 The best @-@ known role of proteins in the cell is as enzymes , which <unk> chemical reactions . Enzymes are usually highly specific and accelerate only one or a few chemical reactions . Enzymes carry out most of the reactions involved in metabolism , as well as manipulating DNA in processes such as DNA replication , DNA repair , and transcription . Some enzymes act on other proteins to add or remove chemical groups in a process known as <unk> modification . About 4 @,@ 000 reactions are known to be <unk> by enzymes . The rate acceleration conferred by enzymatic catalysis is often enormous — as much as <unk> @-@ fold increase in rate over the <unk> reaction in the case of <unk> <unk> ( 78 million years without the enzyme , 18 <unk> with the enzyme ) . 
