BIO 3C EXAM 2

Be familiar with Insulin Signaling

* A second example of an RTK (receptor tyosine kinase) * Via a receptor tyrosine kinase * Insulin signaling will trigger the movement of glucose receptors such as GLUT4, to the cell membrane so that they can take glucose in, which will trigger the store of glucose as glycogen, it'll promote glycogen synthesis. Insulin signaling inhibits degradation of fat and promotes fat synthesis. * Insulin is the ligand, it will bind to the receptor (blue) at the α subunit. There's the transmembrane domain (the part of the yellow within the membrane), and then the intracellular domain (yellow outside the membrane). The yellow is the β subunit. Binding of insulin results in cross-phosphorylation, and then the phosphorylated sites on the insulin receptor act as binding sites for the protein known as the insulin receptor substrate, (or IRS-1). Then, the IRS protein will be phosphorylated, again by the tyosine kinase activity of the insulin receptor. Now that its phosphorylated, it'll act as an adapter protein. These phosphotyrosine residues will be recognized by other proteins including (shown in green) the lipid kinase that will phosphorylate PIP2, to form PIP3. PIP3 activates PDK1 (PIP3-dependent protein kinase 1), which is yet another kinase that can then phosphorylate and activate a protein known as Akt. Akt is a serine/threonine kinase. It'll phosphorylate enzymes that stimulate glycogen synthesis, and it'll also phosphorylate components to control the trafficking of GLUT4 to the cell surface. When we have more GLUT4 receptors at the surface of our cells, we can bring glucose into cells and get glucose out of the blood. Proteins phosphorylated on serine, threonine, or tyrosine residues, such as insulin-receptor substrates, do not hydrolyze spontaneously; they are extremely stable kinetically. Specific enzymes, called protein phosphatases, are required to hydrolyze these phosphorylated proteins and convert them back into the states they were in before the initiation of signaling. Similarly, lipid phosphatases are required to remove phosphoryl groups from inositol lipids that had been phosphorylated as part of a signaling cascade. In insulin signaling, three classes of enzymes are of particular note: (1) protein tyrosine phosphatases that remove phosphoryl groups from tyrosine residues on the insulin receptor, (2) lipid phosphatases that hydrolyze PIP3 to PIP2, and (3) protein serine phosphatases that remove phosphoryl groups from activated protein kinases such as Akt. Many of these phosphatases are activated or recruited as part of the response to insulin. Thus, binding of the initial signal sets the stage for the eventual termination of the response. Insulin receptor: IRS-1 -> PIP3 kinase -> PDK-1 -> Akt

What role do protein kinases and phosphatases play in signal transduction?

* Kinases are enzymes that phosphorylate a substrate at the expense of a molecule of ATP (Serine/Threonine/Tyrosine residues (-OH)). * We have seen that the activated G protein promotes its own inactivation by the release of a phosphoryl group from GTP. In contrast, proteins phosphorylated on serine, threonine, or tyrosine residues, such as insulin-receptor substrates, do not hydrolyze spontaneously; they are extremely stable kinetically. Specific enzymes, called protein phosphatases, are required to hydrolyze these phosphorylated proteins and convert them back into the states they were in before the initiation of signaling. Similarly, lipid phosphatases are required to remove phosphoryl groups from inositol lipids that had been phosphorylated as part of a signaling cascade. In insulin signaling, three classes of enzymes are of particular note: (1) protein tyrosine phosphatases that remove phosphoryl groups from tyrosine residues on the insulin receptor, (2) lipid phosphatases that hydrolyze PIP3 to PIP2, and (3) protein serine phosphatases that remove phosphoryl groups from activated protein kinases such as Akt. Many of these phosphatases are activated or recruited as part of the response to insulin. Thus, binding of the initial signal sets the stage for the eventual termination of the response.

Be able to fill in this table.

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Be familiar with the epinephrine signaling pathway discussed: (1) activation of adenylate cyclase & (2) activation of phospholipase C. Be able to sketch the pathway.

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Fill in the table (all blank boxes, and add detail when possible).

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Know this table.

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List 5 secondary messengers that are commonly used in animal cells.

