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

CLIP-corticotropin-like

intermediary peptide

(under control of norepinephrine)

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

G protein-coupled receptors

Signal 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 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

GH TSH ACTH LPH β-Endorphin PRL FSH LH MSH

Anterior pituitary

Growth of bone, body

tissues; carbohydrate

and protein

metabolism;

Hyperglycemic

effects

Thyroid hormones

Liver Thyroid Adrenal cortex Mammary gland Ovary

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

norepinephrine

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

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

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

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, uterine contraction

Vasopressin: increases water reabsorption from distal kidney tubule

Hypothalamus

Axonal transport

Neurohypophysis

Oxytocin Vasopressin (ADH)

Biochemical actions

1. GH increases protein synthesis

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

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

Parathyroid hormone (PTH)

PTH

Parathyroid gland (behind thyroid)

STIMULUS: Falling blood

Ca2+ level

Homeostasis: Blood Ca2+ level

(about 10 mg/100 mL)

Blood Ca2+ level rises.

Stimulates Ca2+ uptake in kidneys

Stimulates Ca2+ release from bones

Increases Ca2+ uptake in intestines

Active vitamin D

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

Homeostasis: Blood glucose level

(about 90 mg/100 mL)

Glucagon

STIMULUS: Blood glucose level

falls.

Alpha cells of pancreas release glucagon.

Liver breaks down glycogen and releases glucose.

Blood glucose level rises.

STIMULUS: Blood glucose level

rises.

Beta cells of pancreas release insulin into the blood.

Liver takes up glucose and stores it as glycogen.

Blood glucose level declines.

Body cells take up more glucose.

Insulin

• 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

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

Inactivation and degradation of peptide hormones

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

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 beta-

adrenergic receptors

Glycogen deposits

βreceptor

Vessel dilates.

Epinephrine

(a) Liver cell

Glycogen breaks down and glucose is released.

(b) Skeletal muscle

blood vessel

Same receptors but different intracellular proteins (not shown)

β receptor

Different receptors

Epinephrine

α receptor

Vessel constricts.

(c) Intestinal blood

vessel

Epinephrine

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