Basic concepts of Metabolism Metabolism and metabolic pathway• Metabolic Map• Catabolism • Anabolism- Regulation of Metabolism • Signals from within the cell (Intracellular)• Communication between cells. - Biosignaling: Signal transduction * Transduction by Intracellular receptors * Transduction by Cell-surface receptors

a. Ligand-Gated Ion channels b. Receptor enzyme c. Receptors involving second messenger molecules

i. Adenylate cyclase systemii. Phosphatidylinositol systemiii. Ca+ as second messenger

* Other messenger systemsa. cGMPsb. Nitric oxide

References:chapter 8 of Lippincotschapter 13 of Lehningers

* Enzymes catalyze different reactions that don't occur in isolation but organized into multi step sequence called pathway.

* The product of one reaction will be the substrate of the subsequent reaction

* Different pathways intersect forming an integrated reactions collectively called Metabolism

* Metabolic MapA picture containing the central pathways of energy metabolism. Each pathway is composed of multienzyme sequence and each enzyme has catalytic or regulatory features

The Metabolic MapShows the links between pathways, indicates the intermediates between different cycles (pathways) and shows the effect on different intermediates if one pathway is blocked

* Metabolic MapIt is a picture containing the central pathways of energy metabolism. Each pathway is composed of multienzyme sequence and each enzyme has catalytic or regulatory features

The Metabolic MapShows the links between pathways, indicates the intermediates between different cycles (pathways) and shows the effect on different intermediates if one pathway is blocked

Catabolic and Anabolic Pathways

Pathways can be classified as either catabolic (degradative) or anabolic (synthetic)

Catabolic: degradation of complex molecules (polysaccharides, proteins) into simple molecules like CO2, NH3 and water. Catabolism is convergent process: a wide variety of molecules are transformed into a few common end products

Anabolic reactions are the synthesis of complex molecules from simple precursor.Anabolic is divergent process in which few biosynthetic precursors form a wide variety of polymeric or complex products.

Catabolism- catabolic reactions provide chemical energy in the form of the ATP from the degradation of the energy-rich fuel compound. The catabolism is essential for providing energy necessary for building up the complex compound

Energy generation occur in three stepsStage IStage IIStage IIIEnergy is librated from the transfer of electrons from NADH and FADH2 to O2 through the electron transport chain

Anabolism

- Anabolic reactions combine small

molecules as amino acids to form

large complexes as proteins. These

processes require energy which is

provided by the break down of ATP

to ADP

- The biosynthetic pathway usually

is different from degredative

pathway of the same compound so

the two processes respond to

different regulatory

- Anabolic reactions involve

chemical reduction in which the

reducing power is NADPH

Regulation of Metabolism

•Pathways of the metabolism must be coordinated so that the

catabolism and the anabolism must meet the needs of the cell.

• The cell is not present in isolation it present in a tissue, in which

all the cells communicate together with regulatory signals.

• Regulatory signals including hormones, nervous system and

availability of nutrients which affect the signals generated within

the cell itself.

*Signals from within the cell (Intracellular)

the rate of a metabolic pathway may respond to regulatory

signals from the cell, e.g. the rate of the pathway may be

influenced by the availability of the substrate, product inhibition

or alterations in the level of allosteric activators or inhibitors.

These intracellular signals provide rapid responses and are

important for the moment to moment regulation metabolism

*Communication between cells.

Signals between cells provide for long-range integration of

metabolism and show slower response. Cell communication involves

surface interactions. For metabolism the chemical signaling is

involved like hormones and neurotransmitters released by nervous

system

Biosignaling: Signal transduction

Signal transduction is specific and very sensitive

* Specificity is achieved by precise molecular complementary

between signal and receptor molecule.

Epinephrine affect glycogen metabolism in hepatocyet and not in

erythrocyte because of the absence of the receptors.

The affinity of the signal to the receptor is very high highly

sensitive

Two Basic mechanisms of Signal transduction

Intracellular receptors

Cell-surface receptors

Vit D, steroidal hormones, retinoic acid

and thyroxine act through intracellular

receptors located in the cytosol or the

nucleus. The receptor-ligand complex

inter the nucleus and bind to specific

regions of the DNA (enhancer region)

causing increasing the expression of the

specified gene. These hormones should

penetrate the cell membrane and bind to

specific region. Their effect are not

immediate because time is required for

gene transcription and then mRNA

translation. But the duration of action will

be longer.

Transduction by intracellular receptors steroid

c

Transduction by cell-surface receptors.