1. Cyclic AMP (cAMP) 2. Cyclic GMP (cGMP) 3. Ca2+ 4. IP3 5. Diacyglycerol (DAG)

What are the 3 classes of membrane receptors? Understand their main features, mechanism of action, examples, and how the signal is terminated. First Type

1. Seven-Transmembrane-Helix Receptors 2. Dimeric receptors that recruit protein kinases 3. Dimeric Receptors that are protein Kinases 1. 7TM Helix Receptors (β-adrenergic receptor) * β-adrenergic receptor binds epinephrine. * 7TM Helix Receptors are comprised of 7 alpha helices that pass through the cell membrane. * Epinephrine binds to the 7TM receptor known as the β-adrenergic receptor, and after signal transduction has taken place, the different cellular responses (fight or flight responses) include heart muscle contraction, divert blood flow to skeletal muscle, glycogen breakdown, and more. * Ligand binding causes conformational change in the receptor. * 7 alpha helices each spans the membrane, common receptor motif. Epinephrine binding to that 7TM receptor. When it does so, that receptor changes its conformation. When it changes its conformation (shape), its going to activate the G-protein. This G protein has 3 parts, making it a heterotrimeric G protein. Notice it has α, β, γ subunits. That conformational change in its cytoplasmic domain (highlighted in pink) of the 7TM receptor will activate the G protein. Specifically, the receptor triggers an exchange of GDP (guanosine diphosphate) to GTP (guanosine triphosphate). Additionally, there's a dissociation of the β and γ subunits. Highlighted in yellow is the activated α subunit of this G protein. NOTED: Its not phosphorylation at this point, just an exchange of GDP for GTP. GTP, an activated G protein, will activate the orange enzyme known as Adenylate Cyclase, AKA Adenolil Cyclase. This enzyme catalyzes the formation of Cyclic AMP from ATP, just like the name suggests (cAMP). Cyclic AMP (our secondary messenger) activates a kinase (protein kinase A or pKa). Binding of cyclic AMP to pKa activates it. * Adenylate cyclase converts ATP to cAMP and cAMP will activate pKa. * Kinases phosphorylate substrates. * Upon inactive heterotrimeric G protein binding to the hormone receptor complex (epinephrine bound to the β-adrenergic receptor), GDP is released and GTP binds to that α subunit. It is now in its active form and binds to and activates adenylate cyclase. We don't want the active G protein to stay active forever, we want to turn it off. That happens through GTPase activity. - GTPase activity is not due to a separate enzyme. Instead, the α subunit has intrinsic GTPase activity. It will hydrolyze the bound GTP to GDP plus an organic phosphate, and now it is inactive and will reassociate with the β and γ subunits. * Adenylate cyclase has 12 membrane spanning helices. * Gαs: In the active form of the G protein (α subunit bound to GTP form), s = stimulatory. Gαs stimulates adenylate cyclase. In order to activate pKa, four individual cAMP molecules need to bind to the cAMP binding domains. When that occurs. the 2 regulatory chains will dissociate from pKa, and now you have the C catalytic subunits which become active. These active molecules phosphorylate other proteins, specifically Serine and Threonine residues. 7TM-GPCR: another way we can organize 7TM receptors is by those that subsequently activated a G protein. We call these the 7TM G-protein coupled receptors. The adrenergic receptor can activate other pathways as well. A 7TM-GPCR activates 2 major pathways: 1. Gαs -> adenylate cyclase -> cAMP -> pKa * breaks down glycogen for fuel and halts synthesis of glycogen. Once pKa is activated, glycogen will be broken down for fuel. The synthesis for the storing of glucose in the form of glycogen is going to be halted. This is because when epinephrine activates the β-adrenergic receptor, you have a fight or flight response. Glycogen will be broken down so that we can provide energy for muscle contraction and so forth. 2. Gαq -> Phospholipase C (PLC) -> DAG + IP3 * PLC -> DAG -> pKc * PLC -> IP3 -> Ca2+ -> pKc and/or calmodulin binding * Calmodulin (with Ca2+) -> CaM kinase - breaks down glycogen for fuel and halts synthesis of glycogen. In addition to Gα subunit becoming stimulated (Gαs), other hormones can bind to this receptor and activate a G protein that activates a different enzyme called phospholipase C (PLC). This G protein is called Gαq. Once PLC is activated, two additional second messengers are activated: DAG and IP3. DAG will go on to activate a protein kinase known as protein kinase C (pKc). IP3 will stimulate a calcium release that then can also serve to activate pKc, or it will activate a kinase called CaM kinase (calmodulin kinase). This is a different pathway entirely from the first, but you can see the outcome is the same. We break down glycogen as fuel and we're going to stop storing up glucose in the form of glycogen, we're going to stop glycogen synthesis. * PLC Pathway: A messenger such as vasopressin will bind to the 7TM receptor that will activate the heterotrimeric G-Protein. In doing so, GDP is exchanged for GTP. That αq subunit will activate the enzyme PLC. PLC cleaves the phospholipid PIP2 into two products: Diacylglycerol (DAG) and IP3. DAG will activate protein kinase C, which phosphorylates other proteins will IP3 will bind to receptors in the endoplasmic reticulum. These receptors will open up and cause a release of calcium into the cytosol. Calcium will bind to calmodulin, which will activate protein kinase C, and there are other responses as well such as contraction and secretion. The purple receptor is called the IP3 receptor. * PLC is in the plasma membrane. * DAG stays within the membrane * IP3 can diffuse into the cytoplasm Upon pKc activation, it phosphorylates Serine and Threonine residues. Calmodulin is activated by Ca2+ binding. CaM is a calcium sensor. Activated CaM binds (and activates) CaM kinase and other proteins (CaM kinase is a calmodulin-dependent protein kinase). CaM kinase also adds PO4 to glycogen synthase to inactivate it. * Once IP3 receptor is activated by IP3, calcium will flow from the endoplasmic reticulum out into the cytoplasm. Those high levels of calcium will trigger a variety of biochemical processes such as smooth muscle contraction and glycogen breakdown. * Once calcium is out into the cytoplasm, it can bind to a number of molecules including CaM. All proteins in the EF hand family bind to Ca2+. Activating CaM kinase enables this kinase to go on and phosphorylate a wide variety of target proteins. There are other targets for CaM, not just CaM kinase. Calcium is important in signal transduction. * Calcium is a second messenger localized in space and time. Ending Signaling * G protein "turns off" as GTP -> GDP * cAMP broken down (remove 2nd messenger) by cAMP phosphodiesterase. * Ca2+ pumped out of the cell or stored in the ER. * IP3 degraded or phosphorylated. * DAG converted to phosphotidate or hydrolyzed. * Ligand dissociates from the receptor. * Receptor can be removed from the membrane; sequestered into intracellular vesicles. Signal termination 1 - Gα Protein * GTP (G protein in active form) uses GTPase activity to convert GTP back to GDP, and we have reassociation with the β and γ subunits. Signal termination 2 - at the receptor * Dissociation of epinephrine from the receptor. * Without the epinephrine, the cytoplasmic domain will no longer activate a G protein. More than 700 GPCR's in humans 50% of drugs target different GPCR's

