Thermodynamics and kinetics of the hydrolysis and resynthesis of the reactive site peptide bond in turkey ovomucoid third domain by aspergillopeptidase B

1. Aspergillopeptidase B rapidly hydrolyses the -Leu18-Glu19-reactive site peptide bond in turkey ovomucoid third domain (OMTKY3) within the pH-range of 4.0-8.4. The reaction proceeds to equilibrium between OMTKY3 and its modified form with the reactive site peptide bond cleaved (OMTKY3). 2. The dependence of the equilibrium constant (Khyd) on pH indicates that hydrolysis of the reactive site peptide bond apparently does not perturb the pK-values of any preexistent ionizable groups in OMTKY3. 3. The obtained Khyd0 value indicates that free energies of OMTKY3 and OMTKY3 are essentially the same. 4. Hydrolysis of the reactive site peptide bond by aspergillopeptidase B at neutral pH is about 60 times faster than the same reaction catalyzed by subtilisin (Carlsberg), the enzyme strongly inhibited by OMTKY3. 5. Resynthesis of the reactive site peptide bond at neutral pH catalyzed by aspergillopeptidase B (reverse reaction) is almost four orders of magnitude faster than the forward reaction.