Biochemistry of hormones derived from amino acids and proteins Alice Skoumalová.

Biochemistry of hormones derived from amino acids and

proteins

Alice Skoumalová

Content

1. Definition of peptide hormones

2. Common features: synthesis, interactions with receptors at the cell surface

3. Groups of peptide hormones

4. Hypothalamus-hypophysis hormonal cascade (signal amplification, negative feedback system)

5. Genes and formation of polypeptide hormones (gene superfamilies)

6. Hormones of the hypothalamus and the hypophysis

7. Synthesis and degradation of catecholamines

8. Biochemistry of parathyroid hormone and insulin

9. Degradation of peptide hormones

Definition of peptide hormones

secreted into the blood stream; endocrine functions

synthesized from amino acids according to an mRNA template, which is itself synthesized from a DNA template

precursors (pre-prohormones) - posttranslational modification (endoplasmatic reticulum) - removal of the pre-sequence, sometimes glycosylation - resulting in prohormones

the prohormones - packaged into membrane-bound secretory vesicles - secreted from the cell by exocytosis in response to specific stimuli

mature peptide hormones diffuse through the blood to all of the cells of the body, where they interact with specific receptors on the surface of their target cells

Peptide hormones interact with specific receptors on the cell surface

G protein-coupled receptorsSignal transduction via:

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 IP3-Ca2+ (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 receptorse.g. Tyrosin specific protein kinases (Insulin)

Hormones

1. Amine-derived hormones

Catecholamines and thyroxine

2. Peptide 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)

3. Steroid hormones

Peptide 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)

CNS

Limbic system

Hypothalamus

Anterior pituitary

Target „gland“

Environmental or internal signal

Electrical-chemical signal

Electrical-chemical signal

Releasing hormones (ng)

Anterior pituitary hormone (μg)

Ultimate hormone (mg)

Systemic effects

The gonads, the thyroid gland, the adrenal cortex

Hormonal cascade Signal amplification

CNS

Limbic system

Hypothalamus

Anterior pituitary

Adrenal gland

Environmental stress

A single stressor (change in temperature, noise, trauma) Electrical-chemical signal

Electrical-chemical signal

Corticotropin releasing hormone (CRH) in ng, t1/2 minutes

Adrenocorticotropic hormone (ACTH) in µg, increased t1/2

Cortisol in mg, t1/2 hours

The glucocorticoid receptors in different cells

Portal system

The corticotrophic cells

Systemic effects

CRH-ACTH-Cortisol

CNS

Limbic system

Hypothalamus

Anterior pituitary

Target „gland“

Releasing hormones

Anterior pituitary hormones

Ultimate hormone

Systemic effects

Short feedback loop

Long feedback loop

Hormonal cascade Negative feedback system

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

Step 2 The anterior pituitary must be tested

No response The anterior pituitary is nonfunctional

injecting LH or FSH if sex hormone is elicited, the gonads function properly

i.v. administration of GnRH (secretion of LH and FSH; by RIA)

Normal response The hypothalamus is nonfunctional

Hypothalamic releasing hormones (RH)

Releasing hormone Number of amino acids

Anterior pituitary hormone released or inhibited

Thyrotropin releasing hormone (TRH)

3 Thyrotropin (TSH)

Gonadotropin releasing hormone (GnRH)

10 Luteinizing hormone (LH), Follicle stimulating hormone (FSH)

Corticotropin releasing hormone (CRH)

41 Adrenocorticotropic hormone (ACTH), β-lipotropin, β-endorphin

Growth hormone releasing hormone (GHRH)

44 Growth hormone (GH)

Somatostatin 14 GH release inhibited

Prolactin releasing factor (PRF) Prolactin (PRL)

Prolactin release inhibiting factor (PIF), Dopamine

PRL release inhibited

Hypothalamus

GRH TRH CRH Dopamine PRF, PIF GnRH

GH TSH ACTH LPH β-Endorphin PRL FSH LHMSH

Anterior pituitary

Growth of bone, body tissues; carbohydrate

and protein metabolism;

production of IGFs

Hyperglycemic effects

Thyroid hormones

Liver Thyroid Adrenal cortex Mammary glandOvary

Testis

Corticosteroids

β-Endorphin

Analgesia

Skin darkening

Testis

Cell development,

lactation

Development of follicles, estradiol

Growth of seminal tubules and

spermatogenesis

Ovary

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

Hypothalamus

Oxytocin

Vasopressin (ADH)Axonal transport

Neurohypophysis

Uterine contraction, lactation

Water balance

Oxytocin Vasopressin (ADH)

