PEPTIDE HORMONES& HORMONES THAT USE

CELL SURFACE RECEPTORS

Endocrine mediators

OUTLINE:

1. Sizes, origins and fate.2. Occurance in blood: bound/unbound.3. Listing of major examples.4. Receptors and receptor binding.5. Tour of a selected peptide hormone: insulin.6. Tour of a second peptide hormone:

Sizes, origins and fate:Due to the force of custom, “peptide hormones” isa collective name that has been applied to peptides, polypeptides and proteins that all function ashormones. The name “factor” has also been given to some of these peptides and originates from a time when their peptide/protein nature was still unknown. In the endocrine system of hormonesoriginating from the brain, some of the smaller peptides that exist are from the hypothalamus andanterior and posterior pituitary (hypophysis).

SOME EXAMPLES OF SMALL PEPTIDES:

THYROID RELEASING HORMONE (TRH)FROM THE HYPOTHALAMUS, A PEPTIDE OF ~360 daltonsTARGET: ANTERIOR PITUITARY (RELEASE OF TSH)

Glu-His-Pro-NH2

VASOPRESSINFROM THE POSTERIOR PITUITARY, A PEPTIDE OF ~1040 daltonsTARGET: KIDNEY & ARTERIOLES (INCREASES WATER RE-UPTAKE AND BLOOD PRESSURE)

OTHER PEPTIDE HORMONES FALL WITHINTHE DEFINITION OF A PROTEIN (>10 kD).SOME EXAMPLES:

GROWTH HORMONE SOMATOTROPIN; 21.5 kDFROM THE ANTERIOR PITUITARY; TARGET: LIVER & BONE CAUSESINCREASE IN METABOLISM &BONE GROWTH.

THIS IS A PROTEIN THAT HASCONSIDERABLE -a HELICALAND RANDOM COIL STRUCTURE.

THE BLUE ARROWS INDICATEAPPROXIMATELY WHERE THEHORMONE BINDS TO CELL RECEPTORS.

PROLACTIN22 kD WITH A STRUCTURESOMEWHAT SIMILAR TOGROWTH HORMONE.

NOTE THE EXTENSIVE -a HELICES. THE BINDING

SITES FOR THE RECEPTORSARE FROM THE LOWER SIDES.AS SHOWN BY THE BLUEARROWS.

THIS HORMONE IS MADE INTHE ANTERIOR PITUITARY TOSTIMULATE MILK PRODUCTION.

FATE OF PEPTIDE HORMONES

PEPTIDE HORMONES, AFTER BINDING TO THEIR RECEPTORS, ARE COMMONLY TAKENUP BY THE CELLS THAT THEY BIND TO ANDTRANSPORTED TO LYSOSOMES WHERE THEYARE BROKEN DOWN .

THE LYSOSOME, IS AN INTRACELLULAR SUBORGANELLE THAT CONTAINS SOME30 DEGRADATIVE ENZYMES AS WELL AS ANACIDIC pH ENVIRONMENT THAT FAVORS DESTRUCTION.

PEPTIDE HORMONESIN THE BLOOD

THE ABILITY OF A HORMONE TO REACH ITS TARGETTISSUE IS CRUCIAL. THE FOLLOWING IS KNOWN ABOUTTHAT TASK:

1. SMALL PEPTIDE HORMONES ARE AT A LOW CONCENTRATION AND SOLUBLE ENOUGH TO BE TRANSPORTED UNBOUND TO ANY SOLUBLE CARRIER.

2. LARGE “PEPTIDE” HORMONES, LIKE GROWTH HORMONE, ARE CARRIED BY A SOLUBLE PROTEIN WHICH IS SIMILAR TO THE PROTEIN RECEPTOR IT BINDS TO ON CELL SURFACES.

IMPORTANT PEPTIDE HORMONES(A SAMPLE LIST)

Not part of the hypothalamic “pecking order” of hormones.

