Regulation of Enzyme Activity, Vitamins, and Diagnostic Enzyme 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 with distinct mechanisms and effects on enzyme kinetics.

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 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 Vmax and Km 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.

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

Enzyme Regulation

Mechanisms of Regulation

Enzymes in metabolic pathways 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 an enzyme early in the pathway. Negative feedback inhibits the enzyme, while positive feedback enhances 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 levels through gene expression.

Negative allosteric regulation diagram Feedback control in metabolic pathway Covalent modification of serine residue Phosphorylation and dephosphorylation regulation Zymogen activation diagram

Vitamins and Their Significance to Enzyme Function

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 function as coenzymes.
Fat-Soluble Vitamins: Hydrophobic, composed mainly of carbon and hydrogen, stored in the liver.

Water-soluble vitamins include the B-complex and vitamin C. Fat-soluble vitamins include A, D, E, and K.

Water-soluble and fat-soluble vitamin classification

Fat-Soluble Vitamins

Vitamin	Provitamin	Active Form	Function	Source
A	β-carotene	Retinol	Vision, cell growth, antioxidant	Carrots, dairy, liver
D	7-Dehydrocholesterol	Calciferol	Bone health, Ca2+ absorption	Sunlight, fish, milk
E	None	α-tocopherol	Antioxidant, prevents oxidative damage	Nuts, seeds, oils
K	None	Phylloquinone/K1, menaquinone/K2	Blood clotting, bone metabolism	Leafy greens, bacteria

Vitamin A structure and function Vitamin D synthesis and activation Vitamin K1 and K2 structures

Water-Soluble Vitamins and Coenzymes

Water-soluble vitamins often function as coenzymes, which are organic compounds required for enzyme catalysis. Most coenzymes are derivatives of B vitamins.

Vitamin	Alternate Name	Coenzyme	Function
B1	Thiamine	Thiamine pyrophosphate (TPP)	Decarboxylation, carbohydrate metabolism
B2	Riboflavin	FMN, FAD	Redox reactions
B3	Niacin	NAD+, NADP+	Redox reactions
B5	Pantothenic acid	Coenzyme A	Acetyl group transfer
B6	Pyridoxine	Pyridoxal phosphate (PLP)	Amino acid metabolism
B7	Biotin	Biotin	Carboxylation
B9	Folic acid	Tetrahydrofolate (THF)	Methyl group transfer
B12	Cobalamin	Methylcobalamin	Methyl transfer, nucleic acid metabolism
C	Ascorbic acid	Self	Antioxidant, collagen synthesis

Coenzyme function in enzyme catalysis FMN and FAD structure FMN and FAD reduced forms NAD+ and NADH structure Folic acid and THF structure Vitamin C in collagen synthesis Vitamin C oxidation to dehydroascorbic acid Summary table of water-soluble vitamins and coenzymes

Diagnostic Enzyme Testing

Enzymes as Diagnostic Markers

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

Liver: AST, ALT, GGT, ALP
Heart: CK (CPK), LDH, Troponin
Pancreas: α-amylase
Prostate: PSA, Acid Phosphatase

Isozymes are tissue-specific forms of enzymes that catalyze the same reaction.

Isozyme tissue specificity Enzyme levels after heart attack

Serological Testing and Lateral Flow Assays

Lateral flow assays, such as rapid COVID-19 tests, use antibodies (a class of protective proteins) to detect antigens in a sample. These tests are quick and easy to use.

Lateral flow assay architecture COVID-19 test procedure and result interpretation

Summary Table: Fat-Soluble and Water-Soluble Vitamins

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

Vitamin	Coenzyme	Function
B1 (Thiamine)	Thiamine pyrophosphate	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	Transamination
B12 (Cobalamin)	Methylcobalamin	Methyl group transfer
C (Ascorbic acid)	Vitamin C	Collagen synthesis, healing
B7 (Biotin)	Biocytin	Carboxylation
B9 (Folic acid)	Tetrahydrofolate	Methyl group transfer