 The best @-@ known role of proteins in the cell is as enzymes , which catalyse 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 posttranslational modification . About 4 @,@ 000 reactions are known to be catalysed by enzymes . The rate acceleration conferred by enzymatic catalysis is often enormous — as much as 1017 @-@ fold increase in rate over the uncatalysed reaction in the case of orotate decarboxylase ( 78 million years without the enzyme , 18 milliseconds with the enzyme ) . 
