Control of Metabolic Pathways

1. Control of Metabolic Pathways Higher Human Biology Unit 1 – Human Cells

2. Metabolic Pathways • Consist of several stages • One metabolite is coverted to another and so on • Each step is controlled by an enzyme • Each enzyme (protein) is coded for by a gene (basic)

3. If the enzymes are present, the pathway proceeds • If one enzyme is absent, the pathway comes to a halt Gene(s) Gene(s) Gene(s)

4. This enzyme action can be regulated at the level of gene expression (as follows) or enzyme action (which we’ll come to later)

5. Control by switching genes on and off • To prevent wasted resources, some genes that code for enzymes are switched on and off as required.

6. E.g. Lactose Metabolism in E.coli • Lactose is a sugar found in milk Glucose Galactose

7. Background • Glucose is used in respiration by E.coli for energy release • E.coli can only make use of glucose in lactose if it is released from galactose • This is done by ß-galactosidase (enzyme) • E.coli’s chromosome has a gene that codes for ß-galactosidase • E.coli only produces ß-galactosidase in the presence of lactose (gene is switched on) • It fails to produce ß-galactosidase if lactose is absent (or if only glucose is present) (gene is switched off)

8. LAC operon video • This is the same video that is on the website • We call the lactose an inducer as it prevents the repressor binding to the operator gene. It leads to the induction of ß-galactosidase.

9. Jacob Monod Hypothesis • The hypothesis of gene action was first put forward by two scientists – Jacob and Monod. • It has since been supported by experimental evidence from bacterial investigations.