regulation of enzymes

Enzymes are turned on and off by the binding of subtrates that activate them. By blocking a substrate from binding you can turn off the enzyme. As for why you would want to turn off and enzyme. Lets say for example you are producing too much ATP using glycolysis. The quickest way to stop producing more ATP is to turn off the first enzyme in that pathway which would be hexokinase.

Reversible phosphorylation of enzymes is an important regulatory mechanism that occurs in both prokaryotic and eukaryotic organisms. Enzymes called kinases (phosphorylation) and phosphatases (dephosphorylation) are involved in this process.

Many enzymes and receptors are switched "on" or "off" by phosphorylation and dephosphorylation. Reversible phosphorylation results in a conformational change in the structure in many enzymes and receptors, causing them to become activated or deactivated. Phosphorylation usually occurs on serine, threonine, and tyrosine amino acids in eukaryotic proteins.

In addition, phosphorylation occurs on the basic amino acid residues histidine or arginine or lysine in prokaryotic proteins. The addition of a phosphate (PO4) molecule to a polar R group of an amino acid residue can turn a hydrophobic portion of a protein into a polar and extremely hydrophilic portion of molecule. In this way it can introduce a conformational change in the structure of the protein via interaction with other hydrophobic and hydrophilic residues in the protein.