/pdf/protein-structure-and-function-from-a-colloidal-to-a-1t1d4qfdqn.pdf

Protein structure and function, from a colloidal to a molecular view

Interaction of cobalt(II) complexes with apoarsanilazotyrosine-248 carboxypeptidase A

A 1H NMR study of cobalt(II) arsanilazocarboxypeptidase A

The metal coordination sphere of cobalt-substituted carboxypeptidase A and its complexes with inhibitors has been characterized by X-band electron paramagnetic resonance (EPR) spectroscopy. The temperature dependence of the EPR spectrum of cobalt carboxypeptidase and the g anisotropy are consistent with a distorted tetrahedral geometry for the cobalt ion. Complexes with L-phenylalanine, a competitive inhibitor of peptide hydrolysis, as well as other hydrophobic L-amino acids all exhibit very similar EPR spectra described by three g values that differ only slightly from that of the cobalt enzyme alone. In contrast, the EPR spectra observed for the cobalt enzyme complexes with 2-(mercaptoacetyl)-D-Phe, L-benzylsuccinate, and L-beta-phenyllactate all indicate an approximately axial symmetry of the cobalt atom in a moderately distorted tetrahedral metal environment. Phenylacetate, beta-phenylpropionate, and indole-3-acetate, which exhibit mixed modes of inhibition, yield EPR spectra indicative of multiple binding modes. The EPR spectrum of the putative 2:1 inhibitor to enzyme complex is more perturbed than that of the 1:1 complex. For beta-phenylpropionate, partially resolved hyperfine coupling (122 x 10(-4) cm-1) is observed on the g = 5.99 resonance, possibly indicating a stronger metal interaction for this binding mode. The structural basis for the observed EPR spectral perturbations is discussed with reference to the existing crystallographic kinetic and electronic absorption, nuclear magnetic resonance, and magnetic circular dichroic data.

Characterization of the inhibitor complexes of cobalt carboxypeptidase A by electron paramagnetic resonance spectroscopy.

The interaction of inhibitors with the active site of arsanilazocarboxypeptidase A has been investigated by means of resonance Raman spectroscopy. The resonance Raman bands of the active-site azotyrosine-248 residue have been shown previously to be sensitive to its state of ionization and its interactions with nearby groups. In particular, the azophenol form of azotyrosine-248 can adopt two different coexisting conformations that differ with respect to the presence or absence of an intramolecular hydrogen bond between the phenolic proton and a nitrogen atom of the azo group. Each of these conformations exhibits characteristic vNN and v phi N azo stretching frequencies. The relative concentrations of these two forms, revealed by resonance Raman spectroscopy, are a sensitive probe of the hydrogen bond accepting ability of the local environment. The present study shows that the binding of L-benzylsuccinate, phenylacetate, L-phenyllactate, and beta-phenylpropionate markedly perturbs the distribution of the intra- and intermolecularly hydrogen-bonded forms of azotyrosine-248 in water. In contrast, glycyl-L-tyrosine and L-phenylalanine leave this distribution unperturbed. These results, taken jointly with other data on inhibitor binding, serve to identify common binding sites for groups of inhibitors and result in plausible suggestions concerning the interactions between azotyrosine-248 and these inhibitors that lead to binding.

Resonance Raman spectroscopy of arsanilazocarboxypeptidase A: mode of inhibitor binding and active-site topography.

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Resonance Raman spectroscopy of arsanilazocarboxypeptidase A: mode of inhibitor binding and active-site topography.