“TOUR” OF A SELECTED PEPTIDE HORMONE: INSULIN

INSULIN IS A POLYPEPTIDE WITH A MW OF 5808. IT ACTUALLY CONSISTS OF 2 CHAINS JOINED BY DISULFIDE BONDS (A CHAIN= BLUE; B CHAIN = GREEN)IF YOU STRETCH OUT THEMOLECULE, YOU CAN SEETHAT THER ARE 3 DISULFIDEBONDS (1 INTRACHAIN ONTHE A CHAIN AND 2 INTER-CHAIN BONDS)

SINCE ITS DISCOVERY IN 1921, INSULIN HAS BEEN ONE OF THE MOST HEAVILY STUDIED HORMONES OFALL TIME. THIS IS DUE TO ITS ASSOCIATION WITH DIABETES, A DISEASE IN WHICH GLUCOSE BOTH STARVES SOME CELLS AND POISONS OTHER CELLS WHILE IT DISORGANIZES ASSOCIATED METABOLIC PATHWAYS.

THE ROLE OF INSULIN IS TO FACILITATE THE UPTAKEOF GLUCOSE INTO CELLS BY MEANS OF GLUCOSETRANSPORT PROTEINS. THAT PROCESS IS CONTROLLEDBY TWO ASSOCIATED PEPTIDE HORMONES: GLUCAGONAND SOMATOSTATIN – ALL PRODUCTS OF THE PANCREAS.

INSULIN IS PRODUCEDIN THE ISLET CELLS OFTHE PANCREAS (B CELLS)

GLUCAGON IS MADE INSOME DIFFERENT ISLET CELLS CALLED A CELLS.

SOMATOSTATIN IS MADEIN STILL OTHER ISLETCELLS CALLED D CELLS.

THE ORGAN IS LOCATEDBEHIND THE SPLEEN &ADJACENT TO THE SMALLINTESTINE.

INSULIN IS INITIALLY FORMED AS A SINGLE POLYPEPTIDECALLED PREPROINSULIN. PROTEOLYTIC ENZYMES REMOVE THE SIGNAL PEPTIDE. THEN DISULFIDE BONDFORMATION TAKES PLACE TO FORM PROINSULIN.IN THE LASTSTAGE, INSULINIS FORMED WHENCONTINUEDLYSIS TAKESAWAY THEC PEPTIDE.THE HORMONEIS NOT ACTIVEPREVIOUS TOTHIS STAGE.

BINDING OF INSULIN TO ITS RECEPTOR PROTEIN

THE INSULIN RECEPTORPROTEIN IS A TETRA-PEPTIDE COMPOSED OFTWO ALPHA & TWO BETASUBUNITS. WHEN INSULINBINDS TO THE PROTEIN,IT CAUSES A CONFORMA-TIONAL SHIFT THAT ACTIVATES KINASE ACTIVITY ON THE INSIDE OF THE MEMBRANE (ARROW).THIS PRODUCES A SERIES OF PHOSPHORYLATIONS ASA CASCADE OF MOLECULES (& FOLLOWING EVENTS). ONEEVENT IS THE DELIVERY OF GLUT4 TO THE CELL SURFACE.

POST RECEPTOR ACTIVITY:GLUT-4 SURFACE DELIVERY

SURFACE HORMONES THAT USE cAMP AS A MEDIATOR

CORTICOTROPIN (ACTH) ***DOPAMINEEPINEPHRINEFOLLICLE STIMULATING HORMONE (FSH) ***GLUCAGON ***LUTEINIZING HORMONE (LH) ***MELANOCYTE STIMULATING HORMONE (MSH) ***PARATHYROID HORMONE ***PROSTAGLANDINSSEROTONINSOMATOSTATIN ***THYROID STIMULATING HORMONE (TSH) ***

*** peptides or polypeptides

GENERAL cAMP MECHANISM (stimulatory): THE KEY COMPONENTS ARE THEHORMONE ( -b EFFECTOR); RECEPTOR (in membrane); THE G PROTEIN;ADENYLYL CYCLASE; AND cAMP.

