Bacterial Physiology (Micr430)

Lecture 13Regulation of Gene Expression

(Text Chapter: 6) (Moat book)

Cellular Regulations

Regulation of metabolic pathways Regulation of gene expression

Regulation of metabolic pathways

A cell has thousands of chemical reactions, if not regulated, chaos occurs

Regulation of metabolic pathways is accomplished by adjusting rate of one or more regulatory enzymes that control the overall rate of the pathway Non-covalent binding between enzyme and

pathway intermediate compounds Covalent modification of enzyme activity

Patterns of regulation

Feedback inhibition – for a biosynthetic pathway, end product is usually a negative allosteric effector for a branch point enzyme. Simple feedback; for unbranched

pathway Cumulative feedback; both effectors

exert partial inhibition Concerted feedback; no inhibition

unless both effectors bind Inhibition by isoenzymes

Patterns of feedback inhibition

Branched metabolic pathways

Patterns of regulation

Positive regulation of a pathway By intermediate of a second pathway Precursor activation

Regulatory enzymes usually control branch points of metabolic pathways and reactions are usually irreversible.

Enzyme kinetics

Non-regulatory enzymes Michaelis-Menten equation v = Vmax * S/(Km + S) When S is small compared to Km, v is

proportional to S -> v = Vmax * S/Km

When S is much larger than Km, then

v ~ Vmax

Michaelis-Menten kinetics

Enzyme kinetics

Regulatory enzymes follow sigmoidal curve.

Binding of one substrate molecule increases the affinity of the enzyme for a second substrate molecule or increases rate of product formation from site already occupied – positive cooperactivity.

Regulatory Enzymes

Enzyme kineticsConformational changes in regulatory enzymes

Regulation by covalent modification

Gene expression regulation

Definitions Structural gene Promoter Operator Operon and polycistronic Regulatory gene Inducible Repressible

Gene expression regulation

Negative control: regulatory proteins bind to operator region to prevent transcription

Positive control: regulatory proteins bind to region near promoter to increase transcription

Regulation at transcriptional level relies heavily on DNA binding proteins

The lac Operon

Responsible for utilization of lactose as a carbon source

-galactosidase, encoded by lacZ, cleaves -1,4-linkage of lactose

Permease (encoded by lacY) allows entrance of lactose

lacA encodes a thiogalactoside transacetylase, whose cellular role is not known.