The role of basic residues of the 70-80-loop, Arg(74), Arg(75), and Lys(78) (chymotrypsin numbering) in the catalytic function of activated protein C (APC) was investigated by expressing mutants of protein C in which these residues were replaced with Ala in three separate constructs. Following purification to homogeneity and activation by thrombin, the catalytic properties of the mutants were characterized with respect to their ability to cleave the chromogenic substrate Spectrozyme PCa, react with protein C inhibitor (PCI), and inactivate factor Va. Relative to wild-type APC, the mutants cleaved Spectrozyme PCa with identical or improved catalytic efficiencies. Similarly, PCI inhibited mutants with identical or improved second-order rate constants (k(2)) in the absence of heparin. However, the heparin-catalyzed inhibition of mutants by PCI was impaired approximately 10-fold. Analysis of k(2) values by a ternary complex model revealed that the affinities of mutants for heparin were impaired to a similar extent. Moreover, analysis of the NaCl gradient elution profiles of APC derivatives from Heparin-Sepharose supported this conclusion. An oligosaccharide containing 14 residues efficiently catalyzed the PCI inhibition of APC by a template mechanism. Further studies revealed that the ability of Arg(74) and Arg(75) mutants to inactivate factor Va was markedly impaired. We conclude that basic residues of the 70-80-loop are critical for the catalytic function of APC.