Prescott's Microbiology,4/e

		Microbiology, 4/e Prescott, Harley, Klein
		Instructor Resources

12 Metabolism: The Regulation of Enzyme Activity and Synthesis

CHAPTER OVERVIEW

This chapter introduces the principles of metabolic regulation to demonstrate how the various pathways discussed in previous chapters are coordinated. The chapter begins with a discussion of the need for regulation and then describes three mechanisms by which organisms regulate metabolic activities: metabolic channeling, regulation of enzyme activity, and regulation of enzyme synthesis.

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

o discuss the need for metabolic regulation to maintain cell components at the proper levels and to conserve materials and energy

o discuss regulation by metabolic channeling (localization) of enzymes and metabolites

o discuss regulation of enzyme activity by reversible binding of effector molecules or by covalent modification of the enzyme

o discuss regulation of enzyme activity by feedback (end product) inhibition

o discuss regulation of enzyme synthesis by induction and repression of the genes coding for a set of related enzymes

o discuss regulation of gene expression by sigma factors

o discuss the coordination of DNA replication and cell division

CHAPTER OUTLINE

I. Introduction

A. Metabolic regulation is necessary to maintain cell components at appropriate levels

B. Metabolic regulation is necessary to conserve materials and energy

1. If a particular energy source is not available, the enzymes required for its utilization are unnecessary, and synthesis of these enzymes wastes carbon, nitrogen, and energy that could be better used for other necessary activities

2. If a particular end product were already abundantly present in the environment, it would be wasteful to continue producing enzymes that only manufacture more of that end product

C. Mechanisms of metabolic regulation

1. Metabolic channeling

2. Adjustment of enzyme activity

3. Regulation of gene expression (mRNA synthesis)

II. Metabolic Channeling

A. Compartmentation

1. The phenomenon in which various processes are allocated to separate cell structures or organelles

2. Enables simultaneous but separate operation and regulation of similar pathways

3. Particularly important in eucaryotic cells, which have many separate membrane-bounded organelles

B. Channeling also occurs within any given compartment because enzymes and their metabolites have finite diffusion rates

C. Substrate concentrations are often below saturation levels and, therefore, have a profound effect on metabolic activity

III. Control of Enzyme Activity

A. Allosteric regulation

1. Effector (modulator) molecules bind reversibly and noncovalently to regulatory sites that are separate from the catalytic site

2. Positive effectors increase enzyme activity, while negative effectors decrease activity

3. In some cases, regulatory sites may even reside on polypeptide chains separate from the catalytic site(s)

B. Covalent modification of enzymes

1. Some enzymes can be regulated by reversible covalent attachment of a particular chemical group

2. In this way, enzymes can alternate between two forms, one with the chemical group attached and one without the chemical group attached

3. One form usually exhibits a much higher activity than the other form

C. Feedback (end product) inhibition

1. The first committed step in a metabolic pathway is often catalyzed by a pacemaker enzyme that is regulated by the end product of the pathway

2. This insures balanced production of a pathway end product

a. If the end product becomes too concentrated, it inhibits the regulatory enzyme and slows its own synthesis

b. As the end product concentration decreases, metabolic activity again increases and more product is formed

3. In branched pathways, the end products of each branch should only inhibit the activity of that particular branch; abundance of the end products of all the branches that act in concert with each other should also inhibit the flow of carbon into the whole set of pathways

4. The regulation of multiply branched pathways often involves isoenzymes to catalyze the pacemaker step; these isoenzymes are different enzymes that catalyze the same reaction and each is under separate and independent control; in this situation, an excess of a single end product reduces but does not completely block pathway activity because some isoenzymes are still active

IV. Regulation of mRNA Synthesis

A. Regulation at the level of mRNA (and thereby enzyme) synthesis provides a long-term regulatory mechanism that can respond to major changes in environmental conditions

B. This type of regulation is even more conservative of materials and energy than those mechanisms already discussed, but the response to changing conditions is not as rapid

C. Regulation by Sigma Factors

1. Sigma factors enable the RNA polymerase to recognize and bind to promoters

2. Different sigma factors recognize different sets of promoters

3. Substitution of the sigma factors immediately changes gene expression

4. This has been demonstrated in a number of systems including heat shock response in E. coli and sporulation in B. subtilis

D. Induction and repression

1. Enzymes involved in catabolic pathways are inducible, and the initial substrate of the pathway (or some derivative of it) is usually the inducer

2. Enzymes involved in some anabolic pathways are repressible and the end product of the pathway usually acts as a corepressor

3. Enzymes involved in other anabolic pathways may be regulated by a process known as attenuation

E. The mechanism of induction and repression

1. The rate of mRNA synthesis is controlled by repressor proteins

2. Repressor proteins bind to specific sites on the DNA called operators

3. When bound to the operator, the repressor protein overlaps the promoter region (adjacent to the operator), and thereby prevents RNA polymerase from attaching to the promoter, stopping transcription of the genes downstream of that promoter

4. Repressors must exist in active and inactive forms

a. In inducible systems, the repressor protein is active until bound to the inducer, which renders it inactive

b. In repressible systems, the repressor is inactive until bound to the corepressor, which renders it active

5. The set of structural genes controlled by a particular operator together with the associated operator and promoter is called an operon

F. Positive operon control and catabolite repression

1. Positive control occurs when the operon can function only in the presence of a controlling factor (e.g., the lactose operon is dependent on the presence of cyclic AMP)

2. Catabolite repression occurs when the operon is under control of some catabolite other than the initial substrate of the metabolic pathway

a. Allows organisms to use one source of carbon preferentially over another if both are present in the environment

b. Initial growth occurs using one carbon source until it is gone (e.g., glucose); then after a short lag period, growth resumes using the second carbon source (e.g., lactose) if both are present in the initial environment; this is called diauxic growth

G. Attenuation-in systems where transcription and translation are tightly coupled, ribosome behavior in the leader region of the mRNA can control transcription of operons involved in the biosynthesis of some amino acids

1. If ribosomes actively translate the leader region (attenuator), which contains several codons for the amino acid product of the operon genes, a terminator forms and transcription will not continue

2. If ribosomes stall during translation of the leader region because the appropriate charged aminoacyl-tRNA is absent, the terminator does not form and transcription will continue to produce the appropriate amino acid

V. Gene Regulation by Antisense RNA

A. Antisense RNA is complementary to some RNA component necessary for gene expression and, therefore, will form hydrogen bonds with it, thereby preventing the utilization of that component and blocking the subsequent gene expression

B. Examples include binding with RNA primers to prevent DNA replication, and binding with mRNA to prevent ribosome binding and subsequent translation

C. Antisense RNA regulation has not yet been demonstrated in eucaryotic cells, although there is evidence for its existence

VI. Control of the Cell Cycle

A. The complete sequence of events extending from the formation of a new cell through the next division is called the cell cycle

B. Since each daughter cell receives at least one copy of the genetic material, DNA replication and cell division must be tightly coordinated

C. The precise mechanisms(s) for control of the cell cycle are not known, although several cell-division genes have been identified

D. These appear to be two separate controls for the cell cycle, one sensitive to cell mass and the other responding to cell length

E. A number of proteins have been shown to participate in cell division regulation

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