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.