Transcript Biochemistry of hormones derived from amino acids and proteins

Biochemistry of hormones
derived from amino acids and
proteins
Alice Skoumalová
Content
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Definition of peptide hormones
Common features: synthesis, interactions with receptors at the cell
surface
Groups of peptide hormones
Hypothalamus-hypophysis hormonal cascade (signal amplification,
negative feedback system)
Genes and formation of polypeptide hormones (gene superfamilies)
Hormones of the hypothalamus and the hypophysis
Synthesis and degradation of catecholamines
Biochemistry of parathyroid hormone and insulin
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 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)
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)
Environmental or
internal signal
Hormonal cascade
Signal amplification
CNS
Electrical-chemical signal
Limbic system
Electrical-chemical signal
Hypothalamus
Releasing hormones (ng)
Anterior pituitary
Anterior pituitary hormone (μg)
Target „gland“
The gonads, the thyroid gland,
the adrenal cortex
Ultimate hormone (mg)
Systemic effects
Environmental stress
CRH-ACTH-Cortisol
CNS
A single stressor (change in
temperature, noise, trauma)
Electrical-chemical signal
Limbic system
Electrical-chemical signal
Hypothalamus
Portal system
The corticotrophic
cells
Corticotropin releasing hormone
(CRH) in ng, t1/2 minutes
Anterior pituitary
Adrenocorticotropic hormone (ACTH)
in µg, increased t1/2
Adrenal gland
Cortisol in mg, t1/2 hours
The glucocorticoid receptors in
different cells
Systemic effects
CNS
Hormonal cascade
Negative feedback system
Limbic system
Hypothalamus
Long feedback
loop
Releasing hormones
Anterior pituitary
Anterior pituitary
hormones
Target „gland“
Ultimate hormone
Systemic effects
Short feedback
loop
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
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
Anterior pituitary
GH
Liver
TSH
Thyroid
ACTH LPH β-Endorphin MSH
PRL
FSH
LH
Ovary
Ovary
Mammary gland
Testis
Testis
Adrenal cortex
Skin darkening
β-Endorphin
Corticosteroids
Hyperglycemic
effects
Thyroid hormones
Growth of bone, body
tissues; carbohydrate
and protein
metabolism;
production of IGFs
Analgesia
Ovulation,
corpus luteum,
progesterone
Cell
development,
lactation
Development of
follicles, estradiol
Interstitial cell
development,
testosterone
Growth of seminal
tubules and
spermatogenesis
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
Axonal transport
Vasopressin (ADH)
Neurohypophysis
Oxytocin
Uterine contraction,
lactation
Vasopressin (ADH)
Water balance
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)
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
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
Prolactin (PRL)
 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
The pituitary and placental glycoproteins:
Thyroid-stimulating hormone (TSH), luteinizing
hormone (LH), follicle-stimulating hormone (FSH) a
chorionic gonadotropin (CG)
 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
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
Parathyroid hormone (PTH)
Pre
31
Pro
PTH
6 1
84
Endoplasmic
reticulum
Parathyroid gland
Golgi apparatus
Blood (biological active)
Liver
 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
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 isolated
The 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)
pGlu
C---Peptide
O
N
H
O
Breakage of the pGlu or cleavage of the C-terminal amide can lead to
inactivation of these hormones (this probably accounts for the short halflife of many of these hormones)
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-NH2
1. Cystine aminopeptidase
Step 1: Breakage of the ring structure
Step 2: Cleavage of cystine
Octapeptide
further degradation
amino acids
Oxytocin
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 hypothalamuspituitary cascade (signal amplification, negative feedback interaction)
 Peptide hormones are produced by many different organs and tissues