Question

Discuss the actions taken by prokaryotes when energy substrate conditions and availability concentrations are: 1. glucose present lactose absent, 2. Glucose absent and lactose present, 3. Glucose high and lactose low. Be sure to describe the complete mechanism responsible for the metabolic switch from glucose to lactose metabolism and any factors associated with that change

Answer 1

1. When glucose is present and lactose absent, the cell will utilize glucose as the sole energy source, and keep the lac operon turned off. When lactose is absent, there is no need to synthesize the enzymes needed to degrade it. Absence of lactose keeps the lac repressor active, which then binds to the operator, and prevent RNA polymerase from binding to the promoter and begin transcription of the structural genes (negative control).

2. In absence of glucose and presence of lactose, the cells will turn on the lac operon and synthesize the enzymes needed to breakdown lactose. Lactose binds with the repressor molecules and prevent the formation of an active repressor. Thus, the operator region remains free, allowing RNA polymerase to bind to the promoter region and initiate transcription of the structural genes.

3. When glucose is high and lactose is low, the cells will preferentially utilize glucose as the energy source. This is because, glucose is a simple sugar, and can be directly used by the glycolytic pathway, whereas, lactose needs to be first broken down, and then it can be metabolized by the cells. Here, glucose exerts positive control or catabolite repression on the lac operon. The transcription of lac operon needs the binding of a protein called Catabolite Activator Protein (CAP) to a region upstream of the promoter. CAP can only bind when it is complexed with cyclic AMP. Presence of glucose inhibits the enzyme Adenylate cyclase, as a result of which, intracellular concentration of cAMP is reduced and CAP fails to bind. Thus, lac operon remains switched off in presence of glucose. When glucose is completely utilized, Adenylate cyclase becomes active and cAMP level increases. Binding of the CAP-cAMP complex switches on the lac operon, and the cells prepares themselves to metabolize lactose.