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Biosynthesis of Peptide and Steroid Hormones - Essay Example

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Biosynthesis of Peptide and Steroid Hormones Instructor University Biosynthesis of Peptide and Steroid Hormones Introduction: Hormones are chemical messengers synthesized by endocrine organs and released directly into the blood stream and are carried to distant target organs…
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Biosynthesis of Peptide and Steroid Hormones
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The number of amino acids constituting the hormones can range from three to up to 50. Peptide hormones mediate diverse effects on the body and have a range of target areas and sites of synthesis. The first step involved in the formation of peptide hormones is transcription which results in the formation of a heterogeneous nuclear RNA that is spliced and translated into a peptide chain having a characteristic amino acid sequence. The inactive precursors formed have a single sequence on their N-terminus which binds to signal recognition particles in a GTP dependent process and thereby direct the ribosomes to the rough endoplasmic reticulum. A prohormone is formed by splitting off a single peptide by a single peptidase when the growing peptide chain is transferred to the ER. After formation, the prohormone is folded and undergoes post translational modifications such as disulphide bond formation and glycosylation. At this stage, the cleavage of certain substrates of prohormone convertases occurs however, a major portion of post translational processing occurs in the trans-Golgi network or in the secretory pathway. The folded prohormones are packed in vesicles and are transferred from the ER to the Golgi apparatus. In trans cisternae of the Golgi apparatus, processes such as phosphorylation and sulphation occur. The process of sulphation is mediated by a tyrosyl protein sulphotransferase. The resulting prohormones are packed into secretory vesicles. Once inside the vesicles, the prohormones are converted into biologically active hormones which are released into the blood stream upon the arrival of an appropriate stimulus. (HUGHES 2009). Steroid Hormones: The precursor of all classes of steroid hormones is cholesterol. Glucocorticoids, mineralocorticoids, and sex hormones are all synthesized from cholesterol. The adrenal cortex, ovaries, testes, and placenta are the sites of synthesis and secretion of steroid hormones. The first step in the synthesis of steroid hormones is the shortening of the cholesterol chain and its subsequent hydroxylation. The conversion of cholesterol into 21-carbon pregnenolone is the initial and the rate limiting step in the formation of steroid hormones. The reaction is catalyzed by a cytochrome P450 mixed function oxidase present in the inner mitochondrial membrane and is known as cholesterol side-chain cleavage enzyme complex (desmolase, P450scc).The movement of cholesterol substrate into the inner mitochondrial membrane is mediated by steriodogenic acute regulatory protein. The next step in the formation of steroid hormones involves oxidation and isomerization of pregnenolone to progesterone. Hydroxylation of progesterone in ER and mitochondria results in the formation of steroid hormones. The enzymes involved in hydroxylation are CYP proteins. Once formed, the steroid hormones owing to their lipid solubility diffuse through the cell membrane and enter the blood stream. (HARVEY et al 2011). Similarities: The synthesis of peptide and steroid hormone is not a direct process and involves a series of inter depended enzymatically controlled reactions. Both classes of hormones are initially synthesized as biologically inactive molecules which are later converted into biologically active molecules. The biosynthesis of peptide and steroid hormones involves a set of complex reactions and each of these reactions is controlled by enzymes. Differences: The biosynthesis Read More
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