Vasopressin and oxytocin Synthetized in the hypothalamus (nucleus supraopticus and paraventricularis) Axonal transport with transport proteins (neurophysins) Nonapeptides with disulfide bridge

Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2

Arginine vasopressin

Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Lys-Gly-NH2

Lysine vasopressin

Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Arg-Gly-NH2

Oxytocin

Structural similarity, overlapping functions

Oxytocin: causes milk ejection in lactating female

Vasopressin: increases water reabsorption from distal kidney tubule

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)

Genes and formation of polypeptide hormones

Genes for polypeptide 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

Proopiomelanocortin (a single gen product) is a precursor peptide for eight hormones

ACTH, β-lipotropin, γ-lipotropin, γ-MSH, α-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

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; excessive formation-Cushing‘s syndrome

β-lipotropin: induces lypolysis, 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)

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)

Biochemical actions1. GH increases protein synthesis2. 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

Growth hormone (GH)synthesized in the adenohypophysis, the concentration in the pituitary is 5-15 mg/gsingle polypeptide, two disulfide bridgesis essential for postnatal growth

is secreted in the adenohypophysis

Biochemical actions: the initiation and maintenance of lactation

Pathophysiology: tumors of prolactin-secreting cells cause amenorrhea and galactorrhea in women, gynecomastia and impotence in men

Prolactin (PRL)

structural similarities (common ancestral gene): 2 subunits-α (identical for all of these hormones) and β (determines the specific biologic activity)

synthesized as preprohormones and are subject to posttranslational processing (glycosylation)

LH and FSH are responsible for gametogenesis and steroidogenesis in the gonads

The pituitary and placental glycoproteins:Thyroid-stimulating hormone (TSH), luteinizing

hormone (LH), follicle-stimulating hormone (FSH) a chorionic gonadotropin (CG)

hCG is synthesized in the syncytiotrophoblast cells of the placenta; increases in blood and urine shortly after implantation; its detection is the basis of many

pregnancy tests

hCG- β subunit

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

Catecholamines are rapidly metabolized by catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO)

Different metabolites are formed; two classes have diagnostic significance: metanephrines and 3-methoxy-4-hydroxymandelic acid (vanillylmandelic acid); measurable in urine; elevation in pheochromocytoma

PTH affects calcium homeostasis Increases the rate of dissolution of bone, reduces the renal excretion of Ca2+,

increases the efficienty of calcium absorption from the intestine by promoting the synthesis of calcitriol

Parathyroid hormone (PTH)

PTHPre Pro

841631

Parathyroid gland

Liver

Blood (biological active)

Endoplasmic reticulum

Golgi apparatus

Insulin polypeptide consisting of 2 chains linked by 2 disulfide bridges

Synthesis and posttranslational modification of insulin

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 E

Heterodimer and C-peptide are formed

The human insulin gene has been isolatedThe synthesis of human insulin in bacterial expression systems, using recombinant DNA technology, affords an excellent source of this hormone for diabetic patients

Diagrammatic structure of the human insulin gene

Areas with diagonal stripes correspond to untranslated regions, open regions correspond to intervening sequences, and stippled regions correspond to coding sequences

Inactivation and degradation of peptide hormones

Most polypeptide hormones are degraded to amino acids by hydrolysis in the lysosome

Certain hormones contain modified amino acids

The hypothalamic releasing hormones

Pyroglutamic acid (pGlu)C-terminal amino acid amide (Gly-NH2, Ala-NH2, Leu-NH2)

Breakage of the pGlu or cleavage of the C-terminal amide can lead to inactivation of these hormones (this probably accounts for the short half-life of many of these hormones)

NH

O C---Peptide

O

pGlu

Some hormones contain a ring structure joined by a disulfide bridge (oxytocin, vasopressin, somatostatin)

Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Arg-Gly-NH2 Oxytocin

1. Cystine aminopeptidase

2. Glutathione transhydrogenase

Step 1: Breakage of the ring structure

Step 2: Cleavage of cystine

Octapeptide further degradation amino acids

Summary

Peptide hormones are synthetized in the transcription and translation process (DNA-mRNA-peptid) and further modified (posttranslational modification)

Peptide hormones interact with specific receptors on the cell surface and trigger a cascade of secondary effects within the cytoplasm (cAMP, second messengers)

Peptide hormones form gene families that originate from a common ancestral gene

Several important peptide hormones are secreted from the hypothalamus-pituitary cascade (signal amplification, negative feedback interaction)

Peptide hormones are produced by many different organs and tissues