Another method used by cells to regulate metabolism is by altering the types and numbers of enzymes that are manufactured through the integrated actions of the cell’s genetic and protein synthesizing apparatuses.
This mechanism is also referred to as controlled gene expression.
The primary structures of all cellular enzymes are encoded in the cell’s DNA or genes.
To produce these enzymes, the DNA is first transcribed into messenger RNA (mRNA) and the mRNA is subsequently translated by the cell’s ribosomes into polypeptides that take on the enzymatic properties. This two-step process (transcription followed by translation) provides two additional levels at which cellular metabolism can be regulated.
There can be transcriptional control in which there is selective transcription of specific genes, and there can be translational control operating at the level of mRNA-ribosome interaction. Most of our current understanding of metabolic regulation exercised at these levels stems from research conducted with prokaryotic organisms, for the regulation of gene expression in eukaryotic cells is much more complex.
1. Constitutive and Inducible Enzymes:
Studies using prokaryotes indicate that enzymes fall into two categories with respect to their occurrence and numbers in cells. Those that appear to always be present and that occur in relatively constant concentrations are called constitutive enzymes. For example, the enzymes of glycolysis are usually constitutive. These enzymes are the products of genes that are continuously expressed. The other type of enzyme, called an inducible enzyme, is found lacking in cells or is present only in small amounts.
However, on introduction of a specific metabolite, usually a substrate, the concentration of the enzyme quickly increases. The metabolite that initiates the appearance of the enzyme is called an inducer. Inducible enzymes are the products of genes that are selectively expressed; these genes are referred to as inducible genes. One of the first inducible enzymes to be intensively studied was β-galactosidase. Wild-type E. coli cells metabolize glucose and will metabolize only the glucose even if lactose, another sugar, is also present.
The enzymes of glucose metabolism are constitutive and are always present in the cell, whereas the enzyme needed to initiate lactose metabolism, β-galactosidase, is present in only minor amounts- according to one study, no more than five copies per cell. If wild-type E. coli cells are placed in a growth medium containing only lactose as the carbon source, they are at first unable to utilize this disaccharide (Fig. 11-10).
However, within a few minutes the cells respond to the presence of lactose by synthesizing β-galactosidase, which hydrolyzes the lactose to glucose and galactose; these sugars are then metabolized by glycolysis. The lactose acted as an inducer of the β-galactosidase, and in cells grown on lactose thousands of copies of this enzyme are present.
A number of β-galactosides besides lactose can act as inducers; these include methyl B-galactoside and al- lolactose.
Actually, the presence of these inducers initiates the synthesis of not one but three enzymes in E. coli:
(1) β-galactoside permease, an enzyme formed in the plasma membrane that promotes the transfer of galactosides into the cell;
(2) β-thiogalactoside trans- acetylase, the specific action of which is not understood; and (3) β-galactosidase, the key enzyme for initiating lactose metabolism.
When, as in this case, induction can be brought about by a single agent and results in the appearance of several enzymes, the process is known as coordinate induction. The appearance of inducible enzymes is an example of metabolic regulation through transcriptional control.
2. Repressible Enzymes:
The presence of a specific substance may inhibit the continued production of a specific enzyme or a sequence of enzymes in a metabolic pathway; this process is called enzyme repression (or in the case of the repression of a sequence of enzymes, coordinates repression). Like enzyme induction, enzyme repression represents regulation at the transcriptional level.
Escherichia coli cells growing in a medium in which NH4+ is the only nitrogen source contain all of the enzyme systems necessary to synthesize amino acids from organic acids. However, if one of the amino acids is added exogenously, the synthesis of certain enzymes in the metabolic pathway leading to that amino acid will be repressed, and with continued growth of the culture these enzymes soon become diluted out.
In another bacterium, Salmonella typhimurium, a family of genes encodes nine enzymes that catalyze the reactions of a pathway leading to the synthesis of the amino acid histidine. Histidine is an important constituent of proteins and continuous production of protein in this bacterium depends on the availability of histidine.
The histidine must either be synthesized by the pathway that involves these nine enzymes or be supplied exogenously. When histidine is added to a culture of S. typhimurium, the production of the histidine biosynthetic enzymes is halted (Fig. 11-10).
The genes for the histidine enzymes are said to have been repressed. If the bacteria are then transferred to a medium lacking histidine, the genes are derepressed and the histidine enzymes soon reappear. Enzyme repression is a metabolic control mechanism acting at the level of transcription and should not be confused with allosteric feedback inhibition.