• Signals transduction by hormones and neurotransmitters is

initiated by ligand binding to receptors located in the

plasma membrane.

• This transduction dose not regulate the gene expression

directly. Simply signal interact with receptors activated

receptors interact with cellular machinery producing a

second signal or change in the activity of a cellular protein

metabolic change in target

Three general classes of cell-surface receptors based

on their mechanism of signal transduction.

a. Ligand-Gated Ion channels (Neurotransmitter receptors

linked to ion channels).

b. Receptor enzyme (Catalytic receptors)

c. Receptors involving second messenger molecules.

Three general classes of cell-surface receptors based on their mechanism of signal transduction

Ligand-Gated Ion channels

Commonly called transmitter-gated ion channels or ionotropic receptors

- Involved in rapid synaptic signaling

- Best example for this type is Nicotinic acetylcholine receptors

- Acetylcholine receptors are allosteric protein with two binding sites for

Ach.

- Classically defined by acetylcholine (ACh) receptor at neuromuscular

junction

- Nerve impulse depolarize axon, signal travels to nerve terminal leading

to opening of voltage-gated Ca+ channels, Ca+ flows in and Ach is

released to post synaptic neuron or myocyte.

- Ach binds receptors on muscle cells leading to opening of cation (Ca+,

Na+) channel and Na+ flows in and thus depolarization of the receiving

cell initiates another action potential (neuron) or contraction of the

muscle cell (if myocyte)

Voltage-Gated Ion channels open and close as response to electrical

change

Voltage-gated Na+ channel

Voltage-gated K+ channel

Voltage-gated Ca+ channel

figure 13-5Role of voltage-gated and ligand-gated ion channels in neural transmission. Initially, the plasma membrane of the presynaptic neuron IS polarized (inside negative) through the action of the electrogenic Na+-K+ ATPase. which pumps 3 Na+ out for every 2 K+ pumped Into the neuron , (1) A stimulus to this neuron causes an action potential to move along the axon (white arrow), away from the cell body. The opening of one voltage-gated Na+ channel allows Na+ entry and the resulting local depolarization causes the adjacent Na+ channel to open, and so on. The directionality of movement of the action potential is ensured by the brief refractory period that follows the opening of each voltage-gated Na+ channel. (2) When the wave of depolarization reaches the axon tip, voltage-gated Ca+2 channels open, allowing Ca+2 entry into the presynaptic neuron. (3) The resulting increase in internal [Ca+2] triggers exocytic release of the neurotransmitter acetyleholme into thesynaptic cleft. (4) Acetylcholine binds to a receptor on the postsynaptic neuron, causing its ligand-gated Ion channel to open. (5) Extracellular Na+ and Ca++ enter through this channel, depolarizing the postsynaptic cell. The electrical signal has thus passed to the cell body of the postsynaptic neuron and will move along its axon to a third neuron by the same sequence of events.

Ach binds receptors on muscle cells leading to opening of cation (Ca+,

Na+)

channel and Na+ flows in and thus depolarization of the receiving cell

initiates

another action potential (neuron) or contraction of the muscle cell (if

myocyte)

Neural transmission

Receptor Enzyme•These Transmembrane catalytic receptors have an

inherent enzymatic activity as part of their structure.• have ligand binding domain on the extracellular

surface of the plasma membrane and enzyme active

site on the cytosolic side.• Commonly they are a protein kinase that

phosphorlate Tyr residues in the specific target

proteins• Insulin receptor is prototype for this type • The binding of the a ligand (insulin) to its receptor

activates the tyrosine kinase activity which transfer

the phosphate group from ATP to the –OH group of

the Tyr residues of target proteins and of the receptor

itself

figure 13-6Insulin receptor. The insulin receptor consists of twoα chains on the outer face of the plasma membrane and two chains that traverse tile membrane and protrude from the cytosolic face. Binding of insulin to the α chains triggers a conformational change that allows the autophosphorylation of Tyr residues in the carboxyl-terminal domain of the subunits. Autopllosphorylation further activates the tyrosine kinase domain, which then catalvzes phosphorvlation of other target proteins.

Receptors involving second messenger molecules.

Many signals when bind to their receptors initiate a series of reactions in form of cascade reactions that at the end result in a specific intracellular response. The intracellular messenger systems function as signal amplification.

signal amplification.

