The biosynthesis of wax esters and triglycerides in bacteria is accomplished through the action of the wax ester synthase/acyl-coenzyme A:diacylglycerol acyltransferase (WS/ DGAT or wax ester synthase). A hallmark of these enzymes is the broad substrate profile that accepts alcohols, diglycerides, and fatty acyl-CoAs of various carbon chain lengths and degrees of branching. These enzymes have a broad biotechnological potential due to their role in producing high-value lipids or simple fuels similar to biodiesel through biosynthetic routes. Recently, a crystal structure was solved for the wax ester synthase from Marinobacter aquaeolei VT8 (Maqu_0168), providing a much clearer picture of the architecture of this enzyme and enabling a more precise analysis of the important structural features of the protein. In this work, we used the structure to canvas amino acids lining the proposed substrate-binding pockets and tested the effects of exchanging specific residues on the substrate profiles. We also developed an approach to better probe the residues that alter fatty acyl-CoA selectivity, which has proven more difficult to investigate. Our findings provide an improved blueprint for future efforts to understand how these enzymes position substrates for catalysis and to tailor or improve these enzymes in future biosynthetic schemes.

Enzymes involved in lipid biosynthesis and metabolism play an important role in energy conversion and storage and in the function of structural components such as cell membranes. The fatty aldehyde dehydrogenase (FAldDH) plays a central function in the metabolism of lipid intermediates, oxidizing fatty aldehydes to the corresponding fatty acid and competing with pathways that would further reduce the fatty aldehydes to fatty alcohols or require the fatty aldehydes to produce alkanes. In this report, the genes for four putative FAldDH enzymes from Marinobacter aquaeolei VT8 and an additional enzyme from Acinetobacter baylyi were heterologously expressed in Escherichia coli and shown to display FAldDH activity. Five enzymes (Maqu_0438, Maqu_3316, Maqu_3410, Maqu_3572, and the enzyme reported under RefSeq accession no. WP_004927398) were found to act on aldehydes ranging from acetaldehyde to hexadecanal and also acted on the unsaturated long-chain palmitoleyl and oleyl aldehydes. A comparison of the specificities of these enzymes with various aldehydes is presented. Crystallization trials yielded diffraction-quality crystals of one particular FAldDH (Maqu_3316) from M. aquaeolei VT8. Crystals were independently treated with both the NAD ϩ cofactor and the aldehyde substrate decanal, revealing specific details of the likely substrate binding pocket for this class of enzymes. A likely model for how catalysis by the enzyme is accomplished is also provided.IMPORTANCE This study provides a comparison of multiple enzymes with the ability to oxidize fatty aldehydes to fatty acids and provides a likely picture of how the fatty aldehyde and NAD ϩ are bound to the enzyme to facilitate catalysis. Based on the information obtained from this structural analysis and comparisons of specificities for the five enzymes that were characterized, correlations to the potential roles played by specific residues within the structure may be drawn.KEYWORDS Marinobacter, Maqu_3316, decanal, wax ester, lipid biosynthesis T he fates of fatty compounds within the cell are a central aspect of lipid metabolism. Enzymes involved in the metabolism of fatty compounds can play important roles in disease and have a biotechnological relevance for both the production of lipids or biofuels and the degradation of oils that are released into the environment (1). The bacterium Marinobacter aquaeolei VT8 was isolated from the head of an offshore oil well near Vietnam, where it would be expected to be participating in the biodegrada-

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