Biochemistry of hormones derived from amino acids and proteins Martina Srbová
Hormones derived from amino acids and proteins Hydrophilic signal molecules 1. Amino acid derivatives hormones Catecholamines! thyroxine lipophilic! 2. Protein hormones Small peptide hormones (thyrotropin releasing hormone, oxytocin, vasopressin) Protein hormones (insulin, growth hormone) Glycoprotein hormones (luteinizing hormone, follicle-stimulating hormone and thyroid-stimulating hormone)
Synthesis of protein hormones
Genes and formation of protein hormones Genes for protein hormones contain the information for the hormone 1. More than one hormone is encoded in a gene Proopiomelanocortin peptide family Vasopressin and neurophysin II; oxytocin and neurophysin I 2. Multiple copies of a hormone are encoded in a gene e.g. Enkephalins 3. Only one hormone is encoded in a gene e.g. CRH
1. Proopiomelanocortin (a single gen product) is a precursor peptide for nine hormones ACTH, β-lipotropin, γ-lipotropin, γ-msh, α-, β-msh, CLIP, β-endorphine, enkephalins Proopiomelanocortin occurs in both the corticotropic cells of the anterior pituitary and the pars intermedia cells, the products are different (under control of norepinephrine) CLIP-corticotropin-like intermediary peptide
Proopiomelanocortin peptide family Contains hormones (ACTH, LPH, MSH) and neurotransmitters Precursor molecule involves 285 amino acids Gene expression in the anterior and intermediary pituitary, but also in other tissues (intestine, placenta, male reproductive system) Cleavage into peptides, further modification (glycosylation, acetylation, phosphorylation) ACTH: acts on cells in the adrenal gland to increase cortisol production and secretion; β-lipotropin: induces lipolysis, precursor of β-endorphine Endorphines: endorphines bind to the opioid receptors in CNS, analgesia MSH: acts on skin cells to cause the dispersion of melanin (skin darkening)
2. Multiple copies of a hormone can be encoded on a single gene The gene product for enkephalins (located in the adrenal medulla) Enkephalins are pentapeptides with opioid activity Tyr-Gly-Gly-Phe-Met (methionine-enkephalin) Tyr-Gly-Gly-Phe-Leu (leucine-enkephalin) Model of enkephalin precursor encodes several met-enkephalins (M) molecules and a molecule of leu-enkephalin (L)
Hydrophilic hormones interact with specific receptors on the cell surface Hormone or neurotrasmitter
Signal transduction via: G protein-coupled receptors 1. Protein kinase A pathway (the elevation of camp activates protein kinase A) Corticotropin releasing hormone, thyrotropin, luteinizing hormone, follicle stimulating hormone, adrenocorticotropic hormone, vasopressin, opioid peptides, norepinephrine, epinephrine 2. Protein kinase C and IP 3 -Ca 2+ (inositoltriphosphate) pathway (triggering of the hydrolysis of phosphatidylinositol-4,5-bisphosphate and stimulation of protein kinase C) Thyrotropin releasing hormone, gonadotropic releasing hormone, thyrotropin, norepinephrine, epinephrine, angiotensin 3. Protein kinase G pathway (the elevation cgmp activates protein kinase G) Atrionatriuretic factor Protein kinase receptors e.g. Tyrosin specific protein kinases (Insulin)
Protein hormones Hormones of the hypothalamus-hypophysis cascade Hormones produced by other tissues heart (atrionatriuretic factor) pancreas (insulin, glucagon, somatostatin) gastrointestinal tract (cholecystokinin, gastrin) fat stores (leptin) parathyroid glands (parathyroid hormone) kidney (erythropoietin)
