Enzyme Induction and Repression (World of Microbiology and Immunology)
Microorganisms have many enzymes that function in the myriad of activities that produce a growing and dividing cell. From a health standpoint, some enzymes are vital for the establishment of an infection by the microbes. Some enzymes are active all the time. These are known as constitutive enzymes. However, other enzymes are active only periodically, when their product is required. Such enzymes are known as inducible enzymes.
The ability of microorganisms such as bacteria to control the activity of inducible enzymes is vital for their survival. The constant activity of such enzymes could result in the over-production of a compound, which would be an energy drain on the microorganism. At the same time, inducible enzymes must be capable of a rapid response to whatever condition they are geared to respond.
The twin goals of control of activity and speed of response are achieved by the processes of induction and repression.
Induction and repression are related in that they both focus on the binding of a molecule known as RNA polymerase to DNA. Specifically, the RNA polymerase binds to a region that is immediately "upstream" from the region of DNA that codes for a protein. The binding region is termed the operator. The operator acts to position the polymerase correctly, so that the molecule can then begin to move along the DNA, interpreting the genetic information as it moves along.
The three-dimensional shape of the operator region influences the binding of the RNA polymerase. The configuration of the operator can be altered by the presence of molecules called effectors. An effector can alter the shape of the polymerase-binding region so that the polymerase is more easily and efficiently able to bind. This effect is called induction. Conversely, effectors can associate with the operator and alter the configuration so that the binding of the polymerase occurs less efficiently or not at all. This effect is known as repression.
Enzyme induction is a process where an enzyme is manufactured in response to the presence of a specific molecule. This molecule is termed an inducer. Typically, an inducer molecule is a compound that the enzyme acts upon. In the induction process, the inducer molecule combines with another molecule, which is called the repressor. The binding of the inducer to the repressor blocks the function of the repressor, which is to bind to a specific region called an operator. The operator is the site to which another molecule, known as ribonucleic acid (RNA) polymerase, binds and begins the transcription of the gene to produce the so-called messenger RNA that acts as a template for the subsequent production of protein. Thus, the binding of the inducer to the repressor keeps the repressor from preventing transcription, and so the gene coding for the inducible enzyme is transcribed. Repression of transcription is essentially the default behavior, which is over-ridden once the inducing molecule is present.
In bacteria, the lactose (lac) operon is a very well characterized system that operates on the basis of induction.
Enzyme repression is when the repressor molecules prevent the manufacture of an enzyme. Repression typically operates by feedback inhibition. For example, if the end product of a series of enzyme-catalyzed reactions is a particular amino acid, that amino acid acts as the repressor molecule to further production. Often the repressor will combine with another molecule and the duo is able to block the operation of the operator. This blockage can occur when the repressor duo outcompetes with the polymerase for the binding site on the operator. Alternately, the repressor duo can bind directly to the polymerase and, by stimulating a change in the shape of the polymerase, prevent the subsequent binding to the operator region. Either way, the result is the blockage of the transcription of the particular gene.
The gene that is blocked in enzyme repression tends to be the first enzyme in the pathway leading to the manufacture of the repressor. Thus, repression acts to inhibit the production of all the enzymes involved in the metabolic pathway. This saves the bacterium energy. Otherwise, enzymes would be madet a high metabolic costor which there would be no role in cellular processes.
Induction and repression mechanisms tend to cycle back and forth in response to the level of effector, and in response to nutrient concentration, pH, or other conditions for which the particular effector is sensitive.
See also Metabolism; Microbial genetics