Lec 11 Regulation of Enzyme Activity, Vitamins, and Diagnostic Testing in GOB Chemistry

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 increases the apparent Km (substrate concentration at half-maximal velocity) but does not affect Vmax (maximum reaction rate).

• Noncompetitive Inhibition: The inhibitor binds to a site other than the active site, causing a change in enzyme configuration. This lowers Vmax but leaves Km 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 irreversibly inhibits acetylcholinesterase by covalent modification.

Enzyme Regulation

Mechanisms of Regulation

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

• Feedback Control: A product of a 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 in inactive forms (proenzymes) and activated by removal of a protein segment.

• Genetic Control: Regulation of enzyme levels through gene expression.

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, absorption, and toxicity. Water-soluble vitamins are excreted easily, while fat-soluble vitamins can accumulate and cause toxicity.

Biological Functions and Sources

Each vitamin has a specific biological function and source. Some are ingested as provitamins and converted to active forms in the body.

Vitamin	Alternate Name	Provitamin	Function	Source
A	Retinol	β-carotene	Vision, cell growth, antioxidant	Carrots, dairy, liver
D	Calciferol	7-Dehydrocholesterol	Bone health, Ca2+ absorption	Sunlight, fish, milk
E	α-tocopherol	None	Antioxidant	Nuts, seeds, oils
K	Phylloquinone	None	Blood clotting	Leafy greens, bacteria
C	Ascorbic acid	None	Antioxidant, collagen synthesis	Citrus, vegetables
B1	Thiamine	None	Carbohydrate metabolism	Bread, milk, legumes
B2	Riboflavin	None	Redox reactions	Bread, milk, meat
B3	Niacin	None	Redox reactions	Meat, cereal, nuts
B5	Pantothenic acid	None	Acetyl group transfer	All foods
B6	Pyridoxine	None	Amino acid metabolism	Veggies, grains, meat
B7	Biotin	None	Carbohydrate/lipid metabolism	Eggs, meat, veggies
B9	Folic acid	None	Heme, nucleic acid synthesis	Leafy greens, bacteria
B12	Cobalamin	None	Methyl transfer, nucleic acid metabolism	Meat, milk, bacteria

Coenzymes

A coenzyme is an organic compound required for enzyme catalysis, often derived from water-soluble vitamins. Coenzymes participate in biochemical reactions by carrying atoms or electrons.

Vitamin	Coenzyme	Function
B1 (Thiamine)	Thiamine pyrophosphate (TPP)	Decarboxylation
B2 (Riboflavin)	FAD, FMN	Electron transfer
B3 (Niacin)	NAD+, NADP+	Oxidation-reduction
B5 (Pantothenic acid)	Coenzyme A	Acetyl group transfer
B6 (Pyridoxine)	Pyridoxal phosphate (PLP)	Transamination
B12 (Cobalamin)	Methylcobalamin	Methyl group transfer
C (Ascorbic acid)	Vitamin C	Collagen synthesis, healing
B7 (Biotin)	Biocytin	Carboxylation
B9 (Folic acid)	Tetrahydrofolate (THF)	Methyl group transfer

Diagnostic Enzymes and Serological Testing

Enzyme-Based Diagnostic Tests

Enzyme activity in blood can be used to diagnose organ damage or disease. Specific enzymes are associated with particular organs:

• Heart Attack: Creatine Kinase (CK), Aspartate Transaminase (AST), Lactate Dehydrogenase (LDH)

• Liver Dysfunction: Alanine Transaminase (ALT), γ-Glutamyl transferase (GGT), Glutamate-pyruvate Transaminase (GPT)

• Pancreas: α-amylase

• Bone/Liver: Alkaline Phosphatase (ALP)

• Prostate: Prostate Specific Antigen (PSA), Acid Phosphatase (AP)

Isozymes: Different quaternary forms of the same enzyme, tissue-specific, useful for pinpointing organ damage.

Lateral Flow Assay for COVID-19

The lateral flow assay is a rapid diagnostic test for COVID-19, using antibodies (a class of protective proteins) to bind the spike protein of SARS-CoV-2. The test is visually read by the appearance of lines indicating positive or negative results.

Summary Table: Water-Soluble and Fat-Soluble Vitamins

Water-Soluble Vitamin	Co-factor Form	Basic 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
Biotin	Biocytin	Carboxylation
Folic acid	Tetrahydrofolate	Methyl group transfer

Fat-Soluble Vitamin	Provitamin	Functions
Vitamin A (retinol)	β-carotene	Vision, cell growth, antioxidant for lipids
Vitamin D (calciferol)	7-Dehydrocholesterol	Bone health, Ca2+ absorption
Vitamin E (α-tocopherol)	None	Antioxidant, prevents oxidative damage
Vitamin K (phylloquinone)	None	Blood clotting, bone metabolism