Identification of key amino acid residues in the catalytic mechanism of diaminopropionate ammonialyase from Salmonella typhimurium

Kalyani, Josyula N and Bisht, Shveta and Lakshmikanth, Mariyanna and Murthy, Mathur RN and Savithri, Handanahal S (2013) Identification of key amino acid residues in the catalytic mechanism of diaminopropionate ammonialyase from Salmonella typhimurium. In: FEBS JOURNAL, 280 (20). pp. 5039-5051.

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Abstract

Diaminopropionate ammonialyase (DAPAL), a fold-typeII pyridoxal 5-phosphate-dependent enzyme, catalyzes the ,-elimination of diaminopropionate (DAP) to pyruvate and ammonia. DAPAL was able to utilize both d- and l-DAP as substrates with almost equal efficiency. Mutational analysis of functionally important residues such as Thr385, Asp125 and Asp194 was carried out to understand the mechanism by which the isomers are hydrolyzed. Further, the putative residues involved in the formation of disulfide bond Cys271 and Cys299 were also mutated. T385S, T385D sDAPAL were as active with dl-DAP as substrate as sDAPAL, whereas the later exhibited a threefold increase in catalytic efficiency with d-Ser as substrate. Further analysis of these mutants suggested that DAPAL might follow an anti-E-2 mechanism of catalysis that does not involve the formation of a quinonoid intermediate. Of the two mutants of Asp125, D125E showed complete loss of activity with d-DAP as substrate, whereas the reaction with l-DAP was not affected significantly, demonstrating that Asp125 was essential for abstraction of protons from the d-isomer. By contrast, mutational analysis of Asp194 showed that the residue may not be directly involved in proton abstraction from l-DAP. sDAPAL does not form a disulfide bond in solution, although the position of Cys299 and Cys271 in the modeled structure of sDAPAL favored the formation of a disulfide bond. Further, unlike eDAPAL, sDAPAL could be activated by monovalent cations. Mutation of the cysteine residues showed that Cys271 may be involved in coordinating the monovalent cation, as observed in the case of other fold-typeII enzymes.

Item Type:	Journal Article
Publication:	FEBS JOURNAL
Publisher:	WILEY-BLACKWELL
Additional Information:	copyright for this article belongs to WILEY-BLACKWELL USA
Keywords:	absorbance; circular dichroism; pyridoxal 5-phosphate; site-directed mutagenesis; beta-elimination
Department/Centre:	Division of Biological Sciences > Biochemistry Division of Biological Sciences > Molecular Biophysics Unit
Date Deposited:	28 Dec 2013 06:11
Last Modified:	28 Dec 2013 06:11
URI:	http://eprints.iisc.ac.in/id/eprint/47979

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