steroid structure and function from X-ray crystallographic studies

The characteristic actions of each steroid hormone are known to be contingent on the high-affinity binding of the steroid to specific receptor proteins in the target tissues.^{1 - 3} Furthermore, hormonal function is modulated by interaction of the steroids with synthesizing and metabolizing enzymes as well as binding proteins in the blood and some specific tissues. Steroid function can be altered by synthetic hormones or antihormones that selectively interfere with protein interactions. While receptor binding is essential if certain hormonal responses are to take place, it is not sufficient. Many antihormones bind to steroid receptors but fail to elicit the characteristic responses. Structural similarities between the many natural and synthetic steroids result in a complex pattern of competition for the binding sites on receptor proteins, binding proteins, and metabolizing enzymes. Consequently, a thorough understanding of the molecular basis of these steroid-protein interactions is essential for the rational design of steroid drugs.