THE COORDINATION OF THE CATALYTIC ZINC ION IN ALCOHOL-DEHYDROGENASE STUDIED BY COMBINED QUANTUM-CHEMICAL AND MOLECULAR MECHANICS CALCULATIONS

The coordination number of the catalytic zinc ion in alcohol dehydroge nase has been studied by integrated ab initio quantum-chemical and mol ecular mechanics geometry optimisations involving the whole enzyme. A four-coordinate active-site zinc ion is 100-200 kJ/mol more stable tha n a five-coordinate one, depending on the ligands. The only stable bin ding site for a fifth ligand at the zinc ion is opposite to the normal substrate site, in a small cavity buried behind the zinc ion. The zin c coordination sphere has to be strongly distorted to accommodate a li gand in this site, and the ligand makes awkward contacts with surround ing atoms. Thus, the results do not support proposals attributing an i mportant role to five-coordinate zinc complexes in the catalytic mecha nism of alcohol dehydrogenase. The present approach makes it possible also to quantify the strain induced by the enzyme onto the zinc ion an d its ligands; it amounts to 42-87 kJ/mol for four-coordinate active-s ite zinc ion complexes and 131-172 kJ/mol for five-coordinate ones. Th e four-coordinate structure with a water molecule bound to the zinc io n is about 20 kJ/mol less strained than the corresponding structure wi th a hydroxide ion, indicating that the enzyme does not speed up the r eaction by forcing the zinc coordination sphere into a structure simil ar to the reaction intermediates.