Nicotinic Acid as a ligand affecting leghemoglobin structure and oxygen reactivity.

A small molecule, hitherto called X, which is present in legume root nodules and ligates reversibly to the monomeric protein, leghemoglobin, with formation of a hemochrome structure, is identified as nicotinic acid. The binding constants at pH 5.3 are K = 7.3 x 10(5) M(-1) and K = 3.0 x 10(4) M(-1) for combination of nicotinic acid with ferric and ferrous leghemoglobin, respectively. This high affinity binding of ligand requires an unsubstituted pyridine ring nitrogen atom and an ionized carboxyl group in the 3-position of the ring. Binding of nicotinic acid is favored at acid pH and is reflected by diminished apparent affinity of leghemoglobin for oxygen.