 The molecules bound and acted upon by enzymes are called substrates . Although enzymes can consist of hundreds of amino acids , it is usually only a small fraction of the residues that come in contact with the substrate , and an even smaller fraction — three to four residues on average — that are directly involved in catalysis . The region of the enzyme that binds the substrate and contains the catalytic residues is known as the active site . 
 The molecules bound and acted upon by enzymes are called substrates . Although enzymes can consist of hundreds of amino acids , it <a href='drop_aux' title='1'><mark>is</mark></a>usually only a small fraction of the residues <a href='null_relcl' title='2'><mark>that</mark></a>come in contact with the substrate , and an even smaller fraction — three to four residues on average — <a href='null_relcl' title='3'><mark>that</mark></a>are directly involved in catalysis . The region of the enzyme <a href='null_relcl' title='4'><mark>that</mark></a><a href='uninflect' title='5'><mark>binds</mark></a> the substrate and <a href='uninflect' title='6'><mark>contains</mark></a> the catalytic residues is known as the active site . 
The molecules bound and acted upon by enzymes are called substrates . Although enzymes can consist of hundreds of amino acids , it usually only a small fraction of da residues cum in contact wid tdasubstrate , and an even smaller fraction — three to 4 residues on average — are directly involved in catalysis . The region of thdanzyme bind thedabstrate and contain the catalytic residues is known as the daive site .