 The chief characteristic of proteins that also allows their diverse set of functions is their ability to bind other molecules specifically and tightly . The region of the protein responsible for binding another molecule is known as the binding site and is often a depression or " pocket " on the molecular surface . This binding ability is mediated by the tertiary structure of the protein , which defines the binding site pocket , and by the chemical properties of the surrounding amino acids ' side chains . Protein binding can be extraordinarily tight and specific ; for example , the ribonuclease inhibitor protein binds to human <unk> with a sub @-@ <unk> <unk> constant ( < 10 − 15 M ) but does not bind at all to its amphibian <unk> <unk> ( > 1 M ) . <unk> minor chemical changes such as the addition of a single methyl group to a binding partner can sometimes <unk> to nearly eliminate binding ; for example , the <unk> tRNA <unk> specific to the amino acid <unk> <unk> against the very similar side chain of the amino acid <unk> . 
