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X-ray analyses have defined the three-dimensional structures of crystals of mouse and human renins complexed with peptide inhibitors at resolutions of 1.9 and 2.8 A, respectively. The exquisite specificity of renin arises partly from ordered loop regions at the periphery of the binding cleft. Although the pattern of main-chain hydrogen bonding in other aspartic proteinase inhibitor complexes is conserved in renins, differences in the positions of secondary structure elements (particularly helices) also lead to improved specificity in renins for angiotensinogen substrates.

The X-ray structure of the inhibitor complex of bovine ribonuclease A with cytidylic acid (2'-CMP) has been determined at 1.6 A resolution and refined by restrained least squares to R = 0.17 for 11 945 reflections. Binding of the inhibitor molecule to the protein is confirmed to be in the productive mode associated with enzyme activity. A study of conserved solvent sites amongst high-resolution structures in the same crystal form reveals a stabilizing water cluster between the N and C termini.

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