G protein-coupled receptors (GPCRs) are integral membrane proteins that sense molecules outside the cell and activate intracellular signal transduction. They have seven transmembrane domains and couple to heterotrimeric G proteins inside the cell. When agonists bind to GPCRs, it causes a conformational change activating the G protein, which then activates downstream effectors like adenylyl cyclase. This leads to the formation of second messengers like cyclic AMP (cAMP) and diacylglycerol. Mutations in GPCRs and G proteins can cause disease by disrupting signaling, like loss of function mutations resulting in hormone resistance or constitutively active mutations mimicking hormone excess.

1. G PROTEIN COUPLED
RECEPTORS

2. OVERVIEW
We are going to deal this topic, breaking them up
into three smaller topics
 GPCR
 G-PROTEIN
 SECOND MESSENGERS.
 MUTATIONS IN GPCR AND G-PROTEINS.

3. G PROTEIN COUPLED RECEPTORS/
SERPENTINE RECEPTORS
G protein coupled receptors are also called as the
seven trans membrane receptors.

4. STRUCTURE OF GPCR- Β ADRENERGIC
RECEPTOR

5. GPCR
 As the name suggests, This is a RECEPTOR
coupled with G-Proteins.
 Humans express over 800 GPCR and half of these
are dedicated to sensory perception (smell, taste
and vision).

6. LIGANDS THAT BIND TO THESE RECEPTORS
 Neurotransmitters (eg.Ach)
 Biogenic amines (eg. NE)
 Ecosinoids and other lipid signaling molecule.
 Peptide hormones
 Opioids
 Amino acids (eg. GABA)
 Other miscellaneous peptide and protein ligands.

7. RECEPTOR STRUCTURE
 GPCRs are integral membrane proteins .
 They possess seven membrane-spanning domains
or transmembrane helices.
 The receptors span the cell membrane 7 times.
 These sense molecules outside the cell and
activate signal transduction inside the cell.

9. DIMERIZATION OF GPCR
 GPCRs undergo both homo- and
heterodimerization and possibly oligomerization.
 Heterodimerization can result in receptor units with
altered pharmacology compared with either
individual receptor.
 Eg. opioid receptors can exist as homodimers of or
receptors, or as heterodimers with distinctly
different pharmacodynamic properties than either
homodimer.

10. DIMERIZATION OF GPCR
 Dimerization of receptors may regulate the affinity and
specificity of the complex for G proteins, and regulate
the sensitivity of the receptor to phosphorylation by
receptor kinases and the binding of arrestin, events
important in termination of the action of agonists and
removal of receptors from the cell.
 Dimerization also may permit binding of receptors to
other regulatory proteins such as transcription factors.
 Dimerization of single membrane spanning receptors is
central to their activation.

11. WHY ARE THESE GPCR’S IMPORTANT IN
PHARMACOLOGY?
 Reasons
-Because of their number,
-Their wide array of ligands that bind to them and
-The variety of physiological functions they
mediate.
 GPCR’s are the target of many drugs (Half of all
non antibiotic prescription drugs act on these
receptors).

12. FAMILIES OF GPCR:
3 Families
 A – Rhodopsin family
 B - Secretin/Glucagon receptor family eg. Peptide
hormones.
 C - Metabotropic Glutamate family eg. GABA ,
Glutamate.

13. RHODOPSIN RECEPTOR FAMILY
 RLR are a family of proteins comprise of G protein-
coupled receptors and are extremely sensitive to
light.
 It activates the G protein transducin (Gt) to activate
the visual phototransduction pathway.
 Mutation of the rhodopsin gene is a major
contributor to various retinopathies.

14. SECRETIN RECEPTOR FAMILY
 The secretin-receptor family of GPCRs include
Vasoactive intestinal peptide receptors and
receptors for secretin, calcitonin and parathyroid
hormone/parathyroid hormone-related peptides.
 These receptors activate adenylyl cyclase and the
phosphatidyl-inositol-calcium pathway.

15. METABOTROPIC GLUTAMATE FAMILY
 The metabotropic glutamate receptors (mGluRs)
are family C GPCR that participate in the
modulation of synaptic transmission and neuronal
excitability throughout the central nervous system.
 They have been subdivided into three groups,
based on intracellular signaling mechanisms.
 Group I mGlu receptors (coupled to PLC and
intracellular calcium signaling).

16.  Group II Group III receptors are negatively
coupled to adenylyl cyclase. These receptors are
generally widely distributed throughout the
mammalian brain with high levels in the cerebellum
and thalamus.

17. 7 TRANSMEMBRANE RECEPTOR

18. G PROTEINS

19. G- PROTEINS
 GPCRs couple to a family of heterotrimeric GTP-
binding regulatory proteins termed G proteins.
 G proteins are signal transducers that convey the
information that agonist is bound to the receptor
from the receptor to one or more effector proteins.
 G–protein-regulated effectors include enzymes
such as adenylyl cyclase, phospholipase C, cyclic
GMP phosphodiesterase (PDE6), and membrane
ion channels selective for Ca2+ and K+ .

