The key residues in the active site of ribonucleases are His12, His119, and Glu111. A sequence alignment of several ribonucleases shows that His12 and His119 are strictly conserved, and the bell-shaped pH dependence curve indicates the presence of two ionizable groups involved in catalysis. Visualizing the ribonuclease structure in JMol reveals that His12, His119, and Glu111 are located in the active site.

1. 1. (10 pts, 15min ) Determine key residues in the active site of ribonucleases by integrating all of
the following information: Briefly justify your answer. a) A sequence alignment of several
ribonucleases is shown below. You can ignore Cys residues and non-polar residues from
consideration. b) The pH dependence of the catalytic rate (kCAT) is shown on the right. Note that
the bell-shaped curve indicates the presence of at least two ionizable groups with different pKa
values. c) Structure of a ribonuclease. You can view the enzyme using JMol using the link on the
problem set page. The JMol page provides instructions on how to visualize the location of
individual residues, or multiple residue