Control of Metabolic Pathways Higher Human Biology Unit

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

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)

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

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

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

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

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 ßgalactosidase • 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)

• 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.

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.