20. STRUCTURE OF G-PROTEINS
 G Protein is a hetrotrimer.
 The G protein heterotrimer is composed of
1. Guanine nucleotide-binding α subunit(which
confers specific recognition to both receptors and
effectors), and
2. An associated dimer of β and γ subunits that helps
confer membrane localization of the G protein
heterotrimer by prenylation of the subunit.

21. MECHANISM OF ACTION OF G PROTEINS
 In the basal state of the receptor-heterotrimer
complex, the α subunit contains bound GDP and
the –α GDP: βγ complex is bound to the unliganded
receptor

24. G PROTEIN- SUBUNITS
 The G protein family is comprised of
 23 α subunits (which are the products of 17 genes)
 7 β subunits, and
 12 γ subunits.
 The α subunits fall into four families (Gs, Gi, Gq, and
G12/13) which are responsible for coupling GPCRs to
relatively distinct effectors.

25. ACTIVATION OF GPCR
 When an agonist binds to a GPCR, there is a
conformational change in the receptor that is
transmitted from the ligand-binding pocket to the
second and third intracellular loops of the receptor
which couple to the G protein heterotrimer.

26.  This conformational change causes the α subunit to
exchange its bound GDP for GTP.
 Binding of GTP activates the α subunit and causes it to
release both the βγ dimer and the receptor, and active
signaling molecules, both the GTP-bound α subunit
and the βγ heterodimer become and active
signaling molecules.
 The interaction of the agonist-bound GPCR with the G
protein is transient; following activation of one G protein,
the receptor is freed to interact with other G proteins.

27.  Depending on the nature of the subunit, the active,
GTP-bound form binds to and regulates effectors such
as adenylyl cyclase (via Gs ) or phospholipase C (via Gq
).
 The subunit can regulate many effectors including ion
channels and enzymes such as PI3-K. The G protein
remains active until the GTP bound to the subunit is
hydrolyzed to GDP.
 The α subunit has a slow intrinsic rate of GTP hydrolysis
that is modulated by a family of proteins termed
regulators of G protein signaling (RGSs).

28.  The RGS proteins greatly accelerate the hydrolysis
of GTP and are potentially attractive drug targets.
 Once the GDP bound to the α subunit is hydrolyzed
to GDP, the βγ subunit and receptor recombine to
form the inactive receptor-G protein heterotrimer
basal complex that can be reactivated by another
ligand-binding event.

31. TARGETS FOR G-PROTEINS
The main targets for G-proteins, through which
GPCRs control different aspects of cell function.
 adenylyl cyclase, the enzyme responsible for cAMP
formation
 phospholipase C, the enzyme responsible for
inositol phosphate and diacylglycerol (DAG)
formation
 ion channels, particularly calcium and potassium
channels
 Rho A/Rho kinase, a system that controls the
activity of many signalling pathways controlling cell
growth and proliferation, smooth muscle
contraction, etc.

32. SECOND MESSENGERS

33. SECOND MESSENGERS
 Cyclic AMP
 PKA
 Cyclic AMP regulated Guanine Nucleotide
Exchange Factors.
 PKG
 PDES

36. CYCLIC AMP
 Cyclic AMP is synthesized by adenylyl cyclase
under the control of many GPCRs; stimulation is
mediated by the Gs subunit, inhibition by the Gi
subunit.
 There are nine membrane-bound isoforms of
adenylyl cyclase (AC) and one soluble isoform
found in mammals.
 Membrane-bound ACs exhibit basal enzymatic
activity that is modulated by binding of GTP-
liganded subunits of the stimulatory and inhibitory G
proteins (Gs and Gi).

37.  Numerous other regulatory interactions are possible,
and these enzymes are catalogued based on their
structural homology and their distinct regulation by G
protein and subunits, Ca2+, protein kinases, and the
actions of the diterpene forskolin.
 Cyclic AMP generated by adenylyl cyclases has three
major targets in most cells, the cyclic AMP dependent
protein kinase (PKA), cAMP-regulated guanine
nucleotide exchange factors termed EPACs (exchange
factors directly activated by cAMP), and via PKA
phosphorylation, a transcription factor termed CREB
(cAMP response element binding protein

38.  It is produced continuously and inactivated by
hydrolysis to 5´-AMP, by the action of a family of
enzymes known as phosphodiesterases (PDEs).
 Many different drugs, hormones and
neurotransmitters act on GPCRs and produce their
effects by increasing or decreasing the catalytic
activity of adenylyl cyclase, thus raising or lowering
the concentration of cAMP within the cell.

39.  Cyclic AMP regulates many aspects of cellular
function including, for example, enzymes involved
in energy metabolism, cell division and cell
differentiation, ion transport, ion channels, and the
contractile proteins in smooth muscle

40. PKA
 PKA can phosphorylate a diverse array of
physiological targets such as metabolic enzymes
and transport proteins, and numerous regulatory
proteins including other protein kinases, ion
channels, and transcription factors.