What are the 3 classes of membrane receptors? Understand their main features, mechanism of action, examples, and how the signal is terminated. Second Type

1. Seven-Transmembrane-Helix Receptors 2. Dimeric receptors that recruit protein kinases 3. Dimeric Receptors that are protein Kinases 2. Dimeric Receptors That Recruit Protein Kinases (Human Growth Hormone Receptor) * Extracellular domains of two HGH receptors bind to one growth hormone. * The ligand is the growth hormone. * Ligand binding changes quaternary structure of the receptor, results in receptor dimerization. When the two receptors come together, they are now activated. JAK2 associates with the intracellular domain of the receptor. * Dimerization brings two JAK2 proteins together. * Each of these kinases will phosphorylate their partner. - JAK2 will phosphorylate tyrosine residues on the other kinase. * Activated JAK2 will phosphorylate a transcription factor known as STAT. - STAT will go to the nucleus and will bind to DNA and regulate gene expression. SUMMARY: The HGH (ligand) binds to the HGH receptor, leading to dimerization of the receptor and the activation of JAK2. THe JAK2's will cross phosphorylate eachother, leading to the activation of STAT5. STAT5 is a transcription factor. Once it's phosphorylated, it can move to the nucleus, bind to DNA, and control transcription. ANOTHER SUMMARY: Hormone binds -> Receptor dimerization -> cross-phosphorylation of JAK2 -> JAK2 phosphorylates other targets -> ex. JAK2 phosphorylates STAT5 and STAT 5 binds to DNA to activate transcription. * You can have other signal proteins that bind to receptors and lead to STAT activation. Human growth hormone receptor: JAK2 -> STAT

