Enzyme Induction

In some cases, metabolites or substrates can turn on inactive genes so that they are transcribed. In the process of enzyme induction, the substrate, or a compound structurally similar to the substrate, evokes the formation of enzyme(s) which are usually involved in the degradation of the substrate. Enzymes that are synthesized as a result of genes being turned on are called inducible enzymes and the substance that activates gene transcription is called the inducer. Inducible enzymes are produced only in response to the presence of a their substrate and, in a sense, are produced only when needed. In this way the cell does not waste energy synthesizing unneeded enzymes.

The best known and best studied case of enzyme induction involves the enzymes of lactose degradation in E. coli. Only in the presence of lactose does the bacterium synthesize the enzymes that are necessary to utilize lactose as a carbon and energy source for growth. Two enzymes are required for the initial breakdown of lactose: lactose permease, which actively transports the sugar into the cell, and beta galactosidase, which splits lactose into glucose plus galactose. The genes for these enzymes are contained within the lactose operon (lac operon) in the bacterial chromosome (Figure 7).

Figure 7. The Lac operon and its control elements

The mechanism of enzyme induction is similar to end product repression in that a regulatory gene, a promoter, and an operator are involved, but a major difference is that the lac Repressor is active only in the absence of the inducer molecule (lactose). In the presence of lactose, the Repressor cannot bind to the operator region, so that the genes for lactose transport and cleavage are transcribed. In the absence of lactose, the Repressor is active and will bind to the operator with the result that the genes for lactose metabolism are not transcribed. The induction (presence of lactose) and the repression (absence of lactose) of the lactose operon is represented in Figure 8. The functions of the components and control elements of the lac operon are shown in Table 3.

Figure 8. Enzyme Induction. Induction (or derepression) of the lac operon.

O = Operator specific nucleotide sequence on DNA to which an active Repressor binds.

P = Promoter specific nucleotide sequence on DNA to which RNA polymerase binds to initiate transcription. (The promoter site of the lac operon is further divided into two regions, an upstream region called the CAP site, and a downstream region consisting of the RNAp interaction site. The CAP site is involved in catabolite repression of the lac operon.). If the Repressor protein binds to the operator, RNAp is prevented from binding with the promoter and initiating transcription. Under these conditions the enzymes concerned with lactose utilization are not synthesized.

Lac Z, Y and A = Structural Genes in the lac operon. Lac Z encodes for Beta-galactosidase; Lac Y encodes the lactose permease; Lac A encodes a transacetylase whose function is not known.

lac = lactose, the inducer molecule. When lactose binds to the Repressor protein, the Repressor is inactivated; the operon is derepressed; the transcription of the genes for lactose utilization occurs.