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Special points of emphasis

Coordination Chemistry

Proteins

Blood

The Digestive System and Nutrition

Let us discuss cofactors and coenzymes in the context of vitamins. A cofactor is non-protein chemical compound bound to an enzyme to assist in catalysis. Here is some good MCAT vocabulary. An enzyme without a cofactor is referred to as an apoenzyme, and the completely active enzyme (in addition to the cofactor) is called a holoenzyme. (This level of terminology often appears in an MCAT passage, but you can almost always decode the meaning from the context). For example, vitamin K is a cofactor for proteases involved in blood clotting. A carboxylation reaction which depends on vitamin K converts glutamate side chains of prothrombin into γ-carboxyglutamate, which is a strong chelator of Ca2+. This allows the calcium to serve as a coordinating bridge, a mordant, to anchor the prothrombin to phospholipid membranes in proximity to the clotting factors that activate the zymogen.

Coenzymes are non-protein molecules that carry chemical groups between enzymes. Some coenzymes are vitamins. For example, folic acid, a water soluble B vitamin, acts as a porter in the cell of various carbon groups, methyl, formyl or methylene.




Coordination Chemistry

Proteins

Bioenergetics and Cellular Respiration

You cannot mention cofactors without mentioning nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide phosphate (NADP+). These cofactors serve in many contexts as carriers of electrons in the transfer of reduction potential. NADH and NADPH are hydride (H-) donors.

NADH is the reduced form of NAD+, and NAD+ is the oxidized form of NADH. The same relationship applies between NADPH and NADP+.

Why do these forms donate a hydride so easily? Because the ring within the molecules that gives up the hydride becomes aromatic in the process.

NADH is used extensively in glycolysis and oxidative metabolism. NADPH is important for the biosynthesis of nucleic acids and certain lipids.








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