There is no other molecule like hCG.
Human Chorionic Gonadotropin (hCG), a glycoprotein hormone produced by the human placenta, is composed of an alpha and a beta sub-unit with a molecular mass of 36.7 kDa. The α (alpha) subunit is 92 amino acids long. The β-subunit of hCG gonadotropin contains 145 amino acids.
The α-subunit has 2 N-linked oligosaccharides, and a non-covalently linked acid ß-subunit with 2 N-linked oligosaccharides and 4 O-linked oligosaccharides. The structure of each variant of hCG varies considerable in carbohydrate structure. hCG molecules also vary greatly in charge, due to variation in sialic acid sugar content.
The α sub-unit is essentially identical to the α sub-units of the human pituitary gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), as well as to the alpha sub-unit of human thyroid-stimulating hormone (TSH). The beta sub-units of these hormones differ in amino acid sequence and thus confer biological specificity of the hormones.
The different composition of these oligosaccharides affects bioactivity and speed of degradation. hCG is the longest circulating molecule in human blood with a circulating half life of 36 hours (2160 minutes). LH has a circulating half-life of ~25 minutes. Effectively, hCG extends its biological activity 2160/25 or 86-fold over LH.
The action of hCG is virtually identical to that of pituitary LH, although hCG appears to have a small degree of FSH activity as well. It stimulates production of gonadal steroid hormones by stimulating the interstitial cells (Leydig cells) of the testis to produce androgens and the corpus luteum of the ovary to produce progesterone.
Human LH (hLH) and CG (hCG) also differ in some structural features, such as the presence of a carboxyl terminal peptide (CTP) and the type and amount of glycosylation. Due to this heterogeneity and derivation from extractive preparations, gonadotropins have been difficult to quantify accurately in the past, and most in vitro experiments have been conducted using urinary hCG calibrated by in vivo bioassay against standard preparations expressed in activity units.
With the advent of recombinant gonadotropins, highly homogeneous and consistent r-hLH and r-hCG can be accurately quantified in molar terms and used to compare their effects in vitro at exactly equimolar concentrations.
Being structurally different, it should be expected that hLH and hCG display different hormone-receptor interaction features and, consequently, might be not equivalent at molecular and cellular level.
This was found to be true in a recent study, “LH and hCG Action on the Same Receptor Results in Quantitatively and Qualitatively Different Intracellular Signalling.” They concluded the LH/hCGr is able to differentiate the activity of hLH and hCG. Any statement that LH & hCG are bioequivalent is clearly wrong.
