2002 The Journal of biological chemistry Mechanism / Preclinical Unspecified

2002 · Avvakumov et al. — Crystal structure of human sex hormone-binding globulin in complex with 2-methoxyestradiol reveals the molecular basis for high affinity interactions with C-2 derivatives of estradiol

Original title: Crystal structure of human sex hormone-binding globulin in complex with 2-methoxyestradiol reveals the molecular basis for high affinity interactions with C-2 derivatives of estradiol.

Super-Abstract

This in-vitro structural biology study resolves the crystal structure of human sex hormone-binding globulin (SHBG) bound to 2-methoxyestradiol, explaining at the atomic level why certain estradiol derivatives bind with high or low affinity. Molecular hydrogen (H₂) appears only as a structural chemistry term (hydrogen bonding between amino acid residues and the steroid). This has no connection to H₂ supplementation or any H₂ health application.

Classified as a Mechanism / Preclinical study using Unspecified. See Methodology for how we grade evidence.

Commentary

SHBG is a blood transport protein that binds sex steroids (estradiol, testosterone) and regulates their bioavailability. This crystallography study describes how 2-methoxyestradiol (2-MeOE2) — a naturally occurring estrogen metabolite with anti-proliferative activity — docks in the SHBG steroid-binding pocket. Key molecular interactions identified include: a hydrogen bond between the C-3 hydroxyl of 2-MeOE2 and Asp65, and an interaction between the C-2 methoxy group and Asn82. These structural insights explain differences in binding affinity among C-2 estradiol derivatives. A zinc-binding site that reduces SHBG affinity for estrogens is also described. The term „hydrogen” here refers exclusively to hydrogen bonds — the non-covalent electrostatic interactions fundamental to all protein-ligand binding — and in no way refers to molecular hydrogen gas (H₂). This paper is a structural pharmacology study that could inform drug design for estrogen-related conditions; it has no therapeutic H₂ dimension.

Key quotes

„The high affinity of SHBG for 2-MeOE2 relies primarily on hydrogen bonding between the hydroxyl at C-3 of 2-MeOE2 and Asp(65) and an interaction between the methoxy group at C-2 and the amido group of Asn(82).“ — „hydrogen bonding” here is a structural chemistry term for intermolecular non-covalent interactions — not H₂ gas
„Understanding how C-2 derivatives of estradiol interact with SHBG could facilitate the design of biologically active synthetic estrogens.“ — the paper's practical implication: guiding synthetic estrogen drug design, not H₂ therapy
„Occupancy of a zinc-binding site reduces the affinity of SHBG for 2-MeOE2 and estradiol in the same way.“ — zinc modulates SHBG binding — a secondary structural finding

Our assessment

A structural biology / crystallography study describing protein-ligand interactions involving hydrogen bonds in the chemical sense. This paper has absolutely no connection to molecular hydrogen (H₂) as a health or supplementation agent. The word „hydrogen” in the abstract refers to hydrogen bonds — a routine structural chemistry term — not dissolved H₂ gas.

Study design

Type: in-vitro structural biology study · Model: recombinant human SHBG amino-terminal domain; X-ray crystallography · H₂ relevance: none — „hydrogen bonding” = protein-ligand non-covalent interactions
Result: crystal structure of SHBG/2-MeOE2 complex resolved; key binding interactions identified (Asp65, Asn82); explains differential affinities of C-2 estradiol derivatives; zinc modulation described

Abstract

In a crystal structure of the amino-terminal laminin G-like domain of human sex hormone-binding globulin (SHBG), the biologically active estrogen metabolite, 2-methoxyestradiol (2-MeOE2), binds in the same orientation as estradiol. The high affinity of SHBG for 2-MeOE2 relies primarily on hydrogen bonding between the hydroxyl at C-3 of 2-MeOE2 and Asp(65) and an interaction between the methoxy group at C-2 and the amido group of Asn(82). Accommodation of the 2-MeOE2 methoxy group causes an outward displacement of residues Ser(128)-Pro(130), which appears to disorder and displace the loop region (Leu(131)-His(136)) that covers the steroid-binding site. This could influence the binding kinetics of 2-MeOE2 and/or facilitate ligand-dependent interactions between SHBG and other proteins. Occupancy of a zinc-binding site reduces the affinity of SHBG for 2-MeOE2 and estradiol in the same way. The higher affinity of SHBG for estradiol derivatives with a halogen atom at C-2 is due to either enhanced hydrogen bonding between the hydroxyl at C-3 and Asp(65) (2-fluoroestradiol) or accommodation of the functional group at C-2 (2-bromoestradiol), rather than an interaction with Asn(82). By contrast, the low affinity of SHBG for 2-hydroxyestradiol can be attributed to intra-molecular hydrogen bonding between the hydroxyls in the aromatic steroid ring A, which generates a steric clash with the amido group of Asn(82). Understanding how C-2 derivatives of estradiol interact with SHBG could facilitate the design of biologically active synthetic estrogens.

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