Douglas C. Rohrer

Molecular conformation and protein binding affinity of progestins.

Analysis of X-ray data concerning 277 estranes, androstanes, and pregnanes and comparison with progesterone receptor binding data have prompted the following observations. In general: 1. The flexibility of natural steroid hormones permits them to take up conformations optimal for binding to sites on proteins that vary in individual structural requirements. 2. When substituents strain the fused ring system, the strain will be delocalized and often transmitted to the most flexible point of the molecule, thus giving rise to conformational transmission effects. Consequently, substituents will generally stabilize a specific conformation, limiting protein interaction and enhancing a specific hormone response. 3. Hydrogen bond patterns in crystals can be used to predict points of active site attachment. 4. Distortions resulting from crystal packing forms are insignificant. Progestin receptor binding affinity: 5. Complementarity of fit is not specific on the alpha and beta faces of the B, C, and D rings. 6. The delta4-3-one composition is the only consistently required element. 7. Five of the eight highest-affinity binders have inverted A rings. Others may be easily converted to it. 8. The inverted A ring is proposed as the optimal conformation and primary factor controlling binding. 9. An A ring binding pattern is apparent in other steroidal hormones. 10. The D-ring region is open to contribute to conformational change in the receptor or genome interaction.

Molecular details of receptor binding and hormonal action of steroids derived from X-ray crystallographic investigations

Abstract Analysis of X-ray crystallographic data on steroids provides information concerning preferred conformations, relative stabilities, and substituent influence on the interactive potential of steroid hormones. Analysis of the data on the 4-ene-3-one ring indicates that it normally has a conformation midway between the 1α,2β-half chair and the 1α-sofa forms. Strain introduced into the molecule by substitution on the fused ring system shifts the A-ring conformation toward the more symmetric forms with an attendant change in the conjugation of the 4-ene-3-one system. Crystallographic data on 85 pregnane structures having a 20-one substituent provide information on 17β-side chain flexibility. In eighty-one structures the C(16)-C(17)-C(20)-O(20) torsion angle is between 0° and −46°. This is consistent with CD, i.r. and n.m.r. solution spectra and more precisely defines the side chain conformation in solution. The four structures whose side chains lie outside this range do so because of the presence of a 16β-substituent and not because of crystal packing forces. The steroid A ring appears to be primarily responsible for initiating and maintaining hormone binding to the estrogen and progestin receptors. When the structures of agonists and antagonists of specific steroid hormones are compared, they generally exhibit similarities in their A-ring region and dissimilarities in the D-ring region further suggesting that the steroid A-ring bears responsibility for receptor binding while the D-ring controls expression of activity. Antihormoncs that compete for the receptor site of a steroid hormone may be expected to have structural features appropriate for receptor binding (A-ring composition and conformation) and lack structural features that induce or stabilize subsequent receptor functions (D-ring conformatjonal features and functional groups).

Crystal structure of steroids: molecular conformation and biological function.

Publisher Summary Many biological functions depend directly upon the presence of a particular steroid. However, even in the most thoroughly studied systems, the nature of the molecular level events involved in this structural–functional dependence is not fully understood. Steroid function is dependent upon molecular composition, constitution, configuration, and conformation. Composition defines the number and kinds of atoms that make up the molecule and is readily represented by its chemical formula. Constitution refers to the connectivity of the molecule and is best illustrated by diagrammatically showing which atoms are bonded to one another. Molecular configuration defines the chirality of all asymmetric centers in the molecule. Conformation refers to the total geometric distribution or disposition of the atoms in three dimensions. Cortisol is active and cortisone is inactive primarily as a result of compositional and constitutional differences at carbon C(11). The functional dissimilarity between testosterone and epitestosterone stems from a configurational difference at carbon C(17). The conformational feature of the individual hormones contains subtler aspects of structural dependence. The most precise and accurate details of molecular composition, configuration and conformation are provided by X-Ray crystal structure determinations. This chapter summarizes principal conclusions of comprehensive analysis of steroid structural data.

