Biosynthesis of cellular components

Transmethylation is important not only in the biosynthesis of cellular components but also in detoxification. Although the resulting metabolites are less toxic to the cell, they are often more lipophilic and may have serious adverse effects on other biota. Toxic volatile arsenic compounds... 

The main isotope-discriminating steps during biological carbon fixation are (1) the uptake and intracellular diffusion of CO2 and (2) the biosynthesis of cellular components. Such a two-step model was first proposed by Park and Epstein (1960) ... 

The overall rate of deterioration of a chemical will depend upon its molecular structure the physicochemical properties of a particular environment the type and quantity of microbes present and whether the metabolites produced can serve as sources of usable energy and precursors for the biosynthesis of cellular components, and hence the creation of more microorganisms. 

When a plant is treated with an herbicide, vital processes of that plant are affected, such as photosynthesis, cell wall formation or the biosynthesis of cellular components. This is reflected by changes in the transcriptome, the set of all mRNAs of a plant. The amount of mRNA of some genes increases, whereas the mRNA amount of other genes decreases. This snapshot of the transcriptional status of a plant is called the Gene Expression Profile (GEP). 

Like all microorganisms, lactic bacteria cells multiply when conditions are favorable presence of nutritional factors, absence of toxic factors, and adequate temperature. All of the principal reactions of its metabolism are directed towards the biosynthesis of cellular components nucleic acids for the transmission of genetic heritage, carbohydrates, lipids, structure proteins and of course biologically active proteins. To ensure these syntheses, the cell must first find the necessary chemical elements in the medium carbon, nitrogen and minerals—in usable forms. Since aU of these synthesis reactions are endergonic, the medium must... 

Exposure of cells to elevated temperatures, or heat shock, induces the transcription and translation of a set of proteins known as the heat-shock proteins (HSPs) or the stress proteins. This event usually occurs with the concomitant inhibition of biosynthesis of other cellular components. The response is believed to be an attempt by the cells to protect themselves from injury, and there is some evidence to indicate that it may also be linked to oxidative stress. Evidence in favour of this idea comes from observations such as the following ... 

Other cellular components actively synthesised by neutrophils include a 3.1-kb mRNA encoding a serine-rich protein. This transcript encodes a 32-kDa protein that copurifies with the cytochrome b in some preparations of the oxidase from pig neutrophils. The protein becomes phosphorylated during cell activation its identity, however, is unknown. The biosynthesis of cationic proteins with relative molecular masses of 37 and 57 kDa (identified by one-dimensional PAGE) has also been observed in GM-CSF-treated neutrophils. 

Cytotoxicity is the general term used to describe toxicity at the level of the cell. It can be brought about in many ways, usually by a chemical interaction between the toxic agent and one or more components of the cell. Interactions can be permanently damaging or may lead to temporary injury that the cell is capable of repairing. Perhaps the most important sites of intracellular injury are cell membranes, the cell nucleus (home of DNA), mitochondria (home of energy production), and endoplasmic reticulum (home of the biosynthesis of the all-important protein molecules, essential for cell structure and, as enzymes, for the catalysis of all cellular reactions and for the metabolism of foreign chemicals). 

The pathway of the biosynthesis of Neu5Ac demonstrates the origin of sialic acids from the cellular hexose and hexosamine pools. These sugars are, therefore, suitable components for the study of the biosynthesis of sialic acid. However, only ManNAc has been shown to be a relatively specific precursor of sialic acids, as may be seen from the distribution of radioactivity between the individual monosaccharides of glycoconjugates after incubation. Injections of radioactive ManNAc into animals, or incubation of surviving tissue slices or individual cells with this compound, give incorporation of label mainly into the sialic acids.226 227... 

The same intermediates of glycolysis and the citric acid cycle that activate isocitrate dehydrogenase are allosteric inhibitors of isocitrate lyase. When energy-yielding metabolism is sufficiently fast to keep the concentrations of glycolytic and citric acid cycle intermediates low, isocitrate dehydrogenase is inactivated, the inhibition of isocitrate lyase is relieved, and isocitrate flows into the glyoxylate pathway, to be used in the biosynthesis of carbohydrates, amino acids, and other cellular components. 

We begin with an account of the main steps in the biosynthesis of cholesterol from acetate, then discuss the transport of cholesterol in the blood, its uptake by cells, the normal regulation of cholesterol synthesis, and its regulation in those with defects in cholesterol uptake or transport. We next consider other cellular components derived from cholesterol, such as bile acids and steroid hormones. Finally, an outline of the biosynthetic pathways to some of the many compounds derived from isoprene units, which share early steps with the pathway to cholesterol, illustrates the extraordinary versatility of isoprenoid condensations in biosynthesis. 

Outline of pathways for the biosynthesis of major cellular lipids (other than cholesterol) in a mammalian cell. Most of the metabolism of these lipids occurs on membrane surfaces because of the insoluble nature of the substrates and products. These lipids play three major roles (l) they act as a storehouse of chemical energy, as with triacylglycerols (2) they are structural components of membranes (boxed compounds) and (3) they act as regulatory compounds (underlined), either as eicosanoids, which act as local hormones, or as phosphorylated inositols and diacylglycerols, which function as second messengers. 

Our studies of the bacterial enzymes utilized in peptidoglycan biosynthesis required an ample supply of both lipid I and lipid II. Isolation of these precursors from bacterial sources presented significant challenges. First, each of these intermediates is present in very low copy numbers. For example, in E. coli, the copy numbers for lipid I and lipid II are estimated to be approximately 700 and 1000-2000 molecules per cell, respectively.9 Second, separation of miniscule amounts of the cell wall precursors from the comparatively large amounts of cellular lipid and membrane components is technically challenging and further compromises the ability to isolate sufficient quantities of the enzyme substrates to enable detailed mechanistic studies.10... 

These compounds are well established as important intermediates in the biosynthesis of phosphoglycerides, such as phosphatidylcholine. The fact that they are formed on agonist stimulation of a cell suggested a role in the cell signaling process (Exton, 1990). In addition, over a period of many years, there have been reports on the interrelation of phosphatidic acid formation and cellular responses (Ca2+ movements to agonists, for example). Thus, these compounds have emerged as important components of cellular metabolism and merit serious attention in any metabolic investigation. 

Both IDL and LDL can be removed from the circulation by the liver, which contains receptors for ApoE (IDL) and ApoB-100 (IDL and LDL). After IDL or LDL interacts with these receptors, they are internalized by the process of receptor-mediated endocytosis. Receptors for ApoB-100 are also present in peripheral tissues, so that clearance of LDL occurs one-half by the liver and one-half by other tissues. In the liver or other cells, LDL is degraded to cholesterol esters and its other component parts. Cholesterol esters are hydrolyzed by an acid lipase and may be used for cellular needs, such as the building of plasma membranes or bile salt synthesis, or they may be stored as such. Esterification of intracellular cholesterol by fatty acids is carried out by acyl-CoA-cholesterol acyltransferase (ACAT). Free cholesterol derived from LDL inhibits the biosynthesis of endogenous cholesterol. B-100 receptors are regulated by endogenous cholesterol levels. The higher the latter, the fewer ApoB-100 receptors are on the cell surface, and the less LDL uptake by cells takes place. 

---