Glu-256 is a main structural determinant for oligomerisation of human arginase I

One determinant that could play a role in the quaternary structure of human arginase is the pair of salt links between the strictly conserved residues R255 from one monomer and E256 from every adjacent subunit. In this work, the ionic interaction between monomers was disrupted by expressing a human arginase where Glu-256 had been substituted by Gin. Biochemical analyses of the mutant protein showed that: (i) it shares the wild-type kinetic parame ters of the arginine substrate; (ii) E256Q arginase behaves as a monomer by gel filtration; (iii) it is drastically inactivated by dialysis in the pre sence of EDTA, an inhibitory effect which is reversed by addition of Mn2+; and (iv) the mutant enzyme loses thermal stability. The lack of oligomerisa tion for E256Q arginase and the conservation of E256 throughout evolution o f the protein family suggest that this residue is involved in the quaternar y structure of arginases. (C) 2001 Federation of European Biochemical Socie ties. Published by Elsevier Science B.V. Ali rights reserved.