Introduction

The book, Our Stolen Future, introduced compounds known as hormone mimics and their related problems to the general public in 1996.¹  Since the release of this book much research has been done to study hormone mimics, endocrine-disrupting chemicals, and the like.  Studies have been done to examine the mechanism by which synthetic mimics interact with hormone receptor proteins and how they disrupt normal endocrine activity in the cells of organisms.

The book gives a detailed account of complications such as birth defects and cancer that children have experienced whose mothers took a drug called diethylstilbestrol (DES) while they were pregnant.  The drug was given to pregnant women to reduce the risk of miscarriage.  At the time scientists and doctors believed that complications during pregnancy, such as miscarriages, were caused by insuffient levels of estrogen in the woman's body during the baby's development.  It turned out that the DES was a good estrogen mimic, but was not broken down by the body and eliminated like estrogen after it performed its function.  The hormone distruption that DES caused turning the baby's development did not nessecarily lead to problems immidiatly after birth, but usually showed up in complications, such as various cansers in the child's sex organs, during puberty.  Although its widespread use has been greatly diminished, DES is still being used and recently there has been talk of  using it to treat breast cancer.¹

Here we present a model for the Human Estrogen Receptor alpha (hERalpha) ligand-binding domain (LBD) bound to DES and a peptide derived from the nuclear receptor (NR) box II region of the coactivator GRIP1.  The derived peptide is a small alpha helix that binds to a hydrophobic groove on the surface of the LBD, which we will not worry about for this demonstration.²

Note: There are buttons imbedded in the text, which you can click on in order to see a view in the protein frame that corresponds to the text.

Protein Characteristics

Primary Structure:  hERalpha consists is a dimer that consists of two asymmetric nearly-identical units. Each subunits contains 261 residues. 

Secondary Structure: Each subunit contains 10-11 alpha helicies and 2 beta sheets.

Size: Molecular weight is 29796 kD per subunit.

Charge:

• View Acid/Base Residues: (Basic Residues - Red; Acidic Residues - Blue; Neutral - White)

Optimal pH = 8.5

Hydrophobic Regions:

• Areas of high hydrophobicity (Green)
• Charged Regions (Blue)

Solvent Accessible Surface Area

• MLP Surface Plot -- This will take a long time, but it interesting to explore the protein by moving it and zooming on it.  Look for the estrogen binding domain and the NR box II peptide fragment binding groove!

Binding Domains

Note: In the following demonstration, the animation is meant to be followed in sequence, so you should reload the molecule by clicking the Reload button now, then Starting View below the protein image.  In order for the demonstration to work best allow each step, after you click a button, to complete before clicking the next button.

DES Binding Site

Before we get into the details of how DES binds to the protein let's look at the ligands.  Below is the chemical structures of an estrogen (b-estradiol), which naturally binds to hERalpha, and the mimic DES.  Just looking at the similarities between the molecules, we can image how the DES can mimic the activity of estrogen in the hERalpha protein's binding domain.  Both molecules are planar, are similar in length, contain aromatic rings, and contain hydroxyl groups on each end of the molecules.

According to Shiau et al., DES does indeed bind to hERalpha in a similar fashion as the estrogen does.  One of the phenolic rings of DES lies in the same position as the beta-estradiol aromatic ring does.  In addition, it turns out that the when DES is bound to the receptor protein , the protein including the LBD have a similear conformation, which confirms the conclusion that DES interacts with the protein as an agonist (mimic).

We can see the DES molecule tucked away within the narrower half of the LBD in a predominantly hydrophobic cavity composed of residues from 6 of the helices.  Click to view a close-up of the DES binding site on one the protein's subunits.  Now click the next three buttons to see the binding site from a few different angles: top side bottom .

By removing everything but the amino acids in the binding site, we can get a good look at the interations that hold DES, and presumably estrogen, in the binding pocket.   Click to view the binding amino acids.  The DES aromatic ring A, which is in the upper right hand of the protein screen is engaged by the side chains of Phe-404, Ala-350, Leu-387, and Leu-391 with its phenolic hydroxyl forming hydrogen bonds to the gamma-carboxylate of the Glu-353, to the quanidinium group of Arg-394, and to a structurally concerved water molecule (not shown).  Click to see the hydrogen bonding amino acids highlighted in red.  Finally, click to retreive the entire protein again and display a Molecular Electrostatic Potential.  At this point feel free to use the mouse commands to move and zoom-in on the binding site and the rest of the protein.  It will be in slab view so as you move, you should be able to see into the binding domain.

Note:  Also, you are encouraged to go back into the text, reload, and reset the protein, and explore the structures in greater detail using the mouse commands.  If you have question, feel free to drop us an email by clicking on our names in the header.

THE END!

References:

1. Colborn, T., Dumanoski, D., and Myers, J. P. (1996) Our Stolen Future. Plume Books, New York, NY, USA.  See also their web sites for uptades:  http://www.osf-facts.org

2. A. K. Shiau, D. Barstad, P. M. Loria, L. Cheng, P. J. Kushner, D. A. Agard, G. L. Greene. (1998) The Structural Basis Of Estrogen Receptor/Coactivator Recognition and the Antagonism of this Interaction by Tamoxifen. Cell. 95: 927.

Other Interesting Sites:

• http://www.osf-facts.org/othersrc/enviro.html
• http://www.osf-facts.org/newsci/basemech.html