A bacterial expression system for the inhibitory N-terminal domain of human tissue inhibitor of metalloproteinases 1 (N-TIMP-1) (Huang, W., Suzuki, K., Nagase, H., Arumugam, S., Van Doren, S. R., and Brew, K. (1996) FEBS Lett. 384, 155-161) has been used to produce 20 single- and double-site mutants that probe the roles of different residues in its inhibitory action on metalloproteinases. Mutations that produce the largest increases in the Ki for a C-terminally truncated form of stromelysin 1, MMP-3(DeltaC), but do not disturb the conformation involve substitutions of residues that are located in a ridge that is centered around the disulfide bond between Cys1 and Cys70. Specific residues that have a large influence on activity include Cys1, Thr2, Met66, Val69, and Cys70. Of the mutations introduced, the greatest functional disturbances, reflected in Ki increases of 2-4 orders of magnitude, are generated by changes that disrupt the Cys1-Cys70 disulfide bond and by substitution of Ala for Thr2. Most mutations that perturb the interaction with MMP-3 have parallel effects on the affinity of N-TIMP-1 for MMP-1 (interstitial collagenase) and MMP-2 (gelatinase A). However, the Thr2 to Ala mutation produces an inhibitor that is 17-fold more effective against MMP-3 than MMP-1, suggesting that it is feasible to engineer TIMP-1 variants that are more specifically targeted to selected matrix metalloproteinases. The reactive site identified by these studies is a structurally constrained but elongated region of TIMP that can fit the matrix metalloproteinase substrate-binding site.