Biosynthesis and regulation of enzyme in microorganismsPRESENTED BY: SWATI RAINA IBT SEM 1ST ROLL NO: 10

Bacteria have developed sophisticated mechanisms for the regulation of both catabolic and anabolic pathways

Vibrio cholerae does not produce the cholera toxin that causes diarrhoea unless it is in the human intestinal tract. & Bacillus subtilis does not produce enzymes for tryptophan until it find pre-existing tryptophan in the medium.

Bacteria do not synthesize degradative(catabolic) enzymes unless the substrate for these enzymes are present in their environment.

Similarly for anabolic Bacterial cells shut down biosynthetic pathways when the end product of the pathway is not needed or is readily obtained by uptake from the environment.

 

 bacterial cells  change patterns of enzymes, to adapt a specific environment.

concentration of an enzyme depends on the presence of the substrate for the enzyme.

TYPES OF ENZYMES

CONSTITUTIVE ENZYME : produced independently of the composition of the medium.

INDUCIBLE ENZYME : produced when they are needed, only in the presence of substrate.

REPRESSIBLE ENZYME : this is downregulated (turned off) when there is the formation of the end product.

RegulationOf

Enzyme synthesi

s

End product repression

Enzyme induction

Enzyme activity Feedback

inhibition/ end product

Catabolite repression

Negative control : because it dec rate of transcription positive control : because it inc rate of transcription

POINTS OF REGULATION There are some regulatory processes & there are some

points of regulation at the level of transcription (enzyme induction & repression) which are regulated by different properties of enzyme.

Some examples are:Allosteric proteinEnzyme repression Enzyme induction Catabolite repression

Allosteric Protein An allosteric protein is one which has an active (catalytic)

  site and an allosteric (effector) site.

Feedback InhibitionFeedback inhibition (or end 

product inhibition)  is  a  mechanism for the inhibition of performed enzymes.

The final product is able to feed back to the first step in the pathway and to regulate its own biosynthesis.

Enzyme Repression This prevents the synthesis of the enzyme concerned with the synthesis of the particular end product.

Similar to feedback Inhibition but spends less energy than it.

The product of trpL gene is a trp repressor, an allosteric protein which is regulated by tryptophan.

In absence of tryptophan this repressor is made in inactive form in small amounts, thus transcription occurs and tryptophan is produced.

In presence of tryptophan, trp acts as a corepressor and binds to the inactive repressor making it active thus repressor binds to DNA and blocks transcription Hence tryptophan is not produced.

Enzyme induction Metabolites or substrates, in some cases

can turn on inactive genes so that they are transcribed. The substrate/compound structurally similar to the substrate, 

evokes the formation ofenzyme(s) which are usually involved in  the degradation of the substrate.

Enzymes that are synthesized as a result of genes being turned on are called inducible enzymes and the substance that activates gene transcription is called the inducer.

The enzyme is only produced when it’s substrate is present thus energy is saved. E.g  lactose degradation in E. coli

• Only in the presence of  lactose does the bacterium synthesize the enzymes that are necessary to utilize lactose.

•  In the presence of lactose, the Repressor cannot bind to the operator region, so that the genes for lactose transport and cleavage  are transcribed.

• In the absence of lactose, the Repressor is active and will bind to operator with the result  that the genes for lactose metabolism are not  transcribed.

Catabolic repression Catabolite repression is a type of positive control of

transcription, since a regulatory protein affects an increase (upregulation) in the rate of transcription of an operon.

Example of catabolic repression is the diauxic growth of E.Coli in the presence of glucose & lactose.

Glucose is utilised before lactose because lactose degradation require 2 enzymes.

Thus the glucose represses the enzymes for lactose utilisation this is known as catabolic repression.

IN PRESENCE OF GLUCOSE

Adenylate cyclase (AC) Blocked

AC is required to synthesise cAMP

When cAMP level is low or absent

cAMP can’t bind to CAP

Transcription does not occur

IN ABSENCE OF GLUCOSE

Adenylate cyclase (AC) is active

cAMP is synthesised & level is high

CAP is activated

Transcription occurs ( In presence of inducer i.e lactose)