Introduction 
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Diffraction pattern 
Electron-density 
maps of sections 
tlirougli the molecule 
Electron-density map 
I 
Figure 25. The use of x-rays to determine the structure of enzymes. When an enzyme is 
in a crystalline form, its molecules are arranged in regularly repeating arrays. A beam 
of x-rays passed through the enzyme is diffracted to give a pattern of spots on a detec- 
tor screen behind the sample. Each spot contains information about the electron den- 
sity ( and hence the types of atoms: carbon, oxygen, nitrogen, etc.) in various parts of 
the crystal. This information can be used to construct electron density maps whose 
contours indicate the type of atoms present. The contour map shown at the right 
represents a short segment of polypeptide backbone with a tyrosine side chain. 
Adapted from Geoffrey Zubay, Biochemistry, Addison- Wesley, 1983. 
Electron-density map 
with inferred polypeptide 
backbone and tyrosine 
side chain fitted in 
- - "V 
B 
Figure 26. A: The x-ray diffraction pattern given by the enzyme chymotrypsin. 
Courtesy of Thomas A. Steitz. 
B: A representation of the three-dimensional structure of chymotrypsin. Carbon atoms are shown in 
black, nitrogen in blue, oxygen in red, hydrogen in white, and sulfur in yellow. The diameter of the enzyme 
is about 45 A (somewhat less than a millionth of an inch). The hydrophilic side chain of arginine- 145 is 
clearly visible projecting outward from the right side of the molecule. The ridges and grooves on the surface 
of the chymotrypsin molecule are as unique as the mountains and craters of the moon, and herein lies the 
fulfillment of the lock-and-key mechanism hypothesized by Emil Fischer at the turn of the century. 
Courtesy of Polygen Corporation, Waltham, MA. 
Iviii 
