There are several different mechanisms of regulation of enzyme activity. Each has its place and its most common usage.
We learned in Module 4 that the observed rate of an enzymatic reaction is proportional to the amount of enzyme present. So obviously we can effect the observed rate of flux through a metabolic pathway if the concentration of its enzymes are altered. This can be accomplished by synthesizing more enzyme or by increasing the rate of their degradation back to their component amino acids. This a realtively time consuming process (We must proceed through the entire process of transcribing mRNA and transporting and processing it and then translating it to a protein structure) as well as energetically expensive (each and every RNA base transcribed and peptide bond formed requires several ATP equivalents). The end result is, that this is mostly a response of a major change in environment (change in food source say from glucose to galactose for a bacterium.) But is not generally used to respond to the rapidly changing internal metabolic state of a cell. I will have nothing more to say about this method of regulation for the rest of the course.
Allosteric Binding is most frequently used to respond to a cells internal metabolic state. "Allosteric" simply means "Other Site". Enzymes that are allosterically regulated have two binding sites: (1) The active where chemistry on the substrates occurs; (2) Allosteric binding stite where no chemistry occurs but binding influences protein structure (as well as that of the active site) and therefore activity! In these proteins, binding of some compound at the allosteric site or maybe even of a substrate molecule at the active site can make the enzyme either more or less efficient. It all depends on how they effect the structure of the enzyme. We will spend quite a few pages trying to describe this and how it works.
Covalent Modification of an enzyme can be used to alter its kinetic protperties as well. Recall that we mentioned in Module 3 that proteins can be modified in several ways. One of the most important is through the ATP dependent phosphorylation. This occurs ONLY in cellular proteins. Those that are in extracellular fluids (blood serum for instance) do not have their activty modified through ATP dependent phosphorylation. (primarily because ATP is not present outside of cells) This modification, too, engenders an alteration of protein structure and may make an enzyme either more or less catalytically efficient. It can be "reversed" through the hydrolysis of the phosphate so that the original protein structure is obtained. Frequently this modification is in response to a change in ORGANISMAL metabolic state. It is usually mediated through a cell signalling pathway via hormones. We will take quite a few pages to describe the glucose depende hormone system run by the adrenal glands (insulin/glucagon)
Some extracellular proteins (digestive or blood serum proteins for instance) are also covalently modified, but not by a phosphate. Protease enzymes are frequently synthesized in an inactive "zymogen" structure. Upon a specific cleavage of a limited number of amide bonds (1 or 2 specific amide bonds) these proteins expess their latent activity. Since the amide bond cleavage is a hydrolysis it is not viewed as being a reversible modification. The equlibrium lies far enough toward the free acid and amine that reversal back to the amide cannot be observed (in water). This has its utility in reaction pathways that, once started, need to go virtually to completion --- like blood clotting for instance. It would be kind of a bummer if once blood flow slowed down a bit that the blood clot just dissolved and restarted blood flow again.