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Extended x ray absorption fine structure investigations of zinc in 5 aminolevulinate dehydratase


Biochemical Journal 230(3): 625-634
Extended x ray absorption fine structure investigations of zinc in 5 aminolevulinate dehydratase
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. (a) In the native enzyme, zinc is considered to be co-ordinated to three sulphur atoms at 0.228(2)nm [2.28(2).ANG.] and a lower-Z atom at 0.192(5)nm [1.92(5).ANG.] (if nitrogen) or 0.189(5)nm [1.89(5).ANG.] (if oxygen). (b) 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).ANG.], a nitrogen atom at 0.193(5)nm [1.93(5).ANG.] and a nitrogen atom from the inhibitor at 0.214(5)nm [2.14(5).ANG.]. (c) 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.


Accession: 005432717

PMID: 4062868



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