1. The first thing that could effect the performance of an enzyme is whether or not there is a costly/negligble mutation, such as a point mutation. As you may know, if there is a point mutation, where one nucleotide is exchanged for another, a different amino acid, than expected may be synthesized. The types of amino acids coded by triplets (codons) which consists of 3 bases, determine how a protein, like an enzyme folds. If the enzyme does not fold properly, the enzyme may not function well, or not at all. On the other hand if there is a point mutation where a base change does not change the amino acid, then the enzymatic activity will be normal as expected.
2. Another possibility of course is temperature. With a decrease in temperature, the enzyme will function slower. Whereas with an increase in temperature, the molecular forces that hold the enzyme together (essentially changes the shape or conformation of the enzyme) may be interrupted, break apart, and thus the enzyme has no function.
3. pH is another gimme--enzymes function at both high (basic environments) and low (acidic environments) so this condition depends on what enzyme we are talking about. Regardless, because acids and bases take away and add H+ (protons), this can disrupt the charge of the enzyme, where there will be intermolecular interference, such as that seen when an enzyme is exposed to high temperature (will denature, most likely if the pH change is too drastic).
4. An enzyme's performance will change it there is the a presents of a competitive inhibitor. This means that on the substrate which the enzyme will bind to is actually two active sites. If the substrate target is already bound by another ligand, or enzyme, then the activity of the enzyme of interest will decrease its rate (an enzyme functions best when it can saturate the substrate entirely..this rate of enzymatic activity is called the enzyme's Vmax).
5. Overall concentration of the enzyme. If there is a low concentration of enzyme and a high concentration of substrate, the performance of the enzyme will not be as strong. There will be less filling of the active site and the reaction that the enzyme catalyses will be slower.
6. Overall concentration of the substrate. If there is not enough substrate, the enzyme essentially has nothing to bind to---as a result, the rate of the enzymatic reaction will decrease, as opposed to if there was equal amounts of enzyme and substrate.
