Regulation and Control of Metabolism in Bacteria Yunika S 10406024

Bacterial Adaptation to the Nutritional and Physical Environment Within limits, bacteria can react to changes in their environment through changes in patterns of structural proteins, transport proteins, toxins, enzymes, etc., which adapt them to a particular ecological situation. Bacteria have developed sophisticated mechanisms for the regulation of both catabolic and anabolic pathways. Generally, bacteria do not synthesize degradative (catabolic) enzymes unless the substrates for these enzymes are present in their environment. Similarly, bacteria have developed diverse mechanisms for the control of biosynthetic (anabolic) pathways. Bacterial cells shut down biosynthetic pathways when the end product of the pathway is not needed or is readily obtained by uptake from the environment.

Conditions Affecting Enzyme Formation in Bacteria Constitutive enzymes are always produced by cells independently of the composition of the medium in which the cells are grown. Inducible enzymes are produced ("turned on") in cells in response to a particular substrate; they are produced only when needed. The substrate, or a compound structurally similar to the substrate, evokes formation of the enzyme and is sometimes called an inducer. A repressible enzyme is one whose synthesis is downregulated or "turned off" by the presence of (for example) the end product of a pathway that the enzyme normally participates in. In this case, the end product is called a corepressor of the enzyme.

Regulation of Enzyme Reactions In bacterial cells, enzymatic reactions may be regulated by two unrelated modes: control or regulation of enzyme activity (feedback inhibition or end product inhibition) control or regulation of enzyme synthesis, including end-product repression, which functions in the regulation of biosynthetic pathways, and enzyme induction and catabolite repression, which regulate mainly degradative pathways. The processes which regulate the synthesis of enzymes may be either a form of positive control or negative control.

Regulation of Enzyme Reactions (Cont.) End-product repression and enzyme induction are mechanisms of negative control  because they lead to a decrease in the rate of transcription of proteins. Catabolite repression is considered a form of positive control because it affects an increase in rates of transcription of proteins.

Regulation of Enzyme Reactions (Cont.)

Allosteric Proteins The regulatory proteins that control metabolic pathways involving end product repression, enzyme induction and catabolite repression are allosteric proteins. An allosteric protein has an active (catalytic) site and an allosteric (effector) site. The active site binds to the substrate of the enzyme and converts it to a product. The allosteric site is occupied by some small molecule, called allosteric or effector molecule, which is not a substrate, and can affect the active site.

Allosteric Proteins (cont.)

Allosteric Proteins (cont.) In the case of enzyme repression, a positive effector molecule (called a corepressor) binds to the allosteric regulatory protein and activates its ability to bind to DNA. In the case of enzyme induction a negative effector molecule (called an inducer) binds to the allosteric site, causing the active site to change conformation thereby detaching the protein from its DNA binding site.

Feedback Inhibition In feedback inhibition (or end product inhibition), the end product of a biosynthetic pathway inhibits the activity of the first enzyme that is unique to the pathway, thus controlling production of the end product. The first enzyme in the pathway is an allosteric enzyme. Its allosteric site will bind to the end product of the pathway which alters its active site so that it cannot mediate the enzymatic reaction which initiates the pathway.

The pathway of tryptophan biosynthesis in E. coli. The signal molecule, tryptophan, is a negative effector of Enzyme a in the pathway of tryptophan biosynthesis, because when it binds to Enzyme a, it inactivates the enzyme. The pathway of proline and arginine biosynthesis

Enzyme Repression Enzyme repression is a form of negative control (down-regulation) of bacterial transcription. This process, along with that of enzyme induction, is called negative control because a regulatory protein brings about inhibition of mRNA synthesis which leads to decreased synthesis of enzymes. The genes for tryptophan biosynthesis in Escherichia coli are organized on the bacterial chromosome in the tryptophan operon (trp operon).

Genetic organization of the Trp operon and its control elements.

Repression of the trp operon Repression of the trp operon. In the presence of tryptophan the trp operon is repressed because trp activates the repressor. Transcription is blocked because the active repressor binds to the DNA and prevents binding of RNA polymerase.

Derepression of the trp operon Derepression of the trp operon. In the absence of trp the inactive repressor cannot bind to the operator to block transcription. The cell must synthesize the amino acid.

Enzyme Induction In the process of enzyme induction, the substrate, or a compound structurally similar to the substrate, evokes the formation of enzyme(s) which are usually involved in the degradation of the substrate. Enzymes that are synthesized as a result of genes being turned on are called inducible enzymes and the substance that activates gene transcription is called the inducer.

The Lac operon and its control elements

Enzyme Induction. Induction (or derepression) of the lac operon.

Catabolite Repression Catabolite repression is a type of positive control of transcription, since a regulatory protein affects an increase (upregulation) in the rate of transcription of an operon. The process was discovered in E. coli and was originally referred to as the glucose effect. because it was found that glucose repressed the synthesis of certain inducible enzymes, even though the inducer of the pathway was present in the environment.

The Diauxic Growth Curve of E The Diauxic Growth Curve of E. coli grown in limiting concentrations of a mixture of glucose and lactose Glucose is always metabolized first in preference to other sugars. Only after glucose is completely utilized is lactose degraded. The lactose operon is repressed even though lactose (the inducer) is present. The secondary lag during diauxic growth represents the time required for the complete induction of the lac operon and synthesis of the enzymes necessary for lactose utilization (lactose permease and beta-galactosidase).

Catabolite Repression (cont.) Glucose represses the induction of inducible operons by inhibiting the synthesis of cyclic AMP (cAMP), a nucleotide that is required for the initiation of transcription of a large number of inducible enzyme systems including the lac operon. In the presence of glucose, adenylate cyclase (AC) activity is blocked. AC is required to synthesize cAMP from ATP. Therefore, if cAMP levels are low, CAP is inactive and transcription does not occur. In the absence of glucose, cAMP levels are high, CAP is activated by cAMP, and transcription occurs (in the presence of lactose).

cAMP is required to activate an allosteric protein called CAP (catabolite activator protein) which binds to the promoter CAP site and stimulates the binding of RNAp polymerase to the promoter for the initiation of transcription

Catabolite repression is positive control of the lac operon Catabolite repression is positive control of the lac operon. The effect is an increase in the rate of transcription. In this case, the CAP protein is activated by cAMP to bind to the lac operon and facilitate the binding of RNA polymerase to the promoter to transcribe the genes for lactose utilization.

Terima Kasih