What are the 3 classes of membrane receptors? Understand their main features, mechanism of action, examples, and how the signal is terminated. Third Type

1. Seven-Transmembrane-Helix Receptors 2. Dimeric receptors that recruit protein kinases 3. Dimeric Receptors that are protein Kinases 3. Dimeric Receptors That Are Protein Kinases * The receptor itself is a tyrosine kinase. * Previously, tyrosine kinase was recruited. In this case, the receptor is already a tyrosine kinase. * Two ligands will bind, each to their own receptor. This leads to dimerization. Then we get cross-phosphorylation (autophosphorylation). EGF Receptor is a receptor tyrosine kinase. During autophosphorylation, the kinase domains of these intracellular domains will phosphorylate one another. Again, this is ligand-induced dimerization, and each kinase domain will catalyze the phosphorylation of its partner. After cross phosphorylation, the phosphorylated receptor will bind to the purple protein known as Grb-2. That binds to the blue protein known as Sos. That stimulates the exchange of GTP for GDP in this G protein, known as Ras. Now that it is bound to GTP, its activated, and the signal continues. Activated Ras will bind and activate other protein kinases. * There are many receptor tyrosine kinases. EGF Receptor Domains: * Ligand binds -> Receptor dimerization. * 2 receptor kinase domains will phosphorylate eachother making each active (cross-phosphorylation of Tyr residues). * Once activate, they bind adapter proteins: Grb2. * Protein-protein interactions transmit the signal. * G-protein Ras is activated, Ras continues signal. * The EGF binding domain is the portion of the receptor that's on the cell surface. * The kinase domain is the portion in the cytoplasm. * The C-terminal tail is tyrosine rich and the area that will be phosphorylated through cross-phosphorylation. Ras belongs to a larger family of G proteins called the Ras Superfamily. Ras is a monomeric GTPase, a molecular switch. This is different from the heterotrimeric which we learned about previously with the β-adrenergic receptor pathway. Other than that, the pattern is the same. Ras is a G-Protein, it is inactive when bound to GDP, interaction with Sos will stimulate the exchange of GTP for GDP, once bound to GTP, Ras is now active and it will activate Serine/Threonine kinases. Once again, intrinsic GTPase activity results in a return of that GTP back to GDP. Ras's intrinsic GTPase activity serves to terminate the signal and return the system to the inactive state. NOTE: We're not just popping GDP in and out. We're removing the entire nucleotide during the activation step. But when we return back to the GDP bound form, that's through a different mechanism. That's through GTPase activity. EGF Receptor: Grb -> Sos -> Ras -> kinases activated

Define and differentiate a primary messenger and second messenger.

A stimulus such as a wound or a digested meal triggers the release of the signal molecule (ligand), also called the primary messenger. Structural changes in receptors lead to changes in the concentration of small molecules, called second messengers, that are used to relay information from the receptor-ligand complex. Summary: primary messenger is the signaling molecule while the second messenger relays information, often amplifying the message. 1. Signal molecule (primary messenger, ligand) binds cell receptor. 2. Cell receptor changes conformation and triggers intracellular second messenger molecule. 3. Second messenger activates intracellular responses (often amplifies the message). 4. Activation of effectors that alter the physiological response - Changes in protein activity or gene expression. 5. Termination of the signal Most signal molecules (primary receptors) are too large and too polar to pass through the cell membrane or through transporters. Thus, the info presented by signal molecules must be transmitted across the cell membrane without the molecules themselves entering the cell. Membrane receptors transfer info from the environment to a cell's interior. Such receptors are integralmembrane proteins that have both extracellular and intracellular domains. A binding site on the extracellular domain specifically recognizes the signal molecule (ligand, primary messenger). The formation of the receptor-ligand complex alters the tertiary or quaternary structure of the receptor, including the intracellular domain. However, structural changes in the few receptors that are bound to ligands are not sufficient to yield a response from the cell. The info conveyed by the receptor must be transduced into other forms of info that can alter the cell's biochemisty. Structural changes in receptors lead to changes in the concentration of small molecules, called second messengers, that are used to relay info from the receptor-ligand complex. The use of second messengers have several consequences. One consequence is that second messengers are often free to diffuse to other compartments of the cell, such as the nucleus, where they can influence gene expression and other processes. Another consequence is that the signal may be amplified significantly in the generation of second messengers. Each activated receptor-ligand complex can lead to the generation of many second messengers within the cell. Thus, a low concentration of signal molecules in the environment, even as little as a single molecule, can yield a large intracellular signal and response. Second messengers relay info from the receptor-ligand complex into the cell. These are the intracellular molecules that will change in concentration in response to environmental signals. They're small so they can easily diffuse into other compartments, like the nucleus. They also assist in that amplification. You can have one ligand binding to the receptor, generating many second messenger molecules.

