Section 10
Chapter 9,870

HlyC, the internal protein acyltransferase that activates hemolysin toxin: role of conserved histidine, serine, and cysteine residues in enzymatic activity as probed by chemical modification and site-directed mutagenesis

Trent, M.S.; Worsham, L.M.; Ernst-Fonberg, M.L.

Biochemistry 38(11): 3433-3439


ISSN/ISBN: 0006-2960
PMID: 10079090
DOI: 10.1021/bi982491u
Accession: 009869090

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HlyC is an internal protein acyltransferase that activates hemolysin, a toxic protein produced by pathogenic Escherichia coli. Acyl-acyl carrier protein (ACP) is the essential acyl donor. Separately subcloned, expressed, and purified prohemolysin A (proHlyA), HlyC, and [1-14C]myristoyl-ACP have been used to study the conversion of proHlyA to HlyA [Trent, M. S., Worsham, L. M., and Ernst-Fonberg, M. L. (1998) Biochemistry 37, 4644-4655]. HlyC and hemolysin belong to a family of at least 13 toxins produced by Gram-negative bacteria. The homologous acyltransferases of the family show a number of conserved residues that are possible candidates for participation in acyl transfer. Specific chemical reagents and site-directed mutagenesis showed that neither the single conserved cysteine nor the three conserved serine residues were required for enzyme activity. Treatment with the reversible histidine-modifying diethyl pyrocarbonate (DEPC) inhibited acyltransferase activity, and acyltransferase activity was restored following hydroxylamine treatment. The substrate myristoyl-ACP protected HlyC from DEPC inhibition. These findings and spectral absorbance changes suggested that histidine, particularly a histidine proximal to the substrate binding site, was essential for enzyme activity. Site-directed mutageneses of the single conserved histidine residue, His23, to alanine, cysteine, or serine resulted in each instance in complete inactivation of the enzyme.

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