The intracellular biosynthetic pathway is usually composed of a series of enzymatic reactions that produce a variety of intermediates before the end product is obtained. As shown in Figure 1, the substrate is first changed to intermediate A by the action of enzyme E1, and then other intermediates are formed by the action of a series of other enzymes to finally obtain the end product. When the end product concentration exceeds a certain level, it will have a feedback regulation effect on the metabolic pathway (including feedback inhibition and feedback blocking). Feedback inhibition inhibits the activity of key enzymes in the metabolic pathway to reduce the production of end products. Inhibitory effects inhibit the biosynthesis of a range of enzymes in the metabolic pathway, including key enzymes, thereby inhibiting end-product synthesis more completely.
In a hyperosmotic environment, the most rapid response is an increase in the influx of potassium ions and the main anionic compound involved in osmoregulation is glutamate. There are compounds that, when present in the environment, promote the growth rate of cells in a hyperosmotic environment, called osmoprotectants, which are amphoteric in nature and are similar to glycine betaine and proline.
Hypotonic environments temporarily increase the permeability of cell membranes and the growth of cells under hypotonic conditions induces the production of some compounds and their distribution in the periplasmic space of cells. In addition, hypertonic and hypotonic environments induce cells to express some outer membrane proteins to control the entry and exit of substances.
Most aerobic microorganisms can protect themselves from damage by superoxide and hydrogen peroxide through superoxide dismutase (SOD) and hydrogen peroxidase. Organisms that do not respire aerobically can use a unique flavoprotein-NADH oxidase to reduce oxygen to water. In addition, hydrogen peroxide and superoxide can induce cells to produce enzymes that can undo the oxidative damage that occurs during oxygen stress.
One way that microorganisms respond to pH changes is by producing enzymes that convert acidic metabolites to neutral metabolites or neutral metabolites to basic metabolites. The typical mechanisms employed to control pH in Gram-negative bacteria during growth have been shown to involve the regulation of major proton pumps as well as potassium/sodium and potassium/sodium transport channels. Other adaptive mechanisms are also employed to survive pH conditions beyond the growth range. In addition, some microorganisms can establish an acid-tolerant system by exocytosis of protons through an ATP hydrolase that transports protons.
Some of the physiological changes that occur during the starvation stress response include the collection of carbon sources or other new or higher affinity nutrients from the environment for systemic expression; degradation of intracellular RNA, proteins and fatty acids; reduction in the number of ribosomes; changes in the number and type of lipid components of the cell membrane; and coalescence of chromosomal DNA that occurs to protect DNA from damage.
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