Bacterial Triacylglycerols

ALVAREZ, HÉCTOR M

Triglycerides and Cholesterol Research

Nova Science Publishers, Inc.

Lugar: New York; Año: 2006; p. 159 - 176

Triacylglycerols (TAG) are non-polar, water-insoluble fatty acid triesters of glycerol. They are found in most eukaryotic organisms, including animals, plants, yeast and fungi. In contrast, the accumulation of TAG is an unusual property for bacteria, although these lipids seem to be widespread among bacteria belonging to the actinomycetes group such as Mycobacterium, Rhodococcus, Nocardia or Streptomyces, and in cells of Acinetobacter species. Only recently, some important new knowledge has emerged from research on bacterial TAG, mainly using Rhodococcus opacus PD630 and Acinetobacter calcoaceticus ADP1 as model prokaryotic cells. Bacterial TAG differ from eukaryotic TAG according to the identity and placement of their three fatty acid residues and the enormous variability of fatty acid composition depending on the carbon source used for the cultivation of cells. In addition, the properties of the stored lipids may be changed by metabolic engineering. Molecular data on the TAG-biosynthetic genes are now available for Acinetobacter calcoaceticus ADP1 and Mycobacterium tuberculosis, whereas the characterization of TAG-biosynthetic genes of Rhodococcus opacus PD630 is in course. So far, all bacterial enzymes involved in the final acylation step of TAG biosynthesis belong to a novel type of long-chain acyl-CoA acyltransferase, which are not related to known acyltransferases present in yeast, plants and animals. Bacterial enzymes exhibit at the same time wax ester synthase and acyl-CoA diacylglycerol acyltransferase activities. The bifunctional enzyme of Acinetobacter calcoaceticus ADP1 possesses a remarkably low substrate specificity, which may lead to the production of biotechnological relevant lipids. TAG do not serve solely as an inert lipid depot, but instead may play several key roles in the metabolism of lipid-accumulating bacteria. These storage lipids have important functions in such microorganisms, including: (1) they may play a role in regulating the fatty acid composition of membrane lipids, (2) as a sink for reducing equivalents in cells under limiting-oxygen conditions, (3) as precursor source for mycolic acid biosynthesis during adaptation of cells to environmental stresses, (4) as carbon source for the biosynthesis of antibiotics, (5) as metabolic water reservoir, among other physiological functions. This article aims to summarize the most relevant achievements of basic research in this field, including the most recent knowledge emerged from studies on bacterial TAG and some unpublished results. The article will include the properties of bacterial TAG; the involved biosynthesis pathways and the conditions for their accumulation; the TAG-biosynthetic genes and enzymes; their physiological functions as wells as their potential biotechnological applications.