Protein Synthesis: Enzyme Regulation

I. MICROBIAL GENETICS

E. POLYPEPTIDE AND PROTEIN SYNTHESIS

4. Enzyme Regulation

LEARNING OBJECTIVES FOR THIS SECTION

Enzyme Regulation

In living cells, there are hundreds of different enzymes working together in a coordinated manner. Living cells neither synthesize nor breakdown more material than is required for normal metabolism and growth. All of this necessitates precise control mechanisms for turning metabolic reactions on and off.

Enzymes can be controlled or regulated in two ways: controlling the synthesis of the enzyme (genetic control) and controlling the activity of the enzyme (feedback inhibition).

a. genetic control (def)

Genetic control of enzyme activity refers to controlling transcription of the mRNA needed for an enzyme's synthesis. In prokaryotic cells, this involves the induction or repression of enzyme synthesis by regulatory proteins (def) that can bind to DNA and either block or enhance the function of RNA polymerase, the enzyme required for transcription. The regulatory proteins are part of either an operon or a regulon. An operon (def) is a set of genes transcribed as a polycistronic message that is collectively controlled by a regulatory protein. A regulon (def) is a set of related genes controlled by the same regulatory protein but transcribed as monocistronic units. Regulatory proteins may function either as repressors or activators.

1. repressors (def)

Repressors are regulatory proteins that block transcription of mRNA. They do this by binding to a portion of DNA called the operator that lies downstream of a promoter. The binding of the regulatory protein to the operator prevents RNA polymerase from passing the operator and transcribing the coding sequence for the enzymes. This is called negative control. Repressors are allosteric proteins that have a binding site for a specific molecule. Binding of that molecule to the allosteric site of the repressor can alter the repressor's shape that, in turn affects its ability to bind to DNA. This can work in one of two ways:

a. Some repressors are synthesized in a form that cannot by itself bind to the operator. The binding of a molecule called a corepressor, however, alters the shape of the regulatory protein to a form that can bind to the operator and block transcription.

An example of this type of repression is the trp operon in E. coli that encodes the five enzymes in the pathway for the biosynthesis of the amino acid tryptophan. In this case, the repressor protein, coded for by a regulatory gene, normally does not bind to the operator region of the trp operon and the five enzymes needed to synthesize the amino acid tryptophan are made (see Fig. 1 and Fig. 2). Tryptophan, the end product of these enzyme reactions, however, functions as a corepressor. The tryptophan is able to bind to a site on the allosteric repressor protein, changing its shape and enabling it to interact with the operator region. Once the repressor binds to the operator, RNA polymerase is unable to get beyond the operator and transcribe the genes for tryptophan biosynthesis. Therefore, when sufficient tryptophan is present, transcription of the enzymes that allows for its biosynthesis are turned off (see Fig. 3 and Fig. 4).

GIF animation of the repressible trp operon in the absence of a corepressor.

GIF animation of the repressible trp operon in the presence of a corepressor.

b. Other repressors are synthesized in a form that readily binds to the operator and blocks transcription. However, the binding of a molecule called an inducer alters the shape of the regulatory protein in a way that now blocks its binding to the operator and thus permits transcription.

An example of this is the lac operon that encodes for the three enzymes needed for the degradation of lactose by E. coli. E.coli will only sunthesize the three enzymes it requires to utilize lactose if that sugar is present in the surrounding environment. In this case, lactose functions as an inducer. In the absence of lactose, the repressor protein binds to the operator and RNA polymerase is unable to get beyond the operator and transcribe the genes for utilization of lactose and the three enzymes for degradation of lactose are not synthesized (see Fig. 5 and Fig. 6). When lactose, the inducer, is present, it binds to the allosteric repressor protein and causes it to change shape in such a way that it is no longer able to bind to the operator. Now RNA polymerase can transcribe the three genes required for the degradation of lactose and the bacterium is able to synthesize the enzymes needed for its utilization (see Fig. 7 and Fig. 8).

GIF animation of the inducible lac operon in the absence of an inducer.

GIF animation of the inducible lac operon in the presence of an inducer.

2. activators (def)

Activators are regulatory proteins that promote transcription of mRNA. Activators control genes that have a promotor to which RNA polymerase cannot bind. The promotor lies adjacent to a segment of DNA called the activator-binding site. The activator is an allosteric protein synthesized in a form that cannot normally bind to the activator-binding site. As a result, RNA polymerase is unable to bind to the promoter and transcribe the genes (see Fig. 9). However, binding of a molecule called an inducer to the activator alters the shape of the activator in a way that now allows it to bind to the activator-binding site. The binding of the activator to the activator-binding site, in turn, enables RNA polymerase to bind to the promotor and initiate transcription (see Fig. 10 and Fig. 11). This is called positive control.

GIF animation an activator protein in the absence of an inducer.

GIF animation an activator protein in the presence of an inducer.

3. Bacteria also use translational control of enzyme synthesis. In this case, the bacteria produce antisense RNA (def) that is complementary to the mRNA coding for the enzyme. When the antisense RNA binds to the mRNA by complementary base pairing (def), the mRNA cannot be translated into protein and the enzyme is not made (see Fig. 14).

GIF animation showing antisense RNA.

b. controlling the enzyme's activity (feedback inhibition). Enzyme activity can be controlled by competitive inhibition and noncompetitive inhibition.

1. With what is termed noncompetitive inhibition, the inhibitor is the end product of a metabolic pathway that is able to bind to a second site (the allosteric site) on the enzyme. Binding of the inhibitor to the allosteric site alters the shape of the enzyme's active site thus preventing binding of the first substrate in the metabolic pathway. In this way, the pathway is turned off (see Fig. 12).

GIF animation showing noncompetitive inhibition by an allosteric enzyme.

2. In the case of what is called competitive inhibition, the inhibitor is the end product of an enzymatic reaction. That end product is also capable of reacting with the enzyme's active site and prevents the enzyme from binding its normal substrate. As a result, the end product is no longer synthesized (see Fig. 13).

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