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• W1964595239 abstract "Coordination Chemistry The catalytic zinc ion in horse liver alcohol dehydrogenase (HLADH) can be removed by treating a crystalline suspension of the enzyme with dipicolinic acid [1]. The resulting apo-zinc enzyme (H4-Zn(n)2LADH, where n denotes the noncatalytic metal ion) is inactive. It can be reconstituted with Zn2+, Co2+ [1], Cd2+ [2], Ni2+ [3], Cu2+ [4], Fe2+[5], and Pb2+ [5] to metallo LADHs of the general composition Me(c)2Zn(n)2LADH (where c denotes the catalytic metal ions). Species containing Cu1+ and Fe3+ can be obtained via redox reactions. Catalytic activity is largely preserved with M2+ = Co2+, Ni2+, Cd2+, Cu2+; whereas the Cu2+ and Fe2+ enzymes show activities less than 5% that of the native enzyme and the Pb2+ enzyme less than 1% that of the native enzyme. The process of reconstitution is biphasic according to the equation M2+ + H4Zn(n)2LADH {k1/⇌/k−} M2+ · H4Zn(n)2LADH k2/→ M(c)2 Zn(n)2 LADH + 4H+ where k2 is rate-limiting. The activation parameters as well as k2 and K = k−1/k1 were determined for the insertion of Co2+ and Ni2+. The process is characterized by an exceptionally high negative activation entropy indicating some flexibility in the empty catalytic site of H4Zn(n)2 LADH which leads to a sterically restrictive transition state for the metal insertion pathway. X-ray studies have shown that the tertiary structure of LADH is virtually unchanged upon removal of the catalytic metal ion. In both Co(c)2Zn(n)2LADH and Cd(c)2Zn(n)2LADH the coordination number four and the tetrahedral geometry remain unchanged as compared to the native enzyme. This shows that these (and probably several other) derivatives may serve as true models for the native enzyme. Another X-ray study has revealed that the change from the open to the closed conformation which is induced upon binding of NADH occurs in H4Zn(n)2LADH as well as in the native enzyme which shows that the structural transition is independent of the presence of the catalytic metal ion. Mechanistic Investigation The use of metallo LADHs has helped to clarify several important mechanistic questions which had previously caused long-standing debates. First, NMR relaxation dispersion measurements on solvent and substrate protons have shown that no significant differences exist between the relaxation rates of the Co2+, Ni2+, Fe3+, Zn2+, and demetallized enzymes. This implies that previous models of outer-sphere substrate binding, based on NMR relaxation data obtained with Co4LADH or Zn(c)2Co(n)2LADH, can be safely ruled out. Secondly, spectroscopic and kinetic investigations of the binding and turnover of the chromophoric substrates (notably trans-4-N,N(dimethylamino)cinnamaldehyde) to metallo LADHs containing Cd2+, Zn2+, Ni2+, Co2+ have provided strong evidence for inner-sphere coordination of the substrate's carbonyl group to the catalytic metal ion. Third, optical and NMR measurements on Co(c)2Zn(n)2LADH have shown that at least three proton equilibria are linked to groups in the immediate vicinity of the metal ion. They include the free enzyme (pK ∼ 9.5), the binary complex enzyme · NAD+ (pK ∼ 8) and the productive complex enzyme · NAD+·ethanol (pKa ∼ 6.3). Probably none is due to the postulated ionization of the metal-bound water molecule." @default.
• W1964595239 created "2016-06-24" @default.
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• W1964595239 date "1983-01-01" @default.
• W1964595239 modified "2023-10-14" @default.
• W1964595239 title "Coordination chemistry and function of the catalytic metal ion in alcohol dehydrogenase" @default.
• W1964595239 cites W1561207830 @default.
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• W1964595239 doi "https://doi.org/10.1016/s0020-1693(00)95052-7" @default.
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