The role of a malate binding site ina concavity external to the more deeply situated active site has been a major my stery of the fumarase reaction. The malate, within12 Å of the active site, was bound by hydrogen bonds to two main-chain amides and to two basic residues, H129 and R126. Mutation of the His of this so-called B site of Escherichia colifumarase had little effect on the overall initial rate kinetics ofthe enzyme, which has obscured an understanding of the criticalrole of the site. Contrary to the WT enzyme, which is rate-limitedin the recycling of free enzyme isoforms that follows productrelease, the enzyme with both basic residues modified israte-limited in the product release step itself. A loss ofcomplexity in the mutated, but still functional, step is indicatedby a greatly reduced sensitivity of its rate to changes intemperature. Unlike the inhibition by glycerol shown with normalenzyme and attributed to a viscogenic effect on the recycling rate,the product release step of the B-site mutants is accelerated byglycerol, suggestive of a structural effect on the 12-Å spacebetween the A and B sites. It is proposed that the‘‘extra’’ malate represents a stage in thetransfer of substrate and product between the solvent and the‘‘buried’’ active site of the enzyme.