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Steady-state kinetic mechanism of bovine brain tubulin: tyrosine ligase.

ABSTRACT: The ATP-dependent resynthesis of tubulin from tyrosine and untyrosinated tubulin was examined to establish the most probable steady-state kinetic mechanism of the tubulin: tyrosine ligase (ADP-forming). Three pair-wise sets of initial rate experiments, involving variation of two substrates pair-wise with the third substrate held at a high (but non-saturating) level, yielded convergent-line data, a behaviour that is diagnostic for sequential mechanisms. Michaelis constants were 14 microM, 1.9 microM and 17 microM for ATP, untyrosinated tubulin and L-tyrosine respectively, and the maximal velocity was 0.2 microM/min. AMP was a competitive inhibitor with respect to ATP, and a non-competitive inhibitor versus either tubulin or tyrosine. Likewise, L-dihydroxyphenylalanine acted competitively relative to tyrosine and non-competitively with respect to either ATP or tubulin. These findings directly support a random sequential mechanism. Product inhibition patterns with ADP were also consistent with this assignment; however, inhibition studies were not practical with either orthophosphate or tyrosinated tubulin because both were very weak inhibitors. Substrate protection of the enzyme against alkylation by N-ethylmaleimide and thermal inactivation, along with evidence of enzyme binding to ATP-Sepharose and tubulin-Sepharose, also supports the idea that this three-substrate enzyme reaction exhibits a random substrate addition pathway.

SUBMITTER: Deans NL

PROVIDER: S-EPMC1133046 | biostudies-other | 1992 Aug

REPOSITORIES: biostudies-other

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Project description:Nicotinamidases are metabolic enzymes that hydrolyze nicotinamide to nicotinic acid. These enzymes are widely distributed across biology, with examples found encoded in the genomes of Mycobacteria, Archaea, Eubacteria, Protozoa, yeast, and invertebrates, but there are none found in mammals. Although recent structural work has improved our understanding of these enzymes, their catalytic mechanism is still not well understood. Recent data show that nicotinamidases are required for the growth and virulence of several pathogenic microbes. The enzymes of Saccharomyces cerevisiae, Drosophila melanogaster, and Caenorhabditis elegans regulate life span in their respective organisms, consistent with proposed roles in the regulation of NAD(+) metabolism and organismal aging. In this work, the steady state kinetic parameters of nicotinamidase enzymes from C. elegans, Sa. cerevisiae, Streptococcus pneumoniae (a pathogen responsible for human pneumonia), Borrelia burgdorferi (the pathogen that causes Lyme disease), and Plasmodium falciparum (responsible for most human malaria) are reported. Nicotinamidases are generally efficient catalysts with steady state k(cat) values typically exceeding 1 s(-1). The K(m) values for nicotinamide are low and in the range of 2 -110 ?M. Nicotinaldehyde was determined to be a potent competitive inhibitor of these enzymes, binding in the low micromolar to low nanomolar range for all nicotinamidases tested. A variety of nicotinaldehyde derivatives were synthesized and evaluated as inhibitors in kinetic assays. Inhibitions are consistent with reaction of the universally conserved catalytic Cys on each enzyme with the aldehyde carbonyl carbon to form a thiohemiacetal complex that is stabilized by a conserved oxyanion hole. The S. pneumoniae nicotinamidase can catalyze exchange of (18)O into the carboxy oxygens of nicotinic acid with H(2)(18)O. The collected data, along with kinetic analysis of several mutants, allowed us to propose a catalytic mechanism that explains nicotinamidase and nicotinic acid (18)O exchange chemistry for the S. pneumoniae enzyme involving key catalytic residues, a catalytic transition metal ion, and the intermediacy of a thioester intermediate.

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Steady-state kinetic mechanism of bovine brain tubulin: tyrosine ligase.

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