Figure 23-17 1 X-Ray structure of human dihydrofolate reductase in complex with folic acid. The polypeptide is colored in rainbow order from its N-terminus (blue) to its C-terminus (red) The folic acid is drawn in space-filling form with C green, N blue, and O red. Based on an X-ray structure by Joseph Kraut, University of California at San Diego. PDBid 1DHF) see Interactive Exercise 34.