(cascade starts here)

An example of a hormone and its receptor: THE THYROID STIMULATINGHORMONE AND THE TSHR PROTEIN

THE MODEL SHOWS THE OUTERMOSTPORTION OF THE RECEPTOR BOUND TOTHE HORMONE ON THE LEFT AT THE LEUCINE RICH DOMAIN OF THE RECEPTOR.

AS WITH ALL HORMONE-RECEPTOR BINDINGS, THE RECEPTOR WILL BEINDUCED TO AFFECT A NEARBY G PROTEINPROBABLY IN THE VICINITY OF THE C-TERMINAL END OF THE RECEPTOR.

IT IS CHARACTERISTIC OF THE HORMONETO INDUCE A CHANGE IN THE CONFORMATIONOF THE RECEPTOR THAT RESULTS IN SOMEACTIVITY OF THE RECEPTOR SUCH AS –ENZYME ACTIVATION OR PROTEIN BINDING.

WHEN A G PROTEIN IS AFFECTED, AT THIS POINT, THE G PROTEIN “PICKS UP” THE MESSAGE INITIATED BY THE HORMONE. TYPICALLY GDP (WHICH IS BOUND TO THE G PROTEIN) IS RELEASED AND REPLACED BYGTP AS A NEW ASSOCIATION WITH THE TRANSFORMED RECEPTOR PROTEIN.WHEN THIS OCCURS, THE g AND b SUBUNITS DIFFUSE

AWAY FROM THE ORIGINALG PROTEIN.

AT 1, THE ACTIVATIONBEGINS. BY 3, THE ACTIVATED PROTEINDIFFUSES TO ADENYLYLCYCLASE TO ACTIVATE IT. IN STEP 4, THE G PROTEINIS BACK TO ITS ORIGINAL INACTIVE FORM.

G PROTEINS COME IN SEVERAL VARIETIES (= ISOFORMS) TO INACT HORMONAL TRANSDUCTION MECHANISMS:

NOTES: GS = STIMULATORY G PROTEIN SINCE IT STIMULATES ADENYLYLCYCLASE. GI = INHIBITORY G PROTEIN AS IT INHIBITS ADENYLYL CYCLASE.NOTE THAT OTHER TYPES ARE FOUND IN SPECIALIZED TISSUES SUCH ASBLOOD VESSELS, NOSE AND EYE.

(cascade starts here)

AT THIS STAGE, GTP HAS BEEN FORMED AND HAS EITHER STIMULATEDOR INHIBITED ADENYLYL CYCLASE.

cAMP may be broken down at this point BEFORE it enters into any cascade. THIS IS ACCOMPLISHED WITH THE ENZYME cAMP PHOSPHO-DIESTERASE (PDE). HOWEVER, THE ENZYME CAN BE INHIBITED BY METHYL XANTHINES SUCH AS CAFFEINE AND THEOBROMINE (FOUND IN COFFEE, TEA, CHOCOLATE AND COLA DRINKS. THIS MAY RESULT,FOR EXAMPLE, IN INCREASED GI MOTILITY

CASCADE MECHANISMS:

EXAMPLES OF THESE MECHANISMSHAVE BEEN SEEN BEFORE. A WELL-KNOWN ONE IS CONCERNED WITH THE BREAKDOWN OF GLYCOGEN INTO GLUCOSE WHEN THE HORMONE IS EPINEPHRINE. THAT’SSHOWN OFTEN SINCE MANY, MANYOF THE OTHERS HAVE NOT BEENWORKED OUT WITH CERTAINTY –THOUGH EVIDENCE FOR THEIREFFECTS ARE KNOWN.

NOTE: IN PARTICULAR, THE ENZYMEPROTEIN KINASE A (PKA) AND THE OVERALL AMPLIFICATION FROM xMOLECULES TO 10,000x MOLECULESIN THE CASCADE.

