Transition carboxypeptidase

Carbon atom, 4. See also Atomic orbitals Carbon dioxide hydration, 197-199. See also Carbonic anhydrase Carbonic anhydrase, 197-199,200 Carbonium ion transition state, 154, 159 Carboxypeptidase A, 204-205 Catalysis, general acid, 153,164,169 in carboxypeptidase A, 204-205 free energy surfaces for, 160, 161 in lysozyme, 154... 

Figure 2. Transition-state analogs of a) adenosine deaminase, b) sucrase (and other glucosidases), and c) carboxypeptidase A.

This analysis reveals that enzymes bind the transition state more tightly than the ground state by a factor approximately equal to the rate of acceleration (ie, Kjs/Ks kuncaJkcat)- This method has been used to show, for example, that the peptide phos-phonate inhibitors of carboxypeptidase A are true transition state analogs. 

U. H. Mortensen, S. J. Remington, K. Breddam, Site-Directed Mutagenesis on (Serine) Carboxypeptidase Y. A Hydrogen Bond Network Stabilizes the Transition State by Interaction with the C-Terminal Carboxylate Group of the Substrate , Biochemistry 1994, 33, 508-513. 

The enzyme carboxypeptidase A is particularly amenable to structural investigation crystal structures of the enzyme, of complexes of the enzyme with substrates, substrate analogues and inhibitors, and of transition-state analogues are available. To isolate an enzyme-substrate complex for a one-substrate enzyme reaction, or for an enzyme reaction where water is a... 

Several other studies revealed alternative biological effects of sulfoximines. For example. Mock found 3 to be a transition-state analogue of carboxypeptidase... 

P. A. Bartlett, Phosphonate analogs of carboxypeptidase A substrates are potent transition-state analog inhibitors, Biochemistry, 1989, 28, 6294—305. 

Stable compounds which resemble the transition-state structure of a substrate in an enzymatic reaction are expected to behave as potent reversible inhibitors (1 ). Based on the X-ray crystallographic structure of the active site of carboxypeptidase A (CPA) (2), a mechanism was proposed in which a water molecule adds directly to the scissile carbonyl group of the substrate to give the tetrahedral intermediate 1, which collapses to products (3). We proposed to mimic this tetrahedral intermediate, similar to the transition state, with the stable tetrahedral phosphonic acid derivatives 2,... 

Liu J, Wulff G (2008) Functional mimicry of carboxypeptidase A by a combination of transition state stabilization and a defined orientation of catalytic moieties in molecularly imprinted polymers. J Am Chem Soc 130 8044-8054... 

The combination of Lewis add and nucleophile activations in 12-14 requires four-membered ring transition states. Interestingly, the mechanism for carboxypeptidase A (CPA) catalyzed hydrolysis of peptides also appears to involve joint Lewis add and nucleophile activations that lead to the formation of a four-membered ring transition state. Christianson and Lipscomb [57] have determined the crystal structure (15) of a ketone bound to CPA. Surprisingly, the ketone is in its hydrated form with both oxygens of the gem-diol bound to the active-site zinc of CPA (Figure 6.12). 

Figure 6.12 Four-membered ring transition state analog at the active site of carboxypeptidase A.

Generally speaking, the role of the enzyme consists of the selective and specific attraction of substrate and the highly efficient catalysis. Every enzyme has its own characteristic feature for example, the specificity in the binding and a charge-relay action in the catalysis in a-chymotrypsin, the contribution of the imidazole moiety as an electron donor to the electrophilicity of zinc ion in carboxypeptidase, the change in the spin state and the reactivity of the transition metal ion by the coordination of the imidazole in the hemochrome. These typical characteristic features are the result of the cooperative actions of the constituents. 

In Section 2 mechanisms of a few enzymes are briefly reviewed as a model. a-Chymotrypsin is one of the well-studied enzymes which has the nature of the nucleophilicity in the catalysis. Carboxypeptidase has the nature of electrophilicity in the catalytic center at which the zinc ion is supported and controlled by the imidazole moiety. Hemochrome has the characteristic of the coordination of imidazole with transition metal ion. 

Unlike chymotrypsinogen and trypsinogen, carboxypeptidase A is not inhibited by DFP and cannot therefore be considered as a serine enzyme. It is actually a metallo enzyme containing one zinc atom per molecule. When zinc is removed by dialysis against acidic buffers or 1,10-phenanthro-line (132, 133), an inactive apoenzyme is obtained which does not differ from the enzyme itself, except for activity and zinc content. Activity is gradually restored by addition of zinc up to one equivalent (133, 134). Some other metals of the first transition period (Mn++, Fe++, Co++, Ni++) can also restore activity in the apoprotein (134). Relative affinities of the apoenzyme for various metal anions have been investigated by equilibrium dialysis against solutions of radioactive salts (134). 

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