Expression, purification and crystallization of the Vibrio harveyi acyltransferase

Hyperactivity and interactions of a chimeric myristoryl-ACP thioesterase from the lux system of luminescent bacteria

A chimeric myristoyl-ACP thioesterase with much higher catalytic efficiency than the parental enzymes has been generated by ligating the N-terminal half of the lux-specific thioesterase (LuxD) from Photobacterium phosphoreum with the C-terminal half of LuxD from Vibrio harveyi. The LuxD chimera had the same rate-limiting step and specificity, but cleaved esters and thioesters over eight times faster than the native enzymes. LuxD, along with acyl-protein synthetase (LuxE) and reductase (LuxC), comprise a multienzyme complex channeling activated fatty acids into the aldehyde substrate for the bacterial bioluminescence reaction. As P. phosphoreum LuxD and LuxE modulate each of their respective activities, the effects of mixing V. harveyi and the chimeric LuxD with P. phosphoreum LuxE were investigated. The chimeric LuxD stimulated acylation of LuxE to the same extent as V. harveyi LuxD, but to a lower level than that caused by P. phosphoreum LuxD. Conversely, P. phosphoreum LuxE stimulated the thioesterase activity of V. harveyi LuxD by 30% and the chimeric LuxD by 20% while the activity of P. phosphoreum LuxD was increased by over 140%. These results show that the stimulatory effects are unrelated to the level of thioesterase activity and indicate that the carboxyl terminal region of LuxD interacts with LuxE and causes a conformational change.

Tryptophan fluorescence of the lux-specific Vibrio harveyi acyl-ACP thioesterase and its tryptophan mutants: structural properties and ligand-induced conformational change

The lux-specific myristoyl-ACP thioesterase from Vibrio harveyi contains four tryptophan residues, Trp23, Trp99, Trp186, and Trp213. Replacement of each of these residues with tyrosine by site-directed mutagenesis coupled with fluorescence and quenching studies of the purified mutant and wild type thioesterases during catalysis has been used to probe ligand-induced conformational changes. Mutant W99Y retained high enzyme activity (80%) with W213Y and W23Y retaining intermediate activity and W186Y having the lowest activity (20%). The sum of the differential fluorescence spectra of the individual tryptophans was identical to the fluorescence spectrum of the wild type thioesterase, showing that mutation had not caused a major conformational change and energy transfer did not occur between the tryptophans. Fluorescence emission maxima and quenching by acrylamide revealed that Trp213 and Trp23 are in a polar environment and/or exposed to solvent while Trp186 appeared to be buried inside the molecule, consistent with the crystal structure of the thioesterase. The fluorescence intensities of the wild type, W23Y, W99Y, and W186Y thioesterases increased in direct correlation to their degree of acylation with myristoyl-CoA, while the fluorescence of the acylated W213Y mutant remained constant, showing that the enhancement of fluorescence was entirely due to interaction of the acyl group with Trp213. Acrylamide quenching of the acylated mutants showed that the accessibility of the tryptophans to solvent was differentially altered and that the quenching of W23Y was enhanced in contrast to the quenching of the other mutants, supporting a ligand-induced conformational change during enzyme turnover.

Conversion of serine-114 to cysteine-114 and the role of the active site nucleophile in acyl transfer by myristoyl-ACP thioesterase from Vibrio harveyi

The lux-specific myristoyl-ACP thioesterase (LuxD) is responsible for diverting myristic acid into the luminescent system and can function as an esterase and transferase as well as cleave myristoyl-CoA and other thioesters. The recently elucidated crystal structure of the enzyme shows that it belongs to the alpha/beta hydrolase family and that it contains a typical catalytic triad composed of Asp211, His241, and Ser114. What is unusual is that the nucleophilic S114 is not contained within the esterase consensus motif GXSXG although the stereochemistry of the turn involving S114 is almost identical to the nucleophilic elbow found in alpha/beta hydrolases. In contrast to mammalian thioesterases, deacylation of LuxD was the rate-limiting step, with the level of acylated enzyme formed on reaction with myristoyl-CoA and the pre-steady-state burst of p-nitrophenol on cleavage of p-nitrophenyl myristate both being 0.7 mol/mol. Cold chase experiments showed that the deacylation rate of LuxD corresponded closely to the turnover rate of the enzyme with ester or thioester substrates. Replacement of S114 by a cysteine residue generated a mutant (S114C) that was acylated with the same pH dependence as LuxD but had greatly diminished capacity to transfer acyl groups to water or glycerol. The acyl group could be removed from the S114C mutant by neutral hydroxylamine, showing that a cysteine residue had been acylated. Mutation of H241 creating the double mutant, S114C.H241N, decreased acylation of the cysteine residue. These results provide direct kinetic and chemical evidence that S114 is the site of acylation linked to H241 in the charge relay system and have led to the recognition of a more general consensus motif flanking the nucleophilic serine in thioesterases.