Conformational studies of steroids: Correlations with biological data

Abstract X-Ray crystal structure analysis provides precise and accurate data concerning steroid structure and conformation. The crystal and molecular structure data of over 200 steroids are being collected and analyzed in the Atlas of Steroid Structure. This extensive analysis plainly shows that crystal packing forces have little influence upon molecular conformation and that the overall conformation of steroids and the orientation of functional groups are clearly controlled by intramolecular forces. The feasibility of specific interactions with protein molecules is in turn dependent upon overall conformation. Information concerning both of these aspects of steroid behavior is provided by crystal structure studies. The details of molecular mechanisms of conformational transmission and the nature of correlations between molecular structure and function can be proposed on the basis of these data. The most active naturally occurring steroids are observed to possess the greatest degree of conformational flexibility. Significantly different conformers of a hormone are probably responsible for different aspects of its action in the biological setting. In synthetic steroidal hormones further substitution on the steroid nucleus generally removes conformational flexibility. The less flexible molecule may now be a better competitor than the natural hormone for one type of protein interaction, but be much less competitive in other protein interactions, as is suggested by the highly specific activity enhancement of synthetic steroidal hormones.

The molecular structure of an aldosterone 18-glucopyranosiduronate

Abstract The crystal and molecular structure of aldosterone 18R-α- d -glucopyranosiduronic acid tetraacetate methyl ester was determined by X-ray analysis in order to ascertain the stereochemistry of the glycosidic bond and the configuration at C(18). The C(18) stereochemistry was found to be the same as that observed in the crystal structure of aldosterone monohydrate, and the C-ring has similar strain as shown by the C(11)-C(12)-C(13) valency angle of 98°. The sugar ring, which has a 4 C 1 conformation, is approximately parallel to the steroid nucleus and is located above the aldosterone 17β-side chain.

The molecular structure of 3-methoxy-9 (10 → 19)abeo-1, 3, 5 (10)-estratrien-17-one

Abstract The crystal and molecular structure of 3-methoxy-9 (10 → 19) abeo -1, 3, 5 (10)-estratrien-17-one has been determined by X-ray analysis in order to ascertain the configuration at C (9). The seven-membered B-ring has a chair conformation and this causes the molecule to bow towards the α-face rather than towards the β-face as in estradiol.

CONFORMATIONAL STUDIES OF STEROIDS: CORRELATIONS WITH BIOLOGICAL DATA

X-Ray crystal structure analysis provides precise and accurate data concerning steroid structure and conformation. The crystal and molecular structure data of over 200 steroids are being collected and analyzed in the Atlas of Steroid Structure. This extensive analysis plainly shows that crystal packing forces have little influence upon molecular conformation and that the overall conformation of steroids and the orientation of functional groups are clearly controlled by intramolecular forces. The feasibility of specific interactions with protein molecules is in turn dependent upon overall conformation. Information concerning both of these aspects of steroid behavior is provided by crystal structure studies. The details of molecular mechanisms of conformational transmission and the nature of correlations between molecular structure and function can be proposed on the basis of these data. n nThe most active naturally occurring steroids are observed to possess the greatest degree of conformational flexibility. Significantly different conformers of a hormone are probably responsible for different aspects of its action in the biological setting. In synthetic steroidal hormones further substitution on the steroid nucleus generally removes conformational flexibility. The less flexible molecule may now be a better competitor than the natural hormone for one type of protein interaction, but be much less competitive in other protein interactions, as is suggested by the highly specific activity enhancement of synthetic steroidal hormones.

Applications of the PROPHET system in correlating crystallographic structural data with biological information

The goal of the Molecular Biophysics Department at the Medical Foundation of Buffalo is to establish relationships between the physical structures of molecules and their biological activities. At present, the steroid and thyroid hormones, their derivatives, analogs, and inhibitors are the materials of major interest in this research, but work on other groups of biologically active molecules is anticipated or in progress. The PROPHET system provides a powerful medium for the assembly, storage, and analysis of both structural and biological data. Correlative studies of these two types of data are particularly important because they may lead to an understanding of the molecular level mechanisms of action of hormones, drugs, antibiotics, and other biological substances. The PROPHET system is well suited for this type of analysis because it permits communication and interaction among scientists who are experts on various phases of molecular biology.