Binding to hormones is noncovalent and reversible.

Membrane Receptors
 

• Imbedded in the cell membrane, integral proteins, glycoproteins
• At the surface of target cells for protein molecules and charged hormones (peptides or neurotransmitters)


Three major groups
 

• Serpentine (7 transmembrane domains)
• Growth factor, cytokine receptors (1 transmembrane domain)
• Ion channels

 

• In the nucleus for small, neutral, hydrophobic molecules (steroids, thyroid hormones)
• Often involve cycling of R from cytoplasm where it resides as an inactive complex with heat shock proteins (HSPs) until binding of H and/or phosphorylation or dephosphorylation of the R or HSPs to the nucleus where the HR complex binds to specific Hormone Recognition Elements (HREs) or sequences on the DNA in the promoter regions of target genes. The HR often binds as a homodimer to palindromically oriented HREs and bends the DNA at the site of binding to allow transcriptional enzymes access to gene sequences. The HR may disaggregate as H levels fall or the complex may be phosphorylated or dephosphorylated so as to cause H loss and movement of the R back to the cytoplasm where HSPs can again bind.