F-ATPases (also known as ATP synthases, F1F0-ATPase, or H(+)-transporting two-sector ATPase) () are composed of two linked complexes: the F1 ATPase complex is the catalytic core and is composed of 5 subunits (alpha, beta, gamma, delta, epsilon), while the F0 ATPase complex is the membrane-embedded proton channel that is composed of at least 3 subunits (A-C), with additional subunits in mitochondria. Both the F1 and F0 complexes are rotary motors that are coupled back-to-back. In the F1 complex, the central gamma subunit forms the rotor inside the cylinder made of the alpha(3)beta(3) subunits, while in the F0 complex, the ring-shaped C subunits forms the rotor. The two rotors rotate in opposite directions, but the F0 rotor is usually stronger, using the force from the proton gradient to push the F1 rotor in reverse in order to drive ATP synthesis . These ATPases can also work in reverse in bacteria, hydrolysing ATP to create a proton gradient.
Most insects harbor a substantial microbiota, including bacteria, yeast, and protozoa. Microorganisms might be symbiotic or fortuitous contaminants from the external environment. They are found in the lumen, adhering to the peritrophic membrane, attached to the midgut surface, or within cells. Intracellular bacteria are usually found in special cells, mycetocytes, which may be organized in groups, mycetomes. Microorganisms produce and secrete their own hydrolases, and cell death will result in the release of enzymes into the intestinal milieu. Any consideration of the spectrum of hydrolase activity in the midgut must include the possibility that some of the activity may derive from microorganisms.
Membrane phospholipid synthesis is a vital facet of bacterial physiology. Although the spectrum of phospholipid headgroup structures produced by bacteria is large, the key precursor to all of these molecules is phosphatidic acid (PtdOH). Glycerol-3-phosphate derived from the glycolysis via glycerol-phosphate synthase is the universal source for the glycerol backbone of PtdOH. There are two distinct families of enzymes responsible for the acylation of the 1-position of glycerol-3-phosphate. The PlsB acyltransferase was discovered in Escherichia coli, and homologs are present in many eukaryotes. This protein family primarily uses acyl-acyl carrier protein (ACP) endproducts of fatty acid synthesis as acyl donors, but may also use acyl-CoA derived from exogenous fatty acids. The second protein family, PlsY, is more widely distributed in bacteria and utilizes the unique acyl donor, acyl-phosphate, which is produced from acyl-ACP by the enzyme PlsX. The acylation of the 2-position is carried out by members of the PlsC protein family. All PlsCs use acyl-ACP as the acyl donor, although the PlsCs of the γ-proteobacteria also may use acyl-CoA. Phospholipid headgroups are precursors in the biosynthesis of other membrane-associated molecules and the diacylglycerol product of these reactions is converted to PtdOH by one of two distinct families of lipid kinases. The central importance of the de novo and recycling pathways to PtdOH in cell physiology suggest these enzymes are suitable targets for the development of antibacterial therapeutics in Gram-positive pathogens. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.
Bacteria produce a bewildering variety of phospholipids that play critical roles in the adaptation to the environment. Phosphatidic acid (PtdOH) () is the universal precursor required for the production of these molecules. In contrast to the variety of enzymes involved in producing the broad spectrum of bacterial phospholipid structures, there are only a limited number of enzymes required for the formation of PtdOH, the key intermediate in their synthesis. This review covers the enzymes and pathways responsible for the de novo formation of PtdOH and the recycling enzymes that produce PtdOH from the diacylglycerol formed from the utilization of phospholipids in the biosynthesis of other molecules. In the early days of research on bacterial lipid metabolism, the field thought that understanding these pathways would provide important insight into how all cells construct their phospholipids. Indeed, the elucidation of the formation of sn-glycerol-3-phosphate (G3P) and the consecutive acylation of the 1-position followed by the 2-position of G3P in E. coli identified enzymes and genes that have homologous sequences and functions in mammalian systems. More recently, it has become apparent that a primary bacterial enzyme for the acylation of the 1-position of G3P in many human pathogens has no mammalian homologs and uses a different acyl donor that other acyltransferases in biology. This review covers the current knowledge and future directions for research on these two primary acyltransferase pathways to PtdOH and the kinases involved in PtdOH metabolism in bacteria.
The earliest cells, prokaryotes living in an early Earth devoid of free oxygen, used various alternative electron acceptors to carry on anaerobic cellular respiration. After cyanobacteria invented oxygenic photosynthesis and pumped oxygen gas into the oceans and atmosphere, bacteria that adapted their electron transport chains to exploit oxygen as the terminal electron acceptor gained higher energy yield and thus a competitive advantage. One line of aerobic bacteria took up an endosymbiotic relationship within a larger host cell, providing ATP in exchange for organic molecules. The endosymbiont was the evolutionary ancestor of mitochondria. This endosymbiosis must have occurred in the ancestor of all eukaryotes, because all existing eukaryotes have mitochondria (Martin and Mentel, 2010). The evidence for the endosymbiont origin of mitochondria can be found in:
Despite the fact that digestive enzymes of some insects are thought to be derived from the microbiota, there are relatively few studies that show an unambiguous contribution of microbial hydrolases. Best examples are found among wood- and humus-feeding insects like termites, tipulid fly larvae, and scarabid beetle larvae. Although these insects may have their own cellulases (see section 11.4.3), only fungi and certain filamentous bacteria developed a strategy for the chemical breakdown of lignin. Lignin is a phenolic polymer that forms an amorphous resin in which the polysac-charides of the secondary plant cell wall are embedded, thus becoming protected from enzymatic attack (Terra et al., 1996; Brune, 1998; Dillon and Dillon, 2004).