Acyltransferases in bacteria

Abstract

Long-chain-length hydrophobic acyl residues play a vital role in a multitude of essential biological structures and processes. They build the inner hydrophobic layers of biological membranes, are converted to intracellular storage compounds, and are used to modify protein properties or function as membrane anchors, to name only a few functions. Acyl thioesters are transferred by acyltransferases or transacylases to a variety of different substrates or are polymerized to lipophilic storage compounds. Lipases represent another important enzyme class dealing with fatty acyl chains; however, they cannot be regarded as acyltransferases in the strict sense. This review provides a detailed survey of the wide spectrum of bacterial acyltransferases and compares different enzyme families in regard to their catalytic mechanisms. On the basis of their studied or assumed mechanisms, most of the acyl-transferring enzymes can be divided into two groups. The majority of enzymes discussed in this review employ a conserved acyltransferase motif with an invariant histidine residue, followed by an acidic amino acid residue, and their catalytic mechanism is characterized by a noncovalent transition state. In contrast to that, lipases rely on completely different mechanism which employs a catalytic triad and functions via the formation of covalent intermediates. This is, for example, similar to the mechanism which has been suggested for polyester synthases. Consequently, although the presented enzyme types neither share homology nor have a common three-dimensional structure, and although they deal with greatly varying molecule structures, this variety is not reflected in their mechanisms, all of which rely on a catalytically active histidine residue.

Fig 1

Chemical structures of common lipophilic storage compounds in prokaryotes: poly(3-hydroxyalkanoate) (PHA), triacylglycerol (TAG), and wax ester (WE). R₁, alkyl chain with a length ranging from C₁ to C₁₃; R₂, saturated or unsaturated long-chain-length alkyl residue.

Fig 2

Different families of acyltransferases involved in TAG and/or WE synthesis in eukaryotes and prokaryotes.

Fig 3

Synthesis of wax esters or triacylglycerols from acyl-CoA and fatty alcohol or diacylglycerol, respectively, catalyzed by AtfA.

Fig 4

Mechanism of wax ester synthesis from acyl-CoA thioester and fatty alcohol catalyzed by AtfA from A. baylyi strain ADP1. (Based on data from reference .)

Fig 5

Multiple-sequence alignment of AtfA-like proteins from 17 different organisms (for details, see Table 1). Predicted secondary structural motifs of AtfA are schematically displayed above the AtfA sequence, boxes represent putative α helices, and arrows represent β strands. The active-site motif is marked with a red box.

Fig 6

Biosynthesis of (hybrid) isoprenoid wax esters (WE) from phytol and phytanoyl-CoA or acyl-CoA, as catalyzed by WS2 from M. hydrocarbonoclasticus.

Fig 7

First two acylation steps in membrane lipid and triacylglycerol synthesis from glycerol-3-phosphate to the central intermediate phosphatidate (Kennedy pathway).

Fig 8

Overview of pathways for membrane phospholipid and triacylglycerol biosynthesis in bacteria. G3P, glycerol-3-phosphate; FA_ex, exogenous fatty acids; LPA, lysophosphatidic acid; PA, phosphatidic acid; DAG, diacylglycerol; TAG, triacylglycerol.

Fig 9

Lipid A biosynthesis pathway of Escherichia coli (details are described in the text).

Fig 10

Chemical structures of the polyketide-containing complex lipids PDIM (A) and PGL (B) synthesized by PapA5 via esterification of two mycocerosyl residues (green) with both hydroxyl groups of the (phenol)phthiocerol backbone (black).

Fig 11

Chemical structures of trehalose-based polyketide-containing glycolipids synthesized by mycobacteria. R, complex and variable glycan moiety.

Fig 12

Biosynthesis of rifamycin B, a polyketide antibiotic produced by Amycolatopsis mediterranei. The acetyl moiety (marked in red) is attached by the PapA5 homolog Rif-Orf20.

Fig 13

Acetylation of chloramphenicol catalyzed by chloramphenicol acetyltransferase (CAT) by transfer of the acetyl group from acetyl-CoA to the primary hydroxyl group of chloramphenicol, forming 3-acetylchloramphenicol.

Fig 14

Catalytic mechanism of HlyC acyltransferase. Details are described in the text. HD, hydrophobic region; RTX, repeats-in-toxin.

Fig 15

Hydrolysis and (trans)esterification reactions catalyzed by lipases.

Fig 16

Catalytic mechanism of lipases. A detailed description is provided in the text. (Based on data from reference .)

Fig 17

Proposed catalytic mechanism of PHA synthases. A detailed description is provided in the text. (Based on data from references and .)

Fig 18

Metabolic link between de novo fatty acid synthesis and PHA synthesis in pseudomonads. (Based on data from reference .)

Fig 19

Conversion of 3-hydroxydecanoyl-ACP to 3-hydroxydecanoyl-CoA catalyzed by PhaG.

Fig 20

Multiple-sequence alignment of different PhaG proteins (generated with ClustalW). Amino acid residues identical to those of the PhaG_Pp reference protein from Pseudomonas putida are displayed as dots. PhaG protein sequences are derived from the following bacteria: Pp, P. putida; Pn, P. nitroreducens; Pm, P. mendocina; Po, P. oleovorans; Bc, Burkholderia caryophylli; Ps, Pseudomonas sp. 61-3; Pf, P. fluorescens; Pa, P. aeruginosa (for details, see Table 6).

Fig 21

Synthesis of rhamnolipids from 3-hydroxydecanoyl-ACP derived from de novo fatty acid synthesis.