Receptors involving second messenger molecules

I. Adenylate cyclase system-Typical example is the adrenergic receptors β-receptors that bind to

epinephrine and then trigger either an increase or decrease in the

activity of the adenylate cyclase.- Adenylate cyclase convert the ATP into cAMP which act as second

messenger and activate other enzymes to produce the activity. - Many signals (hormones) act through activating the cAMP like

glucagon - the effect of signals on the second messenger is not direct but

mediated through G-proteins. GTP-dependent regulatory proteins. The

inactive form of G-protein binds to GDP while the active form bind to

GTP.- The binding of hormone to its receptor activate the G-protein which

affect the activity of adenylate cyclase- The activity of signal depends one the type of G-proteins. Gs

stimulates the adenylate cyclase while Gi inhibits it. - The actions of G-protein _GTP complex are short lived because G-

protein has an inherent GTPase activity, resulting in rapid hydrolysis of

GTP to GDP and this causes the inactivation of G-protein.

Adenylate cyclase convert the ATP into cAMP which act as second messenger

Receptors involving second messenger

molecules Adenylate cyclase system

Role of G-Proteins in Signal Transduction

Activation Adenylate cyclase system via G-Proteins

cAMP activates Protein Kinases A.

-The next link in the cAMP second messenger system is the activation of Protein Kinases by cAMP.- Family of enzymes called cAMP-dependent protein kinases. - Protein Kinase A is the typical example, consists of 4 monomers. 2 catlytics and 2 regulatory.-The active subunits catalyze the transfer of phosphate from ATP to specific serine or threonine residues of protein substrate.- the phosphorylated protein can act directly or can activate or inhibit other enzymes to produce the effect.- Not all protein kinases respond to cAMP, other types are cAMP-independent protein kinase like proetin kinase C.- The phosphate group can be removed by proetin phosphatases. - cAMP is rapidly hydrolyzed to 5-AMP by phosphodiesterase.

Activation of cAMP-

dependent protein

kinase, PKA

Activation of cAMP-dependent protein

kinase (PKA)

Phosphatidylinositol 4,5 bisphosphate

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IP3

Second messengers derived from Phosphatidylinositol

Role of Phosphatidylinositol 4,5 bisphosphate in Signal transduction

IP3 activates the release of Ca+

Ca+ is a second messenger in many

signal transductions-Ca+ serves as a second messenger that

triggers intracellular responses as exocytosis

in neurons and endocrine cells, contraction

in muscle and others- Ca+ is released from endoplasmic

reticulum in response to signals (hormones,

neurotransmitters)- Ca+ bind to Ca-binding proteins that called

calmodulin- Calmodulin-ca complex binds and activates

protein molecules usually enzymes, - Calmodulin is regulatory subunit of

phosphorylase b kinase of muscle that

activated by Ca activating the break down

of glycogen. - Many enzymes are know to be modulated

by Ca+ through calmodulin.

Ca+ is a second messenger in many signal transductions

Other messenger systems

a. cGMP

b. Nitric oxide

* Cyclic guanosine monophosphate

It is analogous to cAMP pathway

Synthesized from GTP by guanylate cyclase, it an integral

part of the receptor (not separated like Adenylate cyclase), it

contains heme as prosthetic group and stimulated by nitric

oxide.

Activate a spcific form of protein kinase called cGMP-

dependent protein kinase also called protein kinase G

cGMP is hydrolyzed by phosphodiesterase

cGMP is a specialized messenger being involved in smooth

muscle relaxation, platelet aggregation and the visual

system. cAMP affects a wide variety of processes.

Nitric Oxide

NO act as endothelium relaxing factor, causes vasodilatation by relaxing

vascular smooth muscle and also acts as neurotransmitters, prevents

platelet aggregation and has a role in macrophage function.

It is highly toxic. Nitrous oxide ( NO2) the “ laughing gas” that used as

anesthetic.

NO is very short lived and unstable converted into oxygen and nitrate and

nitrite.

Synthesis of NO

-NO synthase catalyzes the formation of NO from amino acid Arginine,

FMN, FAD, and tetrahydrobiopterin are coenzymes for the enzymes

- NO is synthesized in endothelial cells and diffuses to vascular smooth

muscle activate the guanylate cyclase rise in cGMP which causes

muscle relaxation.

- Synthesis of NO is stimulated in the macrophages by bacterial

liopolysaccharides activated macrophages form oxygen free radical that

combine with NO to form compounds that are more bactericidal than NO

itself

Nitric Oxide act as endothelium relaxing factor, causes vasodilatation by relaxing vascular smooth muscle and also acts as neurotransmitters, prevents platelet aggregation and many functions