Extended-X-ray-absorption-fine-structure investigations of zinc in 5-aminolaevulinate dehydratase.

Abstract

The zinc co-ordination in 5-aminolaevulinate dehydratase (5-aminolaevulinate hydro-lyase, EC 4.2.1.24) was investigated by recording and interpreting the extended X-ray-absorption fine structure (e.x.a.f.s.) associated with the zinc K-edge. The enzyme has a molecular mass of 280 000 Da and consists of eight subunits of 35 000 Da each; the samples studied contained approx. 1 g-atom of zinc/mol of subunit. Four forms of the enzyme were investigated and details of the zinc environment were elucidated, as follows. In the native enzyme, zinc is considered to be co-ordinated to three sulphur atoms at 0.228(2)nm [2.28(2)A] and a lower-Z atom at 0.192(5)nm [1.92(5)A] (if nitrogen) or 0.189(5)nm [1.89(5)A] (if oxygen). Reaction of the enzyme with the inhibitor 2-bromo-3-(imidazol-5-yl)propionic acid produced significant changes in the e.x.a.f.s., the nature of which are consistent with co-ordination by about three sulphur atoms at 0.222(2)nm [2.22(2)A], a nitrogen atom at 0.193(5)nm [1.93(5)A] and a nitrogen atom from the inhibitor at 0.214(5)nm [2.14(5)A]. Inactivation of the enzyme by air-oxidation of essential thiol groups and binding of the substrate produce slight changes in the e.x.a.f.s. consistent with slight re-arrangement of ligands with additional lighter ligands (nitrogen or oxygen). These results, when combined with previous findings, are taken to indicate that zinc has a structural rather than a direct catalytic role in 5-aminolaevulinate dehydratase.