PROTEIN KINASES EXIST AT THE START OF MANY CASCADES AND EXIST IN A VARIETY OF ISOFORMS. THE ONE IN THIS CASCADE ISPROTEIN KINASE A (PKA) WHERE “A” IS SHORT FOR: cAMP.

IN THE INACTIVE FORM(BEGINNING OF THECASCADE), PKA HASFOUR SUBUNITS WITHFOUR BINDING SITESFOR cAMP. WHEN cAMPBINDS TO THE SITES ITCAUSES THE C (ORCATALYTIC) SUBUNITSTO DISSOCIATE FROM THE R SUBUNITS ANDFORM AN ACTIVE ENZYME. SHOWN ONTHE RIGHT IS ANACTIVATED SUBUNITWITH ITS SUBSTRATESHOWN IN RED. SOME OTHER PROTEIN KINASES ARE: PKB (USING A DERIVATIVE OF INOSITOL) AND PKC (USING CALCIUM)

THE LAST MECHANISM THAT WE WILL LOOK AT INVOLVES CALCIUM AND CALMODULIN

THIS IS ACTUALLY A DOUBLE MECHANISM THAT REQUIRES TWO MEMBRANES, TWO ENZYMES AS WELL AS THE USUAL HORMONE, RECEPTOR PROTEIN AND A G PROTEIN (Gq) TO OPERATE. CALCIUMAND CALMODULIN PARTICIPATE IN PART OF THE MECHANISM.

HORMONES ASSOCIATED WITH THIS MECHANISM INCLUDE PEPTIDE HORMONES SUCH AS VASOPRESSIN (POSTERIOR PITUITARY) AND THYROIDRELEASING HORMONE (TRH).

THIS IS A MECHANISM THAT MAKES USE OF PROTEIN KINASE C.

CALCIUM / CALMODULIN MECHANISM:

CASCADE 1

CASCADE 2

SUMMARY1. PEPTIDE HORMONES VARY FROM A FEW HUNDRED TO THOUSANDS OF DALTONS IN SIZE.2. MANY OF THEM ORIGINATE IN THE HYPOTHALMIC- PITUITARY REGION.3. EXCEPT FOR THE LARGEST HORMONES (PROTEINS) THEY DO NOT REQUIRE CARRIERS IN THE BLOOD.4. PEPTIDE HORMONES BIND TO RECEPTOR PROTEINS ON THE CELL SURFACE. AFERWARDS, THEY ARE INTERNALIZED AND DESTROYED.5. INSULIN IS A PEPTIDE HORMONE THAT IS MADE IN THE PANCREAS TO TAKE GLUCOSE INTO SPECIFIED INSULIN DEPENDENT CELLS.6. IT IS STORED IN INACTIVE FORM. WHEN RELEASED, IT BINDS TO A RECEPTOR -- AN INACTIVE KINASE.7. THE KINASE STARTS A CASCADE OF EVENTS AMONG WHICH IS GLUCOSE UPTAKE.

8. WHEN cAMP (or cGMP) is used as a 2nd messenger, the process proceeds as follows: hormone > receptor protein > activation of G protein > activation of adenylyl (guanylyl) cyclase > cAMP (cGMP) > cascade. 9. CONTROL MECHANISMS ABOUND FOR THIS MECHANISM.10. WHEN CALCIUM / CALMODULIN ARE USED AS 2ND MESSENGERS, THE MECHANISM PROCEEDS AS FOLLOWS: HORMONE > RECEPTOR PROTEIN > ACTIVATION OF G PROTEIN > ACTIVATION OF PHOSPHOLIPASE C TO RELEASE I tris P WHILE DAG ACTIVATES PKC ALONG WITH CALCIUM FREED BY I tris P. CALMODULIN OPERATES THE 2ND CASCADE WHILE CALCIUM (FREE) OPERATES THE 1ST CASCADE VIA PKC.

WHEW!!