Be familiar with coenzymes common in metabolism and what they carry: NAD+, FAD, CoA, NADP+.

Activated Carriers of Electrons for Fuel Oxidation * NAD+ can carry: H- (hydride ion, 1 proton and 2 e-) * FAD can carry: 2 H's (2 protons and 2 e-) NAD+ is an e- carrier. Here, we have NAD+ in the oxidized form. The nicotinamide ring will accept a hydrogen ion and two e-, to give us NADH (the reduced form). NAD+ is a great e- acceptor. So, in this example, we have a proton and two e- directly transferred to NAD+, and then the other proton appears in the solvent. FAD is another great e- carrier. Just like NAD+, it can also accept 2 e-. However, it takes up two protons as well, unlike NAD+. Here, we see FAD in the oxidized form. In the reduced form, FADH2, two protons have been taken up along with 2 e-. Activated Carriers of Electrons for the Synthesis of Biomolecules Products are more reduced than the reactants. NADPH is the electron donor used (NADH + phosphate) * NADPH used for reductive biosynthesis * NADH used for the generation of ATP We need high potential e- to be used in anabolic processes, compared to catabolic processes. Because the precursors are more reduced than the products. So we need some reducing power in addition to ATP. So, as an example, in the biosynthesis of fatty acids, the keto group is reduced to a methylene group, and that actually requires the input of 4 e-. * NADP+ is the oxidized form. NADPH will carry e- in the same way as NADH did. * CoA-SH (Coenzyme A) carries an acyl group. Important in catabolism and synthesis of fatty acids. Acyl group is linked by a thioster bond that is a 'high energy' bond due to it not being stabilized by resonance (hydrolysis is favored). Bond has a high acyl-group-transfer potential. CoA is a carrier of two carbon fragments. We need acyl groups in the oxidation of fatty acids and for the synthesis of membrane lipids. The hydrolysis of a thioester bond is thermodynamically more favorable than that of an oxygen ester.

Differentiate between aerobic respiration and anaerobic respiration.

Anaerobic and Aerobic Catabolism of Glucose Provides Energy for: 1. Mechanical work 2. Active transport 3. Synthesis of macromolecules While aerobic metabolism generates more ATP and relies on oxygen, anaerobic metabolism does not need oxygen but only creates two ATP molecules per glucose molecule. However, anaerobic and aerobic metabolism are both required to produce cellular energy.

Why are vitamins necessary for metabolism and good health?

Because many of the activated carriers are coenzymes that are derived from water-soluble vitamins. Activated carriers exemplify the modular design and economy of metabolism. ATP is an activated carrier of phosphoryl groups. - Transfer of the phosphoryl group from ATP is energetically favorable (exergonic, negative ΔG) Coenzymes are often activated carriers. Coenzymes carry electrons removed by oxidation of fuels (in glycolysis and the citric acid cycle) - Coenzymes have a higher affinity for e-'s than C fuels, but lower affinity for e-'s than O2.

Differentiate between catabolic reactions and anabolic reactions.

