Assignment #2: Structure of PTH

Structure of PTH

Parathyroid hormone is encoded by a gene on the short arm of chromosome 11 in humans (Many, 2005). The gene for the other calcium homeostasis hormone, calcitonin, is also located on the same arm of this chromosome, however it is not known if this is significant to regulation.

PTH is an 84-amino acid polypeptide derived from a prohormone (Griffin & Ojeda, 2004). The initial preProPTH is a 115-amino acid chain that is cleaved to ProPTH and then the remaining 6 amino acids at the N terminal are cleaved to produce PTH (Many, 2005). It has been shown in various experiments that the conversion of PreProPTH to PTH occurs in the Golgi apparatus of parathyroid gland chief cells and the reaction is catalyzed by a trypsin-like protease (Hadley & Levine, 2006). The final polypeptide, with a molecular weight of 9500 Da, is packaged in a secretory vesicle migrates to the cell periphery when the gland is stimulated. The hormone is released from the cell by exocytosis.




The degree of similarity in the amino-acid sequence of a hormone (or precursor in this case) found across various genera is a valuable tool to assess differences and importance of the hormone’s function. This type of comparison can also be used to assess evolutionary conservation and infer phylogenetic relationship based on the degree of similarity, as seen in Figure 2 below. Blast and ClustalW were used to obtain protein sequence alignments for the PreProParathyroid hormone between cat, pig, horse, human and mouse genera, as shown in Figure 1.

Key:
* Amino acid residues that are identical between all species compared
: Substitutions which are conserved
. Substitutions which are semi-conserved

Color Key:
RED (AVFPMILW) - Small (small+ hydrophobic (including aromatic -Y))
BLUE (DE) - Acidic
MAGENTA (RK) - Basic
GREEN (STYHCNGQ) - Hydroxyl + Amine + Basic – Q

Figure 1. CLUSTAL 2.0.8 multiple sequence alignment for PreProParathyroid hormone in cat, pig, horse, human and mouse genera.



Figure 2. Phylogenetic comparison of Homo, Mus, Eqqus, Canis and Sus based upon the degree of similarity in the Parathyroid hormone precursor

The PTH receptor has a seven-transmembrane helical structure on which both PTH and another protein, PTHrP (PTH related peptide) bind and induce conformational changes (Many, 2005). This receptor is coupled to G-proteins and stimulates adenylyl cyclise and phospholipase C in presence of bound PTH. There is evidence that the N-terminal region of PTH is critical in order to activate the receptor, and amino-acid residues 17 to 31 are needed for the peptide to bind to the receptor with high affinity.

References:

Griffin, J. E., Ojeda, S. R. 2004. Textbook of Endocrine Physiology. Oxford University Press, New York.

Hadley, M. E., Levine, J. E. 2006. Endocrinology, 6th Ed. Pearson Prentice Hall, New Jersey.

Many, T. N. 2005. Molecular Biology of the Parathyroid. Kluwer Academic/Plenum Publishers, New York.

Assignment 1: Description of PTH

Parathyroid Hormone

Parathyroid hormone, in association with calcitonin and vitamin D, is responsible for regulating calcium metabolism in the body (Griffin & Ojeda, 2004). In adults, the primary action of PTH is to raise serum calcium levels when the body senses a low concentration of ionized calcium in the blood. To induce a response that raises extracellular calcium levels, PTH acts on bone, kidneys and intestine (Many, 2005) in the following ways:

(i) Stimulating specific bone cells that release stored calcium
(ii) Increasing calcium absorption from the gut in a pathway involving vitamin D (1,25-(OH)2 D)
(iii) Acting on the kidney to enhance the calcium being reabsorbed from the urine (Hadley & Levine, 2006).

The parathyroid glands, which secrete parathyroid hormone,

are small pea-sized glands found posterior to the thyroid gland (Griffin & Ojeda, 2004). Four of these glands are normally found, but a small percentage of the population can have as few as two or as many as eight. The parathyroid gland produces PTH by first synthesizing pre-pro-PTH which is cleaved to pro-PTH and then to PTH, an 84-amino acid polypeptide. The hormone is secreted to the bloodstream directly after it is produced and has a half-life of less than 5 minutes in the serum.

The amount of PTH produced depends on the ionized calcium concentration of the blood and is regulated by a negative feedback mechanism (Griffin & Ojeda, 2004). In another feedback loop, the amount of vitamin D directly inhibits PTH secretion. These feedback loops are important to protect the individual from hypercalcemia. Due to these processes, the calcium concentration of blood is kept within a narrow range optimal for proper functioning (Hadley & Levine, 2006).

References:

Griffin, J. E., Ojeda, S. R. 2004. Textbook of Endocrine Physiology. Oxford University Press, New York.

Hadley, M. E., Levine, J. E. 2006. Endocrinology, 6th Ed. Pearson Prentice Hall, New Jersey.

Many, T. N. 2005. Molecular Biology of the Parathyroid. Kluwer Academic/Plenum Publishers, New York.