Appleby C A, Wittenberg B A, Wittenberg J B
Division of Plant Industry, Commonwealth Scientific and Industrial Research Organization, Canberra, Australia 2601.
Proc Natl Acad Sci U S A. 1973 Feb;70(2):564-8. doi: 10.1073/pnas.70.2.564.
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.
一种迄今称为X的小分子,存在于豆科植物根瘤中,能与单体蛋白豆血红蛋白可逆结合,形成血色原结构,该小分子被鉴定为烟酸。在pH 5.3时,烟酸与高铁豆血红蛋白和亚铁豆血红蛋白结合的结合常数分别为K = 7.3×10⁵ M⁻¹和K = 3.0×10⁴ M⁻¹。这种配体的高亲和力结合需要一个未被取代的吡啶环氮原子和环3位上的一个离子化羧基。烟酸在酸性pH下更易结合,这表现为豆血红蛋白对氧气的表观亲和力降低。