Gonadotropin Subunits

FSH, LH, and hCG, together with thyroid-stimulating hormone (TSH), comprise the family of glycoprotein hormones. Each hormone is composed of two subunits that are coupled by noncovalent interactions: the common a-subunit of 92 amino acids (aa) and the P-subunit; 110 aa in FSH, 121 aa in LHp, and 145 aa in hCGp (see Fig. 1). The different P-subunits of the glycoprotein hormones are responsible for their functional specificity. hCG is structurally similar to LH, and both hormones have similar actions. Gonadotropins are glycosylated through ^-linked bonds (see Fig. 1), and the carbohydrate content of LH is approx 15%, of FSH is 20%, and of hCG is 30% (1,2). Most conspicuously, the carbohydrate moieties increase the circulatory half-lives of the gonadotropins, which are approx 20 min for LH, approx 2 h for FSH, and 12-24 h for hCG. The long half-life of hCG is primarily explained by the 24-aa C-terminal extension of its P-subunit, in comparison to LHP, which is heavily glycosylated through O-linked glycosylation sites (see Fig. 1). The short half-life of LH is a function of the high proportion of terminal sulphate groups in its carbohydrates; a specific hepatic receptor accelerates the elimination of this type of glycoprotein from the circulation (3).

The common a-subunit gene is located on chromosome 6q12.21, FSHfi is found on chromosome 11p13, and the cluster of LHp/hCGP genes, consisting of one LHP and six hCGP genes and pseudogenes, is located on chromosome 19q13.32 (4,5). The common a-subunit gene consists of four exons, and the p-subunits are composed of three exons. The crystal structures of deglycosylated hCG (6) and FSH (7) are similar and reveal that both subunits contain so-called "cysteine knot structures" that are similar to some remotely related growth factor-type signaling molecules. Each subunit has an elongated shape, with two P-hairpin loops on one side of the central cysteine knot and a long loop on the other side. The noncovalent interaction between the two subunits is stabilized by a segment of the P-subunit that extends like a "seatbelt" around the a-subunit and is "locked" by a disulfide bridge.

Good Carb Diet

Good Carb Diet

WHAT IT IS A three-phase plan that has been likened to the low-carbohydrate Atkins program because during the first two weeks, South Beach eliminates most carbs, including bread, pasta, potatoes, fruit and most dairy products. In PHASE 2, healthy carbs, including most fruits, whole grains and dairy products are gradually reintroduced, but processed carbs such as bagels, cookies, cornflakes, regular pasta and rice cakes remain on the list of foods to avoid or eat rarely.

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