REGULATION OF GENE EXPRESSION

REGULATION OF GENE EXPRESSION

The synthesis and activity of some

enzymes in the cells do not change. These are called structural enzymes.

Some enzymes are affected by environmental conditions.

The cell controls enzymatic reactions mainly in two ways.

1. Regulates enzyme activity

2. Controls enzyme synthesis.

Regulation of Enzyme Activity (posttranslational regulation)

• The enzyme is synthesized as part of a large inactive precursor protein. The enzyme is then separated from this precursor protein.

•The enzyme is degraded and its activity is reduced.

• Feedback inhibition and covalent modification, which reversible and transient regulation of the regulation of enzyme activity, are also used by the cell.

Feedback Inhibition

Inactivation of enzymes with some compounds

In this inhibition, the final product formed in the metabolic pathway inactivates the first enzyme in this pathway.

As a result, the final product is no longer produced.

Allosteric and isoenzymes are used.

Feedback Inhibition

If the amino acid of arginine is over-

synthesized, N-acetyl glutamate synthase (AGS) enzyme is inhibited by arginine,

If proline is excessive, proline inactivates the glutamyl kinase (GK) enzyme.

AGS    Arginine

Glutamic acid

GK    Proline

Covalent Modification

It is a regulatory mechanism that temporarily inactivates a particular enzyme.

Enzyme activity is controlled by adding or removing some small organic molecules.

Similar to allosteric enzymes, enzyme activity is inhibited by a covalently bonded compound that constitutes a structural change in the enzyme.

Once this compound is removed, the enzyme gains normal activity.

The activity of some enzymes is affected by the addition of AMP, ADP, inorganic phosphate or methyl.

Posttranslational modifications of some proteins are irreversible.

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Regulation of Enzyme Synthesis

In the control of enzyme synthesis at the level of transcription;

Reduction of synthesis of some enzymes repression,

The increase in the synthesis of some enzymes is called induction

Regulon

E. coli has multiple operons for maltose usage.

These operons are controlled by maltose activator protein.

This type of operon, where more than one operon is controlled by the same regulatory protein, is called regulon.

Regulons are also found in operons with negative control (for example arginine regulone).

1. Two-element regulator systems

It is detected by a sensor and the cell is activated. (signal transduction)

Signal transduction systems consist of two parts (two-element system):

1. Sensor protein in the cell membrane

2. Regulatory protein in the cell

Sensor kinase  phosphorylation  regulator protein

2. Quorum Sensing

Quorum: Sufficient Number

Sensing: Detection

It is a regulation system

controlled by the cell density of the same species. If there is

sufficient cell number, a specific biological response is generated.

Quorum Sensing

Aliivibrio fischeri  bioluminescent property luciferase  light  lux operons

Autoinductor when acylated homoserine

lactones (AHLs) are in sufficient quantity  Lux R active  lux operons work  light

A long chain aliphatic aldehyde (RCHO) + O2 + Flavin mononucleotide (FMNH2)  Luciferase  FMN + RCOOH + H2O + light

Lux R activator protein  Runs lux operons

Photobacterium spp. Aliivibrio fischeri, Vibrio spp. Facultative,

Pseudomonas aeruginosa, E. coli  biofilm

Staphylococcus aureus  short peptides

Haloalkalifyl archaea Natronacoccus occutus  protease

Eucaryotic Saccharomyces cerevisiae, Candida  aromatic alcohols  single cell or filaments

 Enzyme induction and repression involving DNA binding proteins commonly used by Bacteria and Archaea is the first pathway.

 The other way is mostly used by eukaryotes.

Signals are transmitted by protein subunits of the RNA polymerase enzyme.

 It is interesting to note that Archaea, whose replication and transcription mechanisms

resemble eukaryotes, are similar to bacteria in terms of transcription regulation.

4. Transcription control of Archaea