Enzymes, vitamins and hormones are all important for cellular metabolism. Enzymes are biological catalysts which catalyse biological reactions. Vitamins cannot be synthesized in human body and must be procured from diet. Many vitamins act as coenzymes and are directly required for cellular metabolism. Related Theory Enzyme is a proteinaceous catalyst produced by a cell and responsible for the high rate and specificity of one or more intercellular or intracellular biochemical reactions. All enzymes are proteinaceous except ribozyme (a nuclease enzyme).