Chapter 7: Coenzymes and Vitamins

Enzymes often require additional chemical helpers called cofactors to function effectively, which convert inactive protein forms into catalytically competent enzymes. These cofactors fall into two primary categories: essential inorganic ions and organic coenzymes. Metal-activated enzymes are stimulated by loosely associated ions such as potassium, calcium, and magnesium, while metalloenzymes contain tightly bound transition metals like zinc, iron, copper, or cobalt that directly participate in catalytic mechanisms. Coenzymes serve as chemical group-transfer agents, with some dissociating from the enzyme after each catalytic cycle as cosubstrates, while others remain permanently attached as prosthetic groups. Many coenzymes originate from dietary vitamins, which the body converts into their active forms. Water-soluble vitamin-derived coenzymes include adenosine triphosphate and other nucleotide cosubstrates for phosphoryl transfer, nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate for two-electron redox chemistry, flavin-based coenzymes for flexible one-electron or two-electron transfers, coenzyme A for acyl group activation through thioester formation, thiamine diphosphate for decarboxylation reactions, pyridoxal phosphate for amino acid transformations, biotin for carboxylation reactions, tetrahydrofolate for one-carbon unit transfers critical to nucleic acid synthesis, and cobalamin for intramolecular rearrangements. Vitamin C functions as a reducing agent rather than a true coenzyme and is essential for collagen hydroxylation. Lipid-soluble vitamins possess hydrophobic structures enabling membrane integration and include retinol for vision and gene regulation, vitamin D as a hormone regulating mineral metabolism, alpha-tocopherol as a membrane antioxidant, and phylloquinone as a coenzyme in blood coagulation cascades. Additionally, some organic molecules like ubiquinone function in electron transport within membranes, while certain proteins themselves serve coenzyme roles through reversible chemical modifications, such as thioredoxin with its disulfide bonds and cytochromes with their heme iron centers that shuttle electrons through metabolic pathways.