Hypothalamus GRH TRH CRH PRF, PIF GnRH Anterior pituitary norepinephrine GH TSH ACTH LPH β-endorphin MSH PRL FSH LH Ovary Liver Thyroid Adrenal cortex Mammary gland Corticosteroids Hyperglycemic effects Thyroid hormones Growth of bone, body tissues; carbohydrate and protein metabolism; Skin darkening β-endorphin Analgesia Cell development, lactation Development of follicles, estradiol Testis Growth of seminal tubules and spermatogenesis Ovary Testis Ovulation, corpus luteum, progesterone Interstitial cell development, testosterone GH-Growth hormone, TSH-Thyrotropin, ACTH-Adrenocorticotropic hormone, LPH-Lipotropin, MSH-Melanocyte stimulating hormone, PRL-Prolactin, FSH-Follicle stimulating hormone, LH-Luteinizing hormone
Releasing hormone Thyrotropin releasing hormone (TRH) Gonadotropin releasing hormone (GnRH) Corticotropin releasing hormone (CRH) Growth hormone releasing hormone (GHRH) Hypothalamic releasing hormones (RH) Number of amino acids Anterior pituitary hormone released or inhibited 3 Thyrotropin (TSH) 10 Luteinizing hormone (LH), Follicle stimulating hormone (FSH) 41 Adrenocorticotropic hormone (ACTH), β-lipotropin, β-endorphin 44 Growth hormone (GH) Somatostatin 14 GH release inhibited Prolactin releasing factor (PRF) Prolactin (PRL) Prolactin release inhibiting factor (PIF), Dopamine PRL release inhibited
Clinical correlation of the hormonal cascade Testing the activity of the anterior pituitary For example infertility: which organ is at fault in the hormonal cascade? Step 1 The gonads must be considered injecting LH or FSH if sex hormone is elicited, the gonads function properly Step 2 The anterior pituitary must be tested i.v. administration of GnRH (secretion of LH and FSH; by RIA) Normal response The hypothalamus is nonfunctional No response The anterior pituitary is nonfunctional
Hypopituitarism The deficiency of one or more hormones of the pituitary gland The connection between the hypothalamus and anterior pituitary can be broken by 1. Trauma (automobile accidents) 2. Tumor of the pituitary gland Decreased generation of the pituitary hormones A life-threatening situation The usual therapy involves administration of the end organ hormones (cortisol, thyroid hormone, sex hormones, progestin, growth hormone in children)
Vasopressin and oxytocin Synthetized in the hypothalamus (nucleus supraopticus and paraventricularis) Axonal transport with transport proteins (neurophysins) Nonapeptides with disulfide bridge Hypothalamus Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH 2 Arginine vasopressin Axonal transport Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Lys-Gly-NH 2 Neurohypophysis Lysine vasopressin Oxytocin Vasopressin (ADH) Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Arg-Gly-NH 2 Oxytocin Structural similarity, overlapping functions Oxytocin: causes milk ejection in lactating female, uterine contraction Vasopressin: increases water reabsorption from distal kidney tubule
Growth hormone (GH) synthesized in the adenohypophysis, the concentration in the pituitary is 5-15 mg/g single polypeptide, two disulfide bridges is essential for postnatal growth 1. GH increases protein synthesis Biochemical actions 2. Carbohydrate metabolism: GH antagonizes the effects of insulin (hyperglycemia); decreased peripheral utilization of glucose, increased hepatic production via gluconeogenesis 3. Lipid metabolism: GH promotes the release of free fatty acids and glycerol from adipose tissue, increases circulating free fatty acids, causes increased oxidation of free fatty acids in the liver 4. Mineral metabolism: GH promotes a positive calcium, magnesium, and phosphate balance (promotes growth of long bones) 5. Prolactin-like effects Pathophysiology: dwarfism, gigantism, acromegaly
Parathyroid hormone (PTH)
Increases Ca 2+ uptake in intestines Active vitamin D Stimulates Ca 2+ uptake in kidneys PTH Stimulates Ca 2+ release from bones Parathyroid gland (behind thyroid) Blood Ca 2+ level rises. STIMULUS: Falling blood Ca 2+ level Homeostasis: Blood Ca 2+ level (about 10 mg/100 ml)
