Question

Describe which enzymes are required for lactose and tryptophan metabolism in bacteria when lactose and tryptophan, respectively, are (a) present and (b) absent.

Short answer

Answer: (a) In the presence of lactose, beta-galactosidase is produced, and in the presence of tryptophan, tryptophanase production is inhibited. (b) In the absence of lactose, beta-galactosidase production is inhibited, while in the absence of tryptophan, production of tryptophanase occurs, allowing for tryptophan metabolism.

Step-by-step solution

Lactose Metabolism Enzymes and Regulation

Bacteria, such as Escherichia coli (E. coli), metabolize lactose using the enzyme beta-galactosidase, which is encoded by the lacZ gene. The expression of this enzyme is regulated by the lac operon, a set of genes that are controlled by the lac repressor protein and catabolite activator protein (CAP).

Lactose Metabolism in the Presence of Lactose

When lactose is present, the lac repressor protein releases from the lac operon, allowing the transcription of lacZ, lacY, and lacA. This leads to the production of beta-galactosidase (from lacZ), which is crucial for lactose metabolism, as it breaks lactose into glucose and galactose.

Lactose Metabolism in the Absence of Lactose

When lactose is absent, the lac repressor protein binds to the lac operon and prevents the transcription of lacZ, lacY, and lacA. This means that beta-galactosidase is not produced due to the lack of lactose. As a result, there's no lactose metabolism.

Tryptophan Metabolism Enzymes and Regulation

The metabolism of tryptophan is driven by the enzyme tryptophanase, encoded by the tna gene. The expression of tryptophanase is regulated by the tryptophanase operon (tna operon) in bacteria such as E. coli. The tna operon is controlled by the tryptophan repressor protein (TrpR) and other regulatory elements.

Tryptophan Metabolism in the Presence of Tryptophan

When tryptophan is present, it acts as a corepressor by binding to the TrpR protein, which then becomes capable of binding to the tna operon. This prevents the transcription of the tna gene and subsequent production of tryptophanase. As a result, tryptophan is not metabolized.

Tryptophan Metabolism in the Absence of Tryptophan

In the absence of tryptophan, the TrpR protein is unable to bind to the tna operon, allowing the transcription of the tna gene. This leads to the production of tryptophanase, which is responsible for the catabolism of tryptophan into indole, pyruvate, and ammonia. To summarize, the enzymatic responses to lactose and tryptophan are as follows: (a) In the presence of lactose, beta-galactosidase is produced, and in the presence of tryptophan, tryptophanase production is inhibited. (b) In the absence of lactose, beta-galactosidase production is inhibited, while in the absence of tryptophan, production of tryptophanase occurs, allowing for tryptophan metabolism.