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cussed in Chapters 37 and 38. Here, we describe the chemistry and formation of the gonadotropins.

Like TSH, human FSH and LH are composed of two structurally different glycoprotein subunits, called a and P, which are held together by noncovalent bonds. The p subunit of human FSH consists of a peptide chain of 111 amino acid residues, to which two chains of carbohydrate are attached. The P subunit of human LH is a peptide of 121 amino acid residues. It is also glycosylated with two carbohydrate chains. The combined a and P subunits of FSH and LH give these hormones a molecular size of about 28 to 29 kDa.

As with TSH, the individual subunits of the go-nadotropins have no hormonal activity. They must be combined with each other in a 1:1 ratio in order to have activity. Again, it is the P subunit that gives the go-nadotropin molecule either FSH or LH activity because the a subunits are identical.

FSH and LH are produced by the same gonadotrophs in the anterior pituitary. There are separate genes for the a and P subunits in the gonadotroph; hence, the peptide chains of these subunits are translated from separate mRNA molecules. Glycosylation of these chains begins as they are synthesized and before they are released from the ribosome. The folding of the subunit peptides into their final three-dimensional structure, the combination of an a subunit and a P subunit, and the completion of glycosyla-tion all occur as these molecules pass through the Golgi apparatus and are packaged into secretory granules. As with the thyrotroph, the gonadotroph produces an excess of a subunits over FSH and LH P subunits. Therefore, the rate of P subunit production is considered to be the rate-limiting step in gonadotropin synthesis.

The synthesis of FSH and LH is regulated by the hormones of the hypothalamic-pituitary-gonad axis. For example, gonadotropin production is stimulated by LHRH. It is also affected by the steroid and peptide hormones produced by the gonads in response to stimulation by the gonadotropins. Such hormonally regulated changes in go-nadotropin production are caused mainly by changes in the expression of the genes for the gonadotropin subunits. More information about the regulation of gonadotropin synthesis and secretion is found in Chapters 37 and 38.

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Essentials of Human Physiology

Essentials of Human Physiology

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