In Chapter 2 we reviewed the forces that come to play in chemical reactions,such as those catalyzed by enzymes. In this chapter we introduce the specificmolecular components of enzymes that bring these forces to bear on thereactants and products of the catalyzed reaction. Like all proteins, enzymes arecomposed mainly of the 20 naturally occurring amino acids. We shall discusshow these amino acids link together to form the polypeptide backbone ofproteins, and how these macromolecules fold to form the three-dimensionalconformations of enzymes that facilitate catalysis. Individual amino acid sidechains supply chemical reactivities of different types that are exploited by theenzyme in catalyzing specific chemical transformations. In addition to theamino acids, many enzymes utilize nonprotein cofactors to add additionalchemical reactivities to their repertoire. We shall describe some of the morecommon cofactors found in enzymes, and discuss how they are utilized incatalysis.