G Protein-Phospholipase C Signal System


Answer to Question about Hormones H-05:

 

(g) Ca++, Diacylglycerol and Inositol 1,4,5 triphosphate

 

 

As was discussed in the answer to H-04, some hormones that can not penetrate the plasma membrane, interact with receptors in membrane that are linked to a G-protein. As a result of the interaction hormone-receptor, the a subunit of the G-protein binds to GTP and separates from the bg subunit. The a subunit-GTP complex activates an specific effector protein, depending on the kind of a subunit. In case the a subunit is Gas,  it activates Adenylate Cyclase, increasing the production of cAMP.

 

If the Hormone-receptor complex interacts with a Gaq/11 kind of G protein, then the activated enzyme is Phospholipase C.

 

Hormones that bind to receptors related to this Protein Gaq/11-Phospholipase C system include:

 

-Angiotensine II

 

-Catecholamines (alpha-receptors)

 

-Godatrophin Releasing Hormone GnRH)

 

-Growth Hormone Relaeasing Hormone

 

-Oxytocin

 

-Thyroid-releasing Hormone

 

-Vasopresin

 

Phospholipase C catalyze the hydrolysis of Phosphatidyl inositol 4,5 biphosphate that is forming part of the plasma membrane.

 

 

 

                                  

   

The action of the enzyme on this substrate produces IP3 (Inositol triphosphate) and diacylglycerol.

 

IP3 difuses into the cytosol and binds to its receptor in the sarcoplasmic reticulum and opens a Calcium channel

 

Diacylglycerol remains close to the membrane and, with the participation of Ca++ released by IP3, activates Protein Kinase C, that phosphorylates other proteins, modifying its function (for the general action of Kinases, see related post)

 

 

 

       

 

Ca++ binds to Calmodulin, Troponin C and other Ca++ binding proteins provoking activation of some enzymes, actin myosine interaction, promotes exocytosis, synthesis of NO, and other effects.

 

Recommended sites:

 

http://www3.us.elsevierhealth.com/promo/Guyton/ch74.pdf),

 

http://mcb.berkeley.edu/courses/mcb136/topic/Endocrine_Autonomic/SlideSet1/endo1.pdf

 

An excellent animation here!

 

About Glucagon and cAMP


 

 Answer to Hormone Question H-04

 

Answer (e): Glucagon provokes the formation of cAMP inside the cell, via a G Protein-Adenyl Cyclase mechanism.

 

 

cAMP or 3’5’AMP is a nucleotide formed from ATP by the action of Adenyl cyclase, a transmembrane protein whose cytoplasmatic domain catalyze the following reaction:

 

             

 

 cAMP was the first compound to be described as a second messenger of the endocrine system. Observe that one phosphate group is bound by two ester phosphates linkages to the 3’  and the 5’  positions of the sugar, that is why this compound is called 3’5’ AMP. Observe also that these linkages forms a kind of ring or cyclic structure, limited by the Phosphorus  and the Oxygen residues and the 3’ and 5’ carbons. It explains the name of cAMP (Cyclic AMP).

 

The mechanism used by glucagon to increase the concentration of cAMP inside the cell, is the following:

 

Glucagon receptors are located mainly in the hepatic and kidney tissues. Glucagon binds to the receptors, that are coupled to  G Proteins ( Guanin nucleotide binding Proteins) located in the cytoplasmatic side of the plasma membrane.  G proteins are formed by three subunits: alpha, Beta and Gamma.

 

There are four main families of a subunits. The alpha subunit is responsible of the kind of response inside the cell, since it is specific for the effector protein.

 

The a subunit that interact with Adenyl clyclase is called Gas, and the G protein that contains it is referred as Gs Protein.

 

The interaction Glucagon/Receptor provokes the activation of a Gs Protein, since the binding of Glucagon to the receptor triggers the general process for Hormones that use the receptor-G Protein mechanism:

 

1.- the interchange of a GDP, attached to the alpha subunit, by GTP

 

2.- the dissociation of the formed GTP-alpha subunit complex, of the Beta-Gamma subunits dimmer.

 

3.- The activation of an effector protein by the GTP-a subunit complex.

 

Since Glucagon receptor is associated to a Gs Protein, The released GTP-Gas subunit complex binds to Adenyl cyclase, provoking the formation of cAMP.

 

 

 

 

In the cells stimulated by glucagon, the cAMP initiates an enzymatic cascade that begins with the activation of protein Kinase A (PKA) and whose main results are:

 

a)     increased glycogenolysis

 

b)     Decreased glycogenesis

 

c)      Decreased glycolysis

 

d)     Increased gluconeogenesis

 

e)     increased fatty acid movilization

 

f)       Increased ketogenesis

 

 

About Insulin/Receptor Interaction


 

Answer to Hormones Question H-03

 

(Original Question)

 

Answer (e)

 

 The insulin receptor is a transmembrane protein formed by two subunits linked by disulfide bonds. It is possible to distinguish in the receptor an extracellular domain, related to the binding of insulin, a transmembrane domain, and an intramembrane domain, that shows activity of tyrosine kinase.

 

    

 

    

When the insulin binds to the receptor (insulin binds to the two peptide chains of the dimer) a conformational chain that activates the tyrosin kinase domain is produced.  This domain phosphorylates various tyrosin residues in the receptor (autophosphorylation) and in other proteins (Insulin Receptor Substrates –IRS) that act as “second messengers” of insulin (IRs1, IRs2, IRs3 and IRS4).