Catabolism: reactions that transform fuels into cellular energy; oxidation of glucose. Anabolism: reactions that require energy; synthesis of molecules. Each pathway is usually distinct: * 1 enzyme(s) used for key steps of a catabolic pathway and a different enzyme(s) used for key steps of an anabolic pathway. * Enhances control over these pathways. Cells and food molecules result in useful forms of energy, although some of that is lost as heat. We end up with the building blocks of biosynthesis. These would be catabolic pathways. Those building blocks used for biosynthesis will form many new molecules, but we have to have an input of energy. Those are anabolic reactions. Our cells continually need to make more ATP. ATP is used to power motion, active transport, biosynthesis, signal amplification.: Anabolic rxns. ADP is converted back into ATP through oxidation of fuel molecules or photosynthesis.

Reactions must be thermodynamically favorable in order to occur in cells. How are reactions with positive Delta G values made thermodynamically favorable?

Metabolism is composed of many interconnecting reactions. Metabolism consists of energy-yielding reactions and energy-requiring reactions. A thermodynamically unfavorable reaction can be driven by a favorable reaction. * (-) charged: favorable * (+) charged: unfavorable Metabolic pathways are formed by the coupling (adding together) of enzyme-catalyzed reactions that lead to a thermodynamically favorable reaction. * Negative ΔG values indicate energetically favorable/spontaneous. * Coupling reactions to the hydrolysis of ATP makes many reactions now favorable. ATP is an energy coupling agent. - We couple its hydrolysis to another reaction, and in doing so, we convert an unfavorable reaction -> favorable one. ATP is energy rich due to its two phosphoanhydride linkages. A large amount of energy is released when ATP is hydrolyzed to ADP, or to AMP. Compounds with high phosphoryl transfer potential can couple carbon oxidation to ATP synthesis. Carbon oxidation generates an acyl phosphate known as 1,3-BPG. The e- captured by NAD+ and 1,3-BPG has a high phosphoryl transfer potential. Cleavage of the 1,3-BPG is coupled to the synthesis of ATP. So, the energy of oxidation is initially trapped as the high phosphoryl transfer potential molecule (1,3-BPG), and then its used to form ATP. Phosphoryl transfer can be used to drive otherwise thermodynamically unfavorable reactions.

Understand what is meant by energy charge of the cell.

Metabolism is often controlled by the energy status of the cell. * Energy charge can range from 0 (all AMP) to 1 (all AMP). High energy charge inhibits catabolic reactions (= near 1). Low energy charge inhibits anabolic reactions (= near 0). If I have plenty of energy, why should a catabolic reaction take place? Let me inhibit glycolysis if I already have plenty of ATP. Anabolic reactions shouldn't take place if there isn't ATP available to power them. Relating to Practice Problem Recall energy charge is between 0-1. When the energy charge is low, catabolic reactions will be very active. However, as the energy charge increases within the cell, those catabolic reactions will slow down and will have an increase in anabolic reactions. The reactions that use energy will be active when that energy is available. When energy charge is low, we have very little ATP so catabolic reactions are in full gear. But, as we start to approach 1 on the x-axis, energy charge is very high, and we have lots of ATP. Now, anabolic reactions will be in full gear making glycogen, proteins, fatty acids, using ATP to do so. Catabolic reactions would be breaking down fatty acids, glucose, proteins, all to get ATP because clearly the cell needs ATP.

Compare molecules in a reaction and be able to determine which ones are oxidized vs. reduced over the course of a reaction.

Oxidized means loss of e- (less H/Increase in O) EX.: Gained a double bond Reduced is gain of electrons (more H/decrease in O) EX: Lost a double bond

Sketch and explain the signaling cascade initiated by EGF binding the EGF receptor. How might Ras be mutated in some cancer cells?

Ras might be mutated so that it loses its intrinsic GTPase activity, dissalowing it to terminate the signal and return the system to the inactive state. This could lead to uncontrolled cell growth. Genes that normally regulate cell growth often cause cancer when mutated. The gene encoding Ras, a component of the EGF-initiated pathway, is one of the genes most commonly mutated in human tumors. The most common mutation in tumors is a loss of the intrinsic GTPase activity. Thus, the Ras protein is trapped in the "on" position and continues to stimulate cell growth, even in the absence of a continuing signal.

Which amino acids can be phosphorylated?

