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B Farzami

Publications and source records attributed to B Farzami.

5 recordsLinked to original sources
PubMed2004

Stability, structural and suicide inactivation changes of mushroom tyrosinase after acetylation by N-acetylimidazole.

Modification (acetylation) of Tyr residues with N-acetylimidazole protects outstandingly mushroom tyrosinase (MT) from the suicide inactivation in the presence of its catecholic substrate, 4-[(4-methylbenzo) azo]-1,2-benzenediol. UV spectrophotometric experiments and differential scanning calorimetry (DSC) studies indicated a decrease in kinetic stability of the enzyme alongside with increase in its thermal stability as well as its stability against n-dodecyl trimethylammonium bromide as a denaturizing agent. Pace analysis resulted in standard Gibbs free energy values of 46.54 and 52.09 kJ/mol in the absence of denaturant for native and modified enzyme, respectively. Structural studies by circular dichroism (CD) spectrophotometry showed that modification did not have major impact on the secondary structure of MT; however, induced some changes in its tertiary structure. The near-UV CD results revealed that the modification had enhanced intramolecular van der Waals interactions in the enzyme structure, which was in coincidence with its thermodynamic stability.

Agaricales↗
PubMed1994

Elucidation of pKa values for Ca2+ binding sites in calmodulin by spectrofluorometry.

Calmodulin (CaM) was purified from bovine brain and identified on the basis of its phosphodiesterase activity. Its purity was further tested by electrophoretic migration in polyacrylamide gels in the presence of sodium dodecyl sulfate. Apo-CaM was prepared from holo-CaM using hydroxyapatite chromatography. The Ca2+ binding sites on CaM and the pKa of each of the functional groups bound to Ca2+ were identified from the dependence of Ca2+ interaction with the functional group as a function of pH. EGTA was found to diminish the peaks corresponding to the pKa values of the groups bound to Ca2+. The use of bromophenacyl bromide, a modifier for aspartate and glutamate residues in proteins, diminished the peaks at pH = 3.4 and 4.3. Diethyl pyrocarbonate, a modifier for histidine residues, reduced the peak at pH = 6.2, corresponding to the pKa of the imidazole group in histidine. Furthermore, the peak at pH = 11.6 was eliminated using the specific tyrosine modifier, N-acetylimidazole. Diethylpyrocarbonate also eliminated four small peaks at pH = 7.2, 7.8, 8.2 and 8.8. This effect could be attributed to the binding of threonine and serine residues. The crystallographic results for parvalbumin, which has a similar molecular structure, suggest identical Ca2+ binding sites.

Amino Acid Sequence↗
PubMed1986

Conjugated alpha-keto acids as mechanism-based inactivators of brewer's yeast pyruvate decarboxylase: electronic effects of substituents and detection of a long-lived intermediate.

A series of phenyl substituted E-4-phenyl-2-keto-3-butenoic acid derivatives were synthesized (p-Cl, m-Cl, p-NO2, m-NO2, o-NO2, 3,4-Cl2, 2,6-Cl2, p-CH3O, p-(CH3)2N) and tested as potential irreversible inhibitors of brewer's yeast pyruvate decarboxylase (EC 4.1.1.1). All those derivatives with electron withdrawing substituents were found to be time-dependent inactivators of the enzyme, unlike the p-CH3O- and p-(CH3)2N derivatives. Detailed kinetic studies with the m-nitro derivative (the most potent inhibitor) indicated that this compound formed reversible complexes with the enzyme at two sites (supposed regulatory and catalytic with Ki values of 0.026 and 0.13 mM, respectively) prior to irreversible inactivation of the enzyme. In addition, concurrently with the inactivation, addition of the m-NO2 derivative to the enzyme produced a new VIS absorbance with lambda max near 430 nm. This absorbance was attributed to the enzyme-bound enamine intermediate. The time course of formation and disappearance of the intermediate could be determined and provided detailed information about the mechanism of the enzyme.

Carboxy-Lyases↗
PubMed1977

Solvent effects on thiamin-enzyme model interactions. I. Interactions with tryptophan.

The solvent polarity dependence of the interaction between thiamin and tryptophan was studied by spectrophotometric methods. The ultraviolet (UV) data clearly indicate that the interaction is weakened when the complex is transferred from water to aqueous ethanol or aqueous dioxane. The interaction of thiamin and tryptophan could also be detected by fluorescence-quenching studies (excitation of tryptophan at 287 nm, maximum emission at 348 nm). Appropriate treatment of the quenching data allowed dissection into static and dynamic contributions. A pyrimidine derivative related to thiamin, both in its neutral and protonated states, was shown to interact with tryptophan by fluorescence techniques, but not by UV. A thiazolium model was shown to interact with tryptophan by UV but was an inefficient quencher of the tryptophan fluorescence. Theoretical models are presented to explain the solvent dielectric constant dependence of the association constant between tryptophan and thiamin. Both electrostatic and dispersion forces are found to contribute to the stability of the complex.

Binding Sites↗