Enzymes are organic catalysts. Their role is to allow chemical reactions to occur at body temperature, at a rate that is faster than would occur without the catalyst. They are important proteins with specific shapes that allow them to bind to their substrates(substances they act on) in precise ways. Any change in the enzyme's shape, can cause it to function poorly or not at all. This is called denaturation. Enzymes become denatured when the temperature, or the particular ph needed to function, is not at optimum conditions. Enzymes are part of metabolic pathways, aid in digestion, respiration, photosynthesis, to name a few. An example of how enzyme function is important, is in the autosomal recessive disorder called Phenylketonuria or P.K.U. In this disease, a person has a mutation in the gene for the hepatic enzyme phenylalanine hydroxylase or PAH. It is nonfunctional. This enzyme is part of a metabolic pathway in which the amino acid phenylalanine is metabolized to the amino acid tyrosine. When PAH activity is reduced, pheylalanine builds up in the urine and in the brain which causes mental retardation and seizures. A person with PKU must have a restricted diet low in proteins containing phenylalanine, and supplemented with a formula containing the other amino acids and nutrients they need for health. Without this necessary enzyme performing its action, a person's health is in jeopardy. Therefore, we need our enzymes to function optimally.