Coenzymes: Structure and Biochemical Functions

• Coenzymes are small, organic molecules that bind to enzymes and are essential for their catalytic activity.
• Derived from vitamins, they act as carriers for chemical groups or electrons in enzymatic reactions.

Structure and Examples of Coenzymes

Nicotinamide Adenine Dinucleotide (NAD⁺)

• Structure: Two nucleotides (adenine and nicotinamide) joined by phosphate groups.
• Function: Electron carrier in redox reactions.
• Example: NAD⁺ to NADH in cellular respiration.

Flavin Adenine Dinucleotide (FAD)

• Structure: Riboflavin bound to an adenine nucleotide.
• Function: Electron carrier.
• Example: FAD to FADH₂ in the citric acid cycle.

Coenzyme A (CoA)

• Structure: Pantothenic acid, cysteamine group, and adenine nucleotide.
• Function: Transfers acyl groups.
• Example: Acetyl-CoA in the citric acid cycle.

Thiamine Pyrophosphate (TPP)

• Structure: Thiazole and pyrimidine rings with two phosphate groups.
• Function: Decarboxylation of alpha-keto acids.
• Example: Pyruvate to acetyl-CoA.

Pyridoxal Phosphate (PLP)

• Structure: Aldehyde group, pyridine ring, and phosphate group.
• Function: Amino acid metabolism.
• Example: Aminotransferases in transamination reactions.

Biotin

• Structure: Bicyclic molecule with ureido and tetrahydrothiophene rings.
• Function: Carboxylation reactions.
• Example: Acetyl-CoA carboxylase in fatty acid synthesis.

Tetrahydrofolate (THF)

• Structure: Pteridine ring, para-aminobenzoic acid, and glutamic acid.
• Function: Transfers one-carbon units.
• Example: Nucleotide synthesis.

Biochemical Functions

1. Electron Transfer: NAD⁺ and FAD in redox reactions (e.g., cellular respiration).
2. Group Transfer: CoA and TPP in metabolic pathways (e.g., citric acid cycle).
3. Carboxylation and Decarboxylation: Biotin and TPP in metabolism.
4. One-Carbon Transfer: THF in nucleotide synthesis.
5. Amino Acid Metabolism: PLP in transamination and other amino acid reactions.

• Coenzymes ensure the proper functioning of metabolic pathways and cellular health by facilitating various biochemical processes.

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