Binding of the Recombinant Proteinase Inhibitor Eglin C from LeechHzrudo Medzcznalzsto Human Leukocyte Elastase, Bovine α-Chymotrypsin and Subtilisin Carlsberg: Thermodynamic Study

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Abstract
The effect of pH and temperature on the apparent association equilibrium constant (Ka) for the binding of the recombinant proteinase inhibitor eglin c from leech Hirudo medicinalis to human leukocyte elastase (EC 3.4.21.37), bovine α-chymotrypsin (EC 3.4.21.1) and subtilisin Carlsberg (EC 3.4.21.14) has been investigated. On lowering the pH from 9.5 to 4.5, values of Ka for eglin c binding to the serine proteinases considered decrease thus reflecting the acid-pK shift of the invariant histidyl catalytic residue (His57 in human leukocyte elastase and bovine α-chymotrypsin, and His64 in subtilisin Carlsberg) from ∼ 6.9, in the free enzymes, to ∼ 5.1, in the enzyme inhibitor adducts. At pH 8.0, values of the apparent thermodynamic parameters for eglin c binding are: human leukocyte elastase - Ka = 1.0 ∼ 1010M−1, δGϕS = - 13.4 kcal/mol, δHϕS = + 1.8 kcal/mol, and δSϕS = + 52 entropy units; bovine α-chymotrypsin - Ka = 5.0 ∼ 109 M-1, δGϕS = -13.0 kcal/mol, δHϕS = + 2.0 kcal/mol, and δSϕS = + 51 entropy units; and subtilisin Carlsberg - Ka = 6.6 ∼ 109M−1. δGϕS = -13.1 kcal/mol, δHϕS = +2.0 kcal/mol, and δSϕS = +51 entropy units (values of Ka, δGϕS and δSϕS were obtained at 21∼C; values of δHϕS were temperature independent over the range explored, i.e. between 10∼C and 40∼C; I kcal = 4184 J). Thermodynamics of eglin c binding to the serine proteinases considered has been analyzed in parallel with those of related (pro)enzyme:macromolecular inhibitor systems. Considering the known molecular models, the observed binding behaviour of eglin c was related to the inferred stereochemistry of the proteinase:inhibitor contact regions.