Metabolic Regulation

1. Metabolic Regulation • Genetic level • Cellular level: - Enzyme activity

2. Metabolic Regulation Genetic Level Regulation: Control which protein is synthesized through adjusting the rate of transcription of that gene: Feedback repression: The end product of enzymatic activity accumulates and blocks transcription. For repression, the repressor protein is required which can bind to the operator region and hinder RNA polymerase binding. The repressor protein can block transcription only when bound to the co-repressor (typically the end product of the pathway).

3. Promoter Operator Gene 1 Gene 2 Gene 3 RNA polymerase repressor corepressor Normal Transcription DNA template Promoter Operator Gene 1 Gene 2 Gene 3 RNA polymerase m-RNA repressor inactive Transcription Blocked DNA template active

4. Genetic Organization of the Tryptophan Operon DNA template encoding related enzymes for tryptophan synthesis encoding repressor Only when the repressor binds with tryptophan, it can bind on the operator region and block the transcription. Operon: In prokaryotes, a set of genes, encoding proteins with related functions, under the control of a single promoter-operator.

5. Metabolic regulation Genetic Level Regulation: Induction: a metabolite ( often a substrate for a pathway) accumulates and acts as an inducer of transcription. The inducer will bind the repressor protein, and the complex is inactive as a repressor.

6. Transcription Blocked Promoter Operator Gene 1 Gene 2 Gene 3 RNA polymerase DNA template repressor Transcription Permitted DNA template Promoter Operator Gene 1 Gene 2 Gene 3 RNA polymerase m-RNA repressor Inducer

7. Example • Inducer: allolactose modified from lactose in the cell.

8. e.g. The lactose operon controls the synthesis of three proteins (Lac z (lactase), lac y, lac a ) involved in lactose utilization as a carbon and energy source in E. coli. Lac i Promoter Operator Lac z Lac y Lac a RNA polymerase m-RNA repressor Lac i encoding repressor. allolactose

9. Catabolite Repression (Glucose Effect) • Inducer: allolactose modified from lactose in the cell. • Induction of allolactose might not be sufficient for maximum transcription if a carbon-energy source (e.g. glucose) preferred to lactose is present. • Only when glucose is depleted, the cell will expend energy to create a pathway to utilize the less favorable carbon-energy source lactose.

10. Metabolic Regulation Catabolite Repression (glucose effect) When the cell has an energetically favorable carbon-energy source (e.g. glucose) available, it will not expend significant energy to create a pathway for utilization of a less favorable carbon-energy source; it will not transcript the related enzyme for such reaction.

11. Metabolic Regulation Genetic Level Regulation: • Some genes are regulated. • Others are not (constitutive): their gene products are made at a relatively constant rate irrespective of changes in growth conditions. ( enzymes are expected to use under almost any conditions such as that involved in glycolysis)

12. Metabolic Regulation Cellular Level Regulation - Metabolic Pathway Control: • The metabolic pathway can be controlled by enzyme activity. • The activity of allosteric enzymes can be controlled by effectors including inhibitors and activators. Allosteric enzyme: enzymes have more than one substrate binding site. The binding of one substrate to the enzyme facilitates binding of other substrate molecules. • Most often the first reaction in the pathway is inhibited by accumulation of the product: feedback inhibition or end-product inhibition.

14. What are the differences between feedback repression and feedback inhibition?

15. Feedback repression Feedback inhibition Regulation level Genetic: RNA transcription Cellular: Activity of enzyme Complex formed End product + repressor End product + enzyme Effect Operator on DNA template occupied by the complex Reduced enzyme activity Consequence Blocked Transcription The respective reaction is inhibited.

16. Metabolic Regulation Cellular level- metabolic pathway controls: The activities of a group of enzymes (pathway) can be controlled. - Isozymes - Concerted feedback - Sequential feedback - Cumulative feedback Please refer to the textbook p.123.

17. Metabolic Regulation Cellular level- metabolic pathway controls through: • Isozymes • A number of separate enzymes initially carry out the same conversion, each of which is sensitive to inhibition by a different end product.

18. 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.

19. Metabolic Regulation - Concerted feedback inhibition More than one end product or all end products must be present in excess to repress the first enzyme.

21. P1 M4 X X M1 M2 M3 X M5 P2 Metabolic Regulation - Sequential feedback inhibition The common steps are inhibited by the product before the branch, and the first enzyme of each branch is inhibited by the branch product. High levels of P1 and P2 inhibit enzyme E3 and E4, respectively → M3 will accumulate →the pathway is inactivated if both P1 and P2 are high. E3 E1 E2 E4

22. Sequential Feedback Inhibition

23. Metabolic Regulation - Cumulative feedback inhibition or cooperative feedback inhibition - A single allosteric enzyme may have effector sites for - several end products of a pathway; - each effector causes only partial inhibition. - Full inhibition is a cumulative effect. Concerted Cumulative

24. Inosine 5-mono-phosphate (IMP)

25. Summary of Metabolic Regulation Metabolic regulation: • Genetic level: control transcription of genes (repression, induction and catabolic repression (glucose effect)) • Cellular level: - Enzyme activity: feedback inhibition