Serine, Threonine, Tyrosine (Only amino acids with a terminal -OH group)

List 2 general responses that can result from signal transduction.

Signal transduction allows an organism to sense its environment and formulate the proper biochemical response. Signal transduction is how cells recieve, process, and respond to information from the environment whether the information is in the form of light, smell, or blood-glucose concentration. Signal transduction cascades mediate the sensing and processing of these stimuli. These molecular circuits detect, amplify, and integrate diverse external signals to generate responses such as changes in enzyme activity, gene expression, or ion-channel activity.

Understand the role of proteases in digestion.

The components of our meal must be degraded into small molecules for absorption by the epithelial cells of the intestine and for transport in the blood. Proteins are digested to amino acids by proteolytic enzymes (proteases) secreted by the stomach and pancreas. Polysaccharides such as starchare cleaved into monosaccharides by α-amylase from the pancrease and to a lesser extent in saliva. Lipids are converted into fatty acids by lipases secreted by the pancreas. All of the digestive enzymes are hydrolases; that is, they cleave their substrates by the addition of a molecule of water. Digestion prepares large biomolecules for use in metabolism. Proteases digest proteins into amino acids and peptides. Protein digestion starts in the stomach and continues in the intestine.

How do G proteins become active and inactive? Review similarities and differences between Gs and Gq, and Ras.

The conformational change in the cytoplasmic domain of the receptor activates a GTP-binding protein. This signal-coupling protein is called a G protein. In the unactivated state, the guanyl nucleotide bound to the G protein is GDP. In this form, the G protein exists as a heterotrimer consisting of α, β, γ subunits; the α subunit (referred to as Gα) binds the nucleotide. The α and γ subunits are usually anchored to the membrane covalently attached to fatty acids. The exchange of the bound GDP for GTP is catalyzed by the ligand-bound receptor. The ligand-receptor complex interacts with the heterotrimeric G protein and opens the nucleotide-binding site so that GDP can depart and GTP can bind. The α subunit simultaneously dissociates from the βγ dimer. This dissociation transmits the signal that the receptor has bound to the ligand. The Gα protein binds to adenylate cyclase on the Gα surface that had bound the βγ dimer when the Gα protein was in its GDP form. Gαs ("s" stands for stimulatory) stimulates adenylate cyclase activity, thus increasing cAMP production. Using a different pathway, phosphoinositide cascade, like the adenylate cyclase cascade, converts extracellular signals into intracellular ones. The intracellular messengers formed by activation of the pathway arise from the cleavage of PIP2, a membrane phospholipid. The binding of a hormone such as vasopressin to its 7TM receptor leads to the activation of PLC. The Gα protein that activates PLC is called Gαq. Ras is a signal transduction protein. Sos is the immediate upstream link to Ras in the circuit conveying the EGF signal. Sos binds to Ras, reaches into the nucleotide-binding pocket, and opens it up, allowing GDP to escape and GTP to enter in its place. Ras possesses an intrinsic GTPase activity, which serves to terminate the signal and return the system to the inactive state.

Fuels are oxidized in cells to provide energy. Why does oxidation of fats provide more energy than oxidation of carbs?

We're looking at the free energy of oxidation of some common single carbon compounds. Fuels are oxidized one carbon at a time, and the more reduced a carbon is to begin with, the more free energy will be released. Why? When electrons move from an atom with low affinity for an electron like carbon here, to an atom with high affinity, for an electron like oxygen, energy will be released. Methane has the most energy, most reduced. Carbon dioxide has the least energy. It's the most oxidized. Fatty acids can provide more energy than carbs because the carbon in fats is more reduced. The carbons in fatty acids have more electrons around them compared to the carbons in glucose. A greater number of electrons around these carbon atoms in fatty acids will be transferred to oxygen as fatty acids are oxidized and more energy will be released than when those same processes happen to carbs. Carbon atoms in fatty acids have more e- around them. When e- move from an atom with low affinity for e- (low electronegatvitity, like C) to an atom with high affinity for e- (like O), energy is released. So, the greater the number of e- around the carbon atoms in fatty acids are transferred to O2 (fatty acids are oxidized), more energy is released than when the same process happens to carbs.