Role of “rough endoplasmic
reticulum” and “Golgi apparatus in synthesis and storage of Glycoprotein
Hormones in basophils cells of anterior pituitary,:-
The glycoprotein hormone family includes
TSH, and the two gonadotropins. The three glycoprotein hormones are synthesized and stored
in pituitary basophils .
As their name implies, each contains
sugar moieties covalently linked to asparagine residues
in the polypeptide chains. All three are comprised of two peptide subunits,
designated alpha and beta, which, though tightly coupled, are not covalently
linked. The alpha subunit of all three hormones is identical in its amino acid sequence, and is the product of a single gene located on chromosome 6.
But the beta subunits of each are somewhat larger than the alpha subunit and
confer physiological specificity. Both alpha and beta subunits contribute
to receptor binding and both must be present in the receptor binding
pocket to produce a biological response.
Beta subunits are encoded in
separate genes located on different chromosomes:for example :- TSH β on chromosome 1, FSH β on chromosome 11, and LH β on chromosome 19, but there is a great deal of homology in their amino acid
sequences. Both subunits contain carbohydrate moieties that are considerably
less constant in their composition than are their peptide chains.
Alpha subunits are synthesized in excess over
beta subunits, and hence it is synthesis of beta subunits that appears to be
rate-limiting for production of glycoprotein hormones. Pairing of the two
subunits begins in the rough endoplasmic
reticulum and continues in the Golgi apparatus, where processing of carbohydrate components of the
subunits is completed. The loosely paired complex then undergoes spontaneous
refolding in secretory granules into a stable, active hormone. Control of expression
of the alpha and beta subunit genes is not perfectly coordinated, and free
alpha and the beta subunits of all three hormones may be found in blood plasma.
The glycoproteins. The placental hormone, human chorionic
gonadotropin (hCG), is closely related chemically and functionally to the
pituitary gonadotropic hormones. It, too, is a glycoprotein and consists of an
alpha and a beta chain. The alpha chain is a product of the same gene as the
alpha chain of pituitary glyco-protein hormones. The peptide sequence of the beta chain is identical to that
of LH except that it is longer by 32 amino acids at its carboxyl terminus.
Curiously, although there is only a
single gene for each beta subunit of the pituitary glyco-protein hormones, the human genome contains 7 copies
of the hCG beta gene, all located on chromosome 19 in close proximity to
the LH beta gene. Not surprisingly, hCG has biological actions that are similar
to those of LH, as well as a unique action on the corpus luteum
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