 

 

 

 

The phosphorylation of the tyrosin residues in the IRSs “attracts” proteins containing SH2 domains (domains that bind to phosphorylated tyrosine) activating them.

 

 

 

(For the whole Receptor Itinerary, visit Signal Transduction at the Wellcome Trust Sanger Institute)

 

 

 

The interaction between insulin and its receptor and the resulting activation of IRSs trigger changes at different levels:

        a)     At a membrane level

b)     At a genetic level

c)      At an enzymatic level

 

At a membrane level the activated IRSs provoke phosphorylations and conformational changes in other proteins, resulting in an increase in the quantity of glucose transporters in the plasma membrane in adipose and muscular tissues.

 

At a genetic level they increase the expression of genes that codify some regulatory enzymes of glycolysis, pentose phosphate shunt, and fatty acid and neutral fats synthesis, while decreases the expression of genes that codify some enzymes of gluconeogenesis.

 

At an enzymatic level the IRSs produce covalent modification of other enzymes.  modifying their activity: these IRS proteins provokes conformational changes and activation on enzymes like protein kinases, that produce further covalent modifications in other proteins.

 

IRS-1 in particular has a very important role in triggering insulin effects:

 

1.-  it promotes the fusion of cytosolic vesicles that contain GLUT4,  with the plasma membrane, increasing the concentration of GLUT4 in this membrane in adipose and muscular tissues and consequently the uptake of glucose.

 

2.- Triggers a phosphorylation cascade that produce the activation of  MAPK (Mutagen Activated Protein Kinase) that enters the nucleus and activates, by phosphorylation, various transcription factors.

 

3.- Trough a different mechanism that include the participation of Protein Kinase 3 (PK3), prevents the deactivation of glycogen synthase, favoring glycogenesis.

 

 

For more information about insulin receptors and mechanism of action of insulin, I would recommend to visit these sites:

 

http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/pancreas/insulin_phys.html

 

Insulin’s Mechanism of action

 

Very detailed information can be found in:

http://www.endotext.org/diabetes/diabetes3/diabetes3.htm

 

    

Steroid Hormones Receptors


Question H-02

 

 

Answer: (b)

 

Steroid and thyroid hormones are hydrophobic and diffuse from their binding proteins in the plasma, across the plasma membrane to intracellular localized receptors.

 

These receptors are proteins located in the cytosol and in the nucleus. They belong to the nuclear superfamily of receptors that include receptors for Vitamin A, Vitamin D and other hydrophobic metabolites. Receptors for steroid hormones (Type I receptors) are typically located in the cytosol, associated to Heat Shock Proteins (HSP).

 

When the hormone diffuse into the cytosol,  binds to these receptors, releasing the HSP, and  the resultant complex of steroid and receptor travels to the nucleus.

 

 In the nucleus, the Hormone-receptor complex recognize and bind to specific sequences of nucleotides in the DNA, called Hormone Response Elements (HRE).

 

This interaction Hormone-Receptor-HRE modify the expression of the associated gen, regulating the production of mRNA for specific proteins.

 

 

More information:

 

King, M.W.: The Medical Biochemistry page: Steroid Hormones and receptors

 

 

Kimball’s Biology Pages: Steroid Hormone Receptors

 

Steroid Hormones and receptors

 

 

Recommended videos availables in You Tube:

 

 

1.- A short video explaining the basic of the cellular mechanism of steroid hormones action:

 

 

 

2.- A very detailed lecture about the Nuclear receptors family:

 

 

 

 

Hormones: Answer to H-01


  

Original Question

 

 

The answer is  (a)

 

The mechanism of the “second messenger” is used by hormones that can not cross the plasma membrane, like peptides hormones and hormones derived from amino acids (T3 is an exception, since it has a hydrophobic lateral chain). Since these hormones can not cross the plasma membrane, they interact with a receptor located in the membrane, like this transmembrane protein:

  

 

 

                        

 

 

The interaction between the receptor (a membrane protein that frequently has seven intramembrane domains) and the hormone provokes the activation of one of the G-Proteins, a family of amphipatic proteins associated to the inner surface of the plasma membrane.

 

In this graphic, the receptor is represented as a transmembrane protein in blue, and the G-Protein is represented in pink color:

 

 

 

 

   

  

 

The interaction between the hormone and the receptor provokes changes in conformation of the G-Protein associated to the receptor, facilitating the release of GDP and the binding of GTP in the alpha subunit of the G-Protein.  The alpha subunit-GTP complex dissociates from the Beta-Gamma subunits, and it can produce activation of membrane associated enzymes like Adenyl Cyclase, Phospholipase C or other enzymes, depending on the specific Hormone-Receptor-G Protein system.

 

If the GTP-alpha subunit complex released in the Hormone-Receptor-G Protein System activates Adenyl Cyclase, then this enzyme catalyses the transformation of ATP to cAMP; if the system activates Phospholipase C, then it produces the hydrolysis of phosphatidyl inositol diphosphate, releasing Inositol triphosphate (IP3) and diacylglycerol, that act as second messengers.

 

Molecules as different as cAMP, IP3, diacylglicerol,  Ca++ and Nitric Oxide can function as second messengers.

 

More information about this topic can be found in the following links:

 

Kimball, J.K. :Second Messengers

 

King, M.W. : The Medical Biochemistry Page