Endo 3: Genes and enzymes: hormone synthesis Synthesis of protein and peptide hormones by gene transcription Post transcriptional and post translational modifications, packaging of hormones Hormone diversity Synthesis of steroid hormones from small molecular weight presursors Steroidogenic enzymes Transport and peripheral conversion of steroid hormones

Hormone synthesis Peptides and proteins Water soluble, made from large precursor molecules - prohormones. Steroids and iodinated tyrosines Lipid soluble, made from low molecular weight precursors Morphology: Peptides - secretory granules, RER. Steroids - lipid droplets

Protein/peptide secreting cellSteroid secreting cells

Hormone synthesis

Synthesis of protein and peptide hormones Gene transcription (controlled by transcription factors)  Processing of the initial mRNA transcript to ‘mature’ mRNA – excision of introns – and modifications of the 3’ and 5’ ends  Translation of mRNA Bound to the polyribosomes of RER, the mRNA acts as a template for assembling amino acids by way of tRNA  Post translation processing Cleavage of large pro- hormone; can also include folding of polypeptide chain and addition of carbohydrates (glycosylation)

Transcription Post-transcriptional modifications - removal of introns, 5’ & 3’ modifications Translation of mature RNA Post-translational processing - cleavage of pro-hormone, folding of protein molecule, glycosylation

Packaging of hormones Signal sequence docking protein on RER As amino acids are assembled they are internalized into the RER RER Golgi apparatus Formation of secretory granules storage Secretion by exocytosis

Packaging of hormones Signal sequence finds docking protein on RER As amino acids are assembled they are internalized into the RER RER Golgi apparatus Formation of secretory granules storage Secretion by exocytosis

Membrane traffic in hormone-secreting cells

Synthesis active hormone Signal sequence + prohormone = pre-prohormone Active hormone + redundant sequence prohormone signalhormoneredundant sequence

Synthesis of insulin

Diversification of protein and peptide hormones Gene duplication - homologous families of hormones Processing of mRNA: One gene two hormones Post translational processing One prohormone several hormones

Homologous hormones Growth Hormone Prolactin Ancestral gene Growth hormone Prolactin Human placental lactogen Also oxytocin and vasopressin

Homologous families of hormones

Homologous gl ycoprotein hormones LH(hCG) FSH TSH  and  sub-units.  unit common,  unit biological activity Ancestral gene gonadotrophin TSH LH (hCG) FSH

One gene, two hormones - post transcriptional modifications

One gene several hormones - post translational modificatioons

Steroid synthesis

Steroid hormone synthesis Cholesterol bound to sterol transfer protein - transported to mitochondria StAR protein transports cholesterol to inner mitochondrial membrane Cholesterol to pregnenolone by side chain cleavage enzyme, P450scc Between mitochondria and SER steroids synthesised by hydroxylase enzymes

STEROID SYNTHESIS SIMPLIFIED Cholesterol Pregnenolone Progesterone 17  OH Progesterone Aldosterone Cortisol Androgens Oestrogens Steroids highly conserved

Outline of steroid synthesis in the adrenal cortex and gonads

Steroid synthesis in the adrenal cortex Genes coding for the P450 enzymes are abbreviated to CYP Most steps in steroid synthesis require cytochrome P450 heme- containing enzymes

Oestrone (E 1 ) Oestradiol (E 2 ) AndrostenedioneTestosterone PregnenoloneProgesterone 17  -hydroxyprogesterone 17  -HSD type 5 17  -HSD type 1 3  -HSD Aromatase 17  hydroxylase Conversion of androgens to oestrogens by aromatase

Synthesis of thyroid hormones Active uptake of iodine into follicular cell Oxidation of iodide to iodine Iodination of tyrosine residues of thyroglobulin and apical-colloid interface Storage in colloid Uptake of thyroglobulin droplets into follicle cell Release and secretion of T 3 and T 4

Active uptake of iodine Oxidation of iodide to iodine Iodination of tyrosine residues at apical/colloid interface to form MIT and DIT Uptake of thyroglobulin into the lumen of the follicle

Peripheral conversion of thyroid and steroid hormones T 4  T 3 Testosterone  dihydrotestosterone Testosterone  oestradiol Vitamin D (synthesised in skin)  dihydroxy vitamin D.

Transport of steroid and thyroid hormones An equilbrium exists between bound and free hormone. H + BG H.BG K eq = [H.BG] (H bound) [H] [BG] (H free) [H] = [H.BG] x 1 K eq [BG] e.g. transcortin, sex hormone binding globulin, transthyretin