Residues in the Human Corticosteroid-binding Globulin Reactive Center Loop That Influence Steroid Binding before and after Elastase Cleavage

Corticosteroid-binding globulin (CBG) is a non-inhibitory serine proteinase inhibitor (serpin) that transports cortisol and progesterone in blood. Crystal structures of rat CBG and a thrombin-cleaved human CBG: anti-trypsin (Pittsburgh) chimera show how structural transitions after proteolytic cleavage of the CBG reactive center loop (RCL) could disrupt steroid binding. This ligand release mechanism is assumed to involve insertion of the cleaved RCL into the beta-sheet A of the serpin structure. We have, therefore, examined how amino acid substitutions in the human CBG RCL influence steroid binding before and after its cleavage by neutrophil elastase. Elastase-cleaved wild-type CBG or variants with substitutions at P15 and/or P16 (E334G/G335N or E334A) lost steroid binding completely, whereas deletion of Glu-334 resulted in no loss of steroid binding after RCL cleavage, presumably because this prevents its insertion into beta-sheet A. Similarly, the steroid binding properties of CBG variants with substitutions at P15 (G335P), P14 (V336R), or P12 (T338P) in the RCL hinge were largely unaffected after elastase cleavage, most likely because the re-orientation and/or insertion of the cleaved RCL was blocked. Substitutions at P10 (G340P, G340S) or P8 (T342P, T342N) resulted in a partial loss of steroid binding after proteolysis which we attribute to incomplete insertion of the cleaved RCL. Remarkably, several substitutions (E334A, V336R, G340S, and T342P) increased the steroid binding affinities of human CBG even before elastase cleavage, consistent with the concept that CBG normally toggles between a high affinity ligand binding state where the RCL is fully exposed and a lower affinity state in which the RCL is partly inserted into beta-sheet A.