41. G-PROTEIN MUTATIONS
CAUSING DISEASE

42. MUTATIONS IN GPCR AND G PROTEINS
 LOSS OF FUNCTION MUTATION.
 Block the signaling by the corresponding agonist.
 In endocrine signaling, loss-of-function mutations
cause hormone resistance, mimicking hormone
deficiency, whereas gain-of-function mutations
mimic states of hormone excess. Defective G
protein–mediated signaling can also lead to
neoplasia and developmental and sensory
abnormalities

43. DISEASES CAUSED BY GPCR LOSS-OF-
FUNCTION MUTATIONS
 Cone opsins Color blindness X-linked; autosomal recessive
 Rhodopsin Retinitis pigmentosa Autosomal dominant; recessive
 V2 vasopressin Nephrogenic diabetes insipidus X-linked
 ACTH Familial ACTH resistance Autosomal recessive
 LH Male pseudohermaphroditism Autosomal recessive
 Ca2+ sensing Familial hypocalciuric hypercalcemia Autosomal dominant
 Ca2+ sensing Neonatal hyperparathyroidism Autosomal recessive
 Endothelin-B Hirschsprung disease Complex
 FSH Hypergonadotropic ovarian failure Autosomal recessive
 TSH Congenital hypothyroidism Autosomal recessive
 TRH Central hypothyroidism Autosomal recessive
 GHRH Growth hormone deficiency Autosomal recessive
 GnRH Central hypogonadism Autosomal recessive
 Melanocortin 4 Extreme obesity Codominant
 PTH/PTHrP Blomstrand chondrodysplasia Autosomal recessive

45. CLASSIFICATION OF THE DISEASES ACCORDING
THE MECHANISM
 Inactive or absent Gs (α subunit)
 Constitutively active Gs (α subunit)
 Temperature-sensitive G αs
 Constitutively active Gi α2

46. 1.INACTIVE OR ABSENT GS (A SUBUNIT)
Pseudohypoparathyroidism-- type I (PHP-I), is an
inherited human disease caused by mutational
inactivation of the α subunit of Gs
Multiple endocrine abnormalities in cAMP
regulated organs
Occurs when bad gene inherited from mother
Pseudopseudohypoparathyroidism--clinically less
severe syndrome, same as mutation
in one Gs
Occurs when bad gene inherited from father
Both conditions--as protein levels about half of normal

47. 2. CONSTITUTIVELY ACTIVE GS (A SUBUNIT)
Tumors
Mutations in αs --block GTPase activity, cause
constitutive activity
as candidate oncogene (termed gsp)
Activating mutations found in 40% of growth
hormone secreting pituitary adenomas; found in
other endocrine tumors including pituitary, thyroid
McCune Albright Syndrome
Somatic mutation in αs in early embryonic
development
Patients mosaic for constitutively active Gs (as)

48. MCCUNE-ALBRIGHT SYNDROME.
Somatic mutation of Gs alpha early in development
Effects of activating MSH and gonadotrophin
receptors evident

49. 3.TEMPERATURE-SENSITIVE ΑS
--"TESTOTOXICOSIS"
Testotoxicosis ——the gain-of-function disorder
Symptoms: Males have general features of
precocious puberty and hypoparathyroidism with a
mutant as that is inactive at 37°C and
constitutively active at testicular temperature

50. Both patients were found to contain a single amino
acid substitution in one of the Gαisoforms. The
alternation in amino acid sequence caused two
effects on the mutant G protein.
 precocious puberty--indicating premature testicular
activation (normally testosterone production is
stimulated by LH, a GPCR coupled to cAMP
formation)
 hypoparathyroidism--impaired responses to PTH,
TSH causing PTH and thyroid abnormalities

51.  At temperatures below normal body temperature,
the mutant G protein remained in the active state,
even in the absence of a bound ligand. In contrast,
at normal body temperatures, the mutant G protein
was inactive, both in the presence and absence of
bound ligand, the testes, which are housed outside
of the body’s core, have a lower temperature than
the body’s visceral organs (33℃ versus 37 ℃).

52. PRECOCIOUS PUBERTY
 Normally, the endocrine cells of the testes initiate
testosterone production at the time of puberty in
response to the pituitary hormone LH, which begins
to be produced at that time.
 The circulating LH binds to LH receptors on the
surface of the testicular cells, inducing the
synthesis of cAMP and subsequent production of
the male sex hormone, the testicular cells of the
patients bearing the G protein mutation were
stimulated to synthesize cAMP in the absence of
the LH ligand, leading to premature synthesis of
testosterone and precocious puberty.

53. 4. CONSTITUTIVELY ACTIVE GI Α2
Tumors
Mutations in ai2 --block GTPase activity, cause
constitutive activity
αi2 candidate oncogene (termed gip)
Activating mutations found in >30% of adrenal,
ovarian tumors

54. REFERENCE
 Goodman and Gilman Textbook of Pharmacology.
 Katzung Textbook of Pharmacology.
 Rang and Dale’s Textbook of Pharmacology.
 Internet sources.

55. THANKYOU