(b) The binding site of dUMP, showing the hydrogen a is bonding system responsible for the enzyme's specific binding of are uracil ring. The dUMP and the polypeptide backbone binding it shown in ball-and-stick form with atoms colored as in Part a, hydrogen bonds are indicated by white dotted lines; and a conserved water molecule is represented by a pink sphere. The side chain of a e conserved Tyr is tightly packed against the ribose ring so as to of discriminate against the presence of a 2'-OH group. [Courtesy La John Tainer and Clifford Mol, Scripps Research Institute, The Jolla, California)