, while positive feedback enhances its activity.
Allosteric Regulation: Binding of a regulator molecule at a site other than the active site alters enzyme activity. Positive regulators increase activity; negative regulators decrease it.
Covalent Modification: Addition or removal of functional groups (e.g., phosphorylation) changes enzyme activity.
Zymogen Activation: Enzymes are synthesized as inactive precursors (zymogens) and activated by removal of a protein segment.
Genetic Control: Regulation of enzyme levels through gene expression.

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

Negative allosteric regulation diagram Feedback control in metabolic pathway Covalent modification by phosphorylation Phosphorylation and dephosphorylation of enzymes Zymogen activation diagram

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, composed mainly of carbon and hydrogen, stored in the liver. Examples: Vitamins A, D, E, K.

Vitamins are placed in these categories due to their chemical properties and storage mechanisms in the body.

Fat-Soluble Vitamins

Vitamin A (Retinol): Provitamin: β-carotene. Function: Vision, cell growth, antioxidant for lipids. Deficiency: Blindness, growth retardation. Toxicity: Liver damage, birth defects.
Vitamin D (Calciferol): Provitamin: 7-Dehydrocholesterol. Function: Bone growth, calcium and phosphate absorption. Deficiency: Rickets, bone deformities. Toxicity: Calcification, kidney stones.
Vitamin E (α-Tocopherol): No provitamin. Function: Antioxidant, prevents oxidative damage. Deficiency: Rare. Toxicity: Interferes with blood clotting.
Vitamin K (Phylloquinone, Menaquinone): No provitamin. Function: Blood clotting, bone metabolism. Deficiency: Bleeding disorders. Toxicity: Liver dysfunction.

Structures of Accutane and Retin-A (Vitamin A derivatives) Vitamin D synthesis pathway Structures of Vitamin K1 and K2

Water-Soluble Vitamins

Vitamin C (Ascorbic Acid): Function: Antioxidant, collagen synthesis, disease prevention. Deficiency: Scurvy. Toxicity: Kidney stones.
Vitamin B1 (Thiamine): Coenzyme: Thiamine pyrophosphate (TPP). Function: Carbohydrate metabolism. Deficiency: Beriberi.
Vitamin B2 (Riboflavin): Coenzymes: FMN, FAD. Function: Redox reactions. Deficiency: Dermatitis, glossitis.
Vitamin B3 (Niacin): Coenzymes: NAD+, NADP+. Function: Redox reactions. Deficiency: Pellagra.
Vitamin B5 (Pantothenic Acid): Coenzyme: Coenzyme A. Function: Acetyl group transfer.
Vitamin B6 (Pyridoxine): Coenzyme: Pyridoxal phosphate (PLP). Function: Amino acid metabolism.
Vitamin B7 (Biotin): Coenzyme: Biotin. Function: Carbon carrier in metabolism.
Vitamin B9 (Folic Acid): Coenzyme: Tetrahydrofolate (THF). Function: Carrier of 1-carbon units, nucleic acid synthesis.
Vitamin B12 (Cobalamin): Coenzyme: Methylcobalamin. Function: Methyl transfer, nucleic acid metabolism.

Structure of FMN and FAD (Vitamin B2 derivatives) Reduced and oxidized forms of FMN and FAD Structure of NAD+ and NADH (Vitamin B3 derivatives) Structure of folic acid and THF Collagen synthesis reaction involving Vitamin C Ascorbic acid and dehydroascorbic acid

Coenzymes

A coenzyme is an organic compound required for catalysis, often derived from water-soluble vitamins. Coenzymes assist enzymes in biochemical reactions by carrying chemical groups or electrons.

Examples: Thiamine pyrophosphate (TPP), Flavin adenine dinucleotide (FAD), Nicotinamide adenine dinucleotide (NAD+), Coenzyme A, Pyridoxal phosphate (PLP), Biotin, Tetrahydrofolate (THF), Methylcobalamin.

Coenzyme involvement in enzyme reactions Summary table of water-soluble vitamins and their coenzymes

Diagnostic Enzymes and Serological Testing

Enzyme-Based Diagnostic Tests

Enzyme assays are used in clinical diagnostics to detect organ damage or disease. Specific enzymes are released into the blood following tissue injury, and their activity can be measured to identify the affected organ.

Liver and Heart: Aspartate Transaminase (AST), Alanine Transaminase (ALT), Lactate Dehydrogenase (LDH), Creatine Kinase (CK/CPK).
Pancreas: α-Amylase.
Prostate: Prostate Specific Antigen (PSA).
Bone and Liver: Alkaline Phosphatase (ALP).

Example: Elevated CK and AST levels in blood indicate a heart attack.

Isoenzymes of LDH and CK Enzyme levels after heart attack

Serological Testing: Lateral Flow Assay

Lateral flow assays are rapid diagnostic tests used to detect antigens, such as the spike protein of SARS-CoV-2. These tests use antibodies (a class of protective proteins) to bind specific antigens and provide a visual result.

Procedure: Sample is applied to a test strip, and antibodies capture the target antigen. A colored line appears if the antigen is present.
Applications: COVID-19 testing, pregnancy tests, infectious disease detection.

Lateral flow assay architecture COVID-19 test card

Summary Tables

Water-Soluble Vitamins and Coenzymes

Vitamin	Coenzyme	Function
Thiamine (B1)	Thiamine pyrophosphate	Decarboxylation
Riboflavin (B2)	FAD, FMN	Electron transfer
Niacin (B3)	NAD+, NADP+	Oxidation-reduction
Pantothenic acid (B5)	Coenzyme A	Acetyl group transfer
Pyridoxine (B6)	Pyridoxal phosphate	Transamination
Cobalamin (B12)	Methylcobalamin	Methyl group transfer
Ascorbic acid (C)	Vitamin C	Collagen synthesis, healing of wounds
Biotin	Biocytin	Carboxylation
Folic acid	Tetrahydrofolate	Methyl group transfer

Fat-Soluble Vitamins

Vitamin	Provitamin	Functions
Vitamin A (retinol)	β-carotene	Vision; cell growth & differentiation; antioxidant for lipids
Vitamin D (calciferol)	7-Dehydrocholesterol	Bone health; absorption of calcium & phosphate
Vitamin E (α-tocopherol)	None	Strong antioxidant; prevents oxidative damage
Vitamin K (phylloquinone)	None	Synthesis of prothrombin for blood clotting

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