Regulation of Gene Expression Mic 428 - Lecture# 11 Regulation of Gene Expression Outline Requires review of Chapter 7 before coming to class. The complexity of regulation. Overview of regulation. Constitutive and inducible enzymes. Major modes of regulation in the cell: 1. Control of enzyme activity. Post translational control.

Regulation of Gene Expression Mic 428 - Lecture# 11 Regulation of Gene Expression Outline Feedback inhibition. Allosteric regulation and rate-limiting enzymes. Types of allosteric regulation. Isoenzymes. 2. Control of the amount of enzyme present: transcriptional control and translational control (NEXT CLASS).

Complexity of regulation Hundreds of different enzymatic reactions happening simultaneously. Need to respond rapidly to changes in their environment. Complicated developmental pathways.

Some enzymes are needed in the same amount under any growth condition. CONSTITUTIVE Some enzymes are not permanently needed in the same amount under any growth condition. INDUCIBLE

Two major modes of regulation: Control of enzyme activity After the enzyme has been produced Post translational control Control of the amount of an enzyme At the level of transcription Next class At the level of translation

Overview of regulation Figure: 08-01 Caption: An overview of the mechanisms that can be used in regulation. The product of gene A is enzyme A, which is synthesized constitutively and carries out its reaction. Enzyme B is also synthesized constitutively but its activity can be inhibited. The synthesis of the product of gene C can be prevented by control at the level of translation. The synthesis of the product of gene D can be prevented by control at the level of transcription. The product of gene A is enzyme A, which is synthesized constitutively and carries out its reaction. Enzyme B is also synthesized constitutively but its activity can be inhibited. The synthesis of the product of gene C can be prevented by control at the level of translation. The synthesis of the product of gene D can be prevented by control at the level of transcription.

Regulation of enzyme activity Inhibiting enzyme activity: Enzymes are synthesized with full enzymatic activity and then the activity is reduced or inhibited by certain compounds in the cell. These compounds are usually related to the metabolic pathway in which the enzyme functions.

We have seen some of these cases. Do you remember any?

Feedback inhibition

Feedback inhibition of enzyme activity Figure: 08-02 Caption: Feedback inhibition of enzyme activity. The activity of the first enzyme of the pathway is inhibited by the end product, thus controlling production of end product.

The regulated enzyme is referred to as the rate-limiting enzyme. How is it possible for the end product of a pathway to inhibit the enzyme that acts on a substrate quite unrelated to it? By means of allosteric regulation The regulated enzyme is referred to as the rate-limiting enzyme.

Mechanism of enzyme inhibition by an allosteric effector Figure: 08-03 Caption: Mechanism of enzyme inhibition by an allosteric effector. When the effector combines with the allosteric site, the conformation of the enzyme is altered so that the substrate can no longer bind.

In branched metabolic sequences, the beginning of each branch is often controlled allosterically

Feedback inhibition in a branched biosynthetic pathway Figure: 08-04 Caption: Feedback inhibition (dashed red arrows) in a branched biosynthetic pathway. A key intermediate in each pathway is shown in pink. The enzymes being inhibited are N-acetyl glutamate synthase (AGS) and g-glutamyl kinase (GK).

Types of allosteric regulation: 1. Simple feedback inhibition. 2. Concerted feedback inhibition. 3. Sequential feedback inhibition.

1. Simple feedback inhibition.

More than one metabolite interacts with the enzyme 2. Concerted feedback inhibition. A B C D E F More than one metabolite interacts with the enzyme

3. Sequential feedback inhibition. Two or more metabolites interact with an enzyme B C F D G E H

Some biosynthetic pathways are regulated by isoenzymes that catalyze the same reaction but are subject to different regulatory controls.

The common pathway leading to the synthesis of the aromatic amino acids contains three isozymes. Each of these enzymes is specifically feedback-inhibited by one of the aromatic amino acids. Note how an excess of all three amino acids is required to completely shut off the synthesis of DAHP. Figure: 08-05 Caption: The common pathway leading to the synthesis of the aromatic amino acids contains three isozymes of DAHP synthetase (DAHP is 3-deoxy-d-arabino-heptulosonate 7-phosphate). Each of these enzymes is specifically feedback-inhibited by one of the aromatic amino acids. Note how an excess of all three amino acids is required to completely shut off the synthesis of DAHP.

Processing of preproinsulin Figure: 08-06-01UN Caption: Processing of preproinsulin. The initiating methionine is actually cleaved from the chain before the entire preproinsulin molecule is synthesized. After the leader peptide is removed, the proinsulin molecule folds, and the C-peptide is then removed leaving the A and B chains on insulin. These chains are held together by disulfide bridges.

Protein splicing Figure: 08-06-02UN Caption: Protein splicing. The protein synthesized from the gyrA mRNA in Mycobacterium leprae is 1273 amino acid residues in length (10 times the length of preproinsulin, shown in Fig. 1). The N-extein is the amino terminal extein and the C-extein is the carboxyl terminal extein. Residues 131 to 550 make up an intein, which removes itself in a self-splicing reaction that generates the free intein and the DNA Gyrase A subunit.