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Multiple cage effects in horseradish peroxidase
INTERMEDIATES-FAST
KINETICS
co50 MULTIPLE CAGE EFFECTS IN HORSERADISH PEROXIDASE Debkumar Bandvonadhay,a Teddy G. Traylor,b aDepartment of Chemistry, Indian Institute of Technology, Delhi, India hDepartment of Chemistry, University of California, San Diego The kinetics of tert-butylisocyanide (t-ButNC) complexation with Horseradish Peroxidase (HRP) is followed in pica to second time domain. Two ligand concentration dependent bimolecular processes are observed The rate constants are 0.04 M-%X-~, and (2-6) x l@M-lsec-1 for the slow and fast processes respectively. The faster process seems to have some photochemical origin. In pica and nanoseconds time domain, two ligand concentration independent processes (cage returns) are observed. The data suggest that in the case of HRP at least a five state model is needed to have a meaningful physical picture. According to this analysis the ligand entry into the protein is energetically an uphill process. Unlike myoglobin the ligand in HRP enters protein very slowly. After ligand’s entry into the cage there is not much difference in the dynamics of ligand movement in these two proteins. In case of a smaller ligand, methylisocyanide (MeNC), only one ligand concentration dependent bimolecular rate is observed (k = 6.2 x 103M-lsec-1). Pica and nanosecond rate constants for MeNC and t-ButNC are almost the same. The data suggest that in the case of bulky t-ButNC some conformational change is necessary before ligand can enter into the protein. The smaller size of MeNC allows it to enter the protein mesh without a conformation change.
261
KINETICS
co50 MULTIPLE CAGE EFFECTS IN HORSERADISH PEROXIDASE Debkumar Bandvonadhay,a Teddy G. Traylor,b aDepartment of Chemistry, Indian Institute of Technology, Delhi, India hDepartment of Chemistry, University of California, San Diego The kinetics of tert-butylisocyanide (t-ButNC) complexation with Horseradish Peroxidase (HRP) is followed in pica to second time domain. Two ligand concentration dependent bimolecular processes are observed The rate constants are 0.04 M-%X-~, and (2-6) x l@M-lsec-1 for the slow and fast processes respectively. The faster process seems to have some photochemical origin. In pica and nanoseconds time domain, two ligand concentration independent processes (cage returns) are observed. The data suggest that in the case of HRP at least a five state model is needed to have a meaningful physical picture. According to this analysis the ligand entry into the protein is energetically an uphill process. Unlike myoglobin the ligand in HRP enters protein very slowly. After ligand’s entry into the cage there is not much difference in the dynamics of ligand movement in these two proteins. In case of a smaller ligand, methylisocyanide (MeNC), only one ligand concentration dependent bimolecular rate is observed (k = 6.2 x 103M-lsec-1). Pica and nanosecond rate constants for MeNC and t-ButNC are almost the same. The data suggest that in the case of bulky t-ButNC some conformational change is necessary before ligand can enter into the protein. The smaller size of MeNC allows it to enter the protein mesh without a conformation change.
261