The roles of three Serratia marcescens chitinases in chitin conversion are reflected in different thermodynamic signatures of allosamidin binding

Abstract

Binding of allosamidin to the three family 18 chitinases of Serratia marcescens has been studied using isothermal titration calorimetry (ITC). Interestingly, the thermodynamic signatures of allosamidin binding were different for all three chitinases. At pH 6.0, chitinase A (ChiA) binds allosamidin with a K(d) value of 0.17 +/- 0.06 microM where the main part of the driving force is due to enthalpic change (DeltaH(r) degrees = -6.2 +/- 0.2 kcal/mol) and less to entropic change (-TDeltaS(r) degrees = -3.2 kcal/mol). A large part of DeltaH is due to allosamidin stacking with Trp(167) in the -3 subsite. Binding of allosamidin to both chitinase B (ChiB) (K(d) = 0.16 +/- 0.04 microM) and chitinase C (ChiC) (K(d) = 2.0 +/- 0.2 microM) is driven by entropy (DeltaH(r) degrees = 3.8 +/- 0.2 kcal/mol and -TDeltaS(r) degrees = -13.2 kcal/mol for ChiB and DeltaH(r) degrees = -0.6 +/- 0.1 and -TDeltaS(r) degrees = -7.3 kcal/mol for ChiC). For ChiC, the entropic term is dominated by changes in solvation entropy (DeltaS(conf) = 1 cal/K.mol and DeltaS(solv) = 31 cal/K.mol), while, for ChiB, changes in conformational entropy dominate (DeltaS(conf) = 37 cal/K x mol and DeltaS(solv) = 15 cal/K x mol). Corresponding values for ChiA are DeltaS(conf) = 4 cal/K x mol and DeltaS(solv) = 15 cal/K x mol. These remarkable differences in binding parameters reflect the different architectures of the catalytic centers in these enzymes that are adapted to different types of actions: ChiA and ChiB are processive enzymes that move in opposite directions, meaning that allosamidin binds in to "product" subsites in ChiB, while it binds to polymer-binding subsites in ChiA. The values for ChiC are compatible with this enzyme being a nonprocessive endochitinase with a much more open and solvated substrate-binding-site cleft.