Proteins account for more than 50% of the dry weight of cells, and are involved in almost everything cells do. Proteins are used for structural support, storage, transport, signaling, movement, and defense against foreign substances. As enzymes, proteins accelerate chemical reaction of the cell.
Like most biological substances, proteins are composed of subunits linked together by chemical bonds; in this case the subunits are amino acids. The properties of a protein are determined by the combination and order of attachment of these amino acids into a chain.