How do small changes in pH affect an enzyme's rate of reaction?

I understand that extreme pH levels can denature an enzyme's active site, but why do increases/decreases in rate of reaciton occur with smaller changes....what happens structurally or molecularly to the enzyme?How do small changes in pH affect an enzyme's rate of reaction?
pH is a measure of the concentration of hydrogen ions in a solution. Proteins are made of amino acids. Because all amino acids have an NH2 (or NH-) at one end and a COOH (or COO-) at the other, they can readily accept an extra Hydrogen ion (or lose one). In a protein, the amino acids are linked together by peptide bonds, but then the long chain of amino acids are folded into sheets or helixes by hydrogen bonds. It is the 3-dimensional structure of a protein (part of which comes from these sheets or helixes) that gives it its activity. If additional hydrogen atoms are gained or lost, it can disrupt the folding so that the 3-D structure changes slightly. For example, it may make it a little more difficult for the substrate to fit into the active area of the protein, so the procedure catalyzed by that protein would be slowed down.