Insulin polypeptide consisting of 2 chains linked by 2 disulfide bridges
Insulin Synthesis Hydrophobic pre-sequence (signal peptide) is cleaved after transporting to ER Proinsulin is further transported to GA and cleaved by trypsin-like enzymes and carboxypeptidase like enzyme Heterodimeric insulin and C-peptide are formed Insulin combine with zinc to form hexamers
Signal transduction
Body cells take up more glucose. Insulin Beta cells of pancreas release insulin into the blood. Liver takes up glucose and stores it as glycogen. Blood glucose level declines. STIMULUS: Blood glucose level rises. Homeostasis: Blood glucose level (about 90 mg/100 ml) Blood glucose level rises. STIMULUS: Blood glucose level falls. Alpha cells of pancreas release glucagon. Liver breaks down glycogen and releases glucose. Glucagon
Insulin reduces blood glucose levels by Promoting the cellular uptake of glucose Slowing glycogen breakdown in the liver Promoting fat storage Glucagon increases blood glucose levels by Stimulating conversion of glycogen to glucose in the liver Stimulating breakdown of fat and protein into glucose
Inactivation and degradation of peptide hormones Most polypeptide hormones are degraded to amino acids by hydrolysis in the lysosome Some hormones contain a ring structure joined by a disulfide bridge (oxytocin, vasopressin, somatostatin) 2. Glutathione transhydrogenase Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Arg-Gly-NH 2 Oxytocin 1. Cystine aminopeptidase Step 1: Breakage of the ring structure Step 2: Cleavage of cystine Octapeptide further degradation amino acids
Amino acid derived hormones Catecholamines The adrenal medulla secretes epinephrine (adrenaline) and norepinephrine (noradrenaline) They are secreted in response to stress-activated impulses from the nervous system They mediate various fight-or-flight responses Trigger the release of glucose and fatty acids into the blood Increase oxygen delivery to body cells Direct blood toward heart, brain, and skeletal muscles, and away from skin, digestive system, and kidneys
Biosynthesis of catecholamines in the adrenal medulla 1 2 3 4 1. Tyrosine hydroxylase: oxidoreductase, cofactor tetrahydropteridine; inhibition by the catecholamines, tyrosine derivates, and by chelating iron 2. Dopa decarboxylase: cofactor pyridoxal phosphate; inhibitors α-methyldopa 3. Dopamine β-hydroxylase: mixed function oxidase, ascorbate as an electron donor, copper at the active site 4. Phenylethanolamine-N-methyltransferase: the synthesis is induced by glucocorticoid hormones, S-adenosyl methionin coenzyme
The same hormone may have different effects on target cells that have: Different receptors for the hormone Different signal transduction pathways Different proteins for carrying out the response The biological effects of catecholamines are mediated by two classes of plasma transmembrane receptors, the alfa- and betaadrenergic receptors
Same receptors but different intracellular proteins (not shown) Different receptors Epinephrine βreceptor Glycogen deposits Epinephrine β receptor Epinephrine α receptor Glycogen breaks down and glucose is released. Vessel dilates. Vessel constricts. (a) Liver cell (b) Skeletal muscle blood vessel (c) Intestinal blood vessel
Catecholamines are rapidly metabolized by catechol-o-methyltransferase (COMT) and monoamine oxidase (MAO) Different metabolites are formed: 3-methoxy-4-hydroxymandelic acid (vanillylmandelic acid); measurable in urine; elevation in pheochromocytoma
Literature Devlin, T. M. Textbook of biochemistry: with clinical correlations. 6th edition. Wiley-Liss, 2006. Marks Basic Medical Biochemistry, A Clinical Approach, third edition, 2009 (M. Lieberman, A.D. Marks) Color Atlas of Biochemistry, second edition, 2005 (J. Koolman and K.H. Roehm) Harper s Biochemistry 23rd edition1993