CMB Exam 2 - Processes Flashcards
How does insulin reverse the action of glucagon?
Via phosphodiesterase-mediated breakdown of cAMP (PKA inactivation) and unregulated enzyme dephosphorylation by protein phosphatases. Also, activation of: PFK2 (and PFK1 indirectly, by synthesis of F-2,6BP), pyruvate kinase, and glycogen synthase. Inactivation of: F-2,6 bisphosphatase, phosphorylase kinase, glycogen phosphorylase, and hormone-sensitive lipase.
Outline the steps of glycolysis, including names of enzymes.
What is the limiting factor of glycolysis? How is this factor replenished?
NAD+ supply is limited for glyceralehyde-3-phosphate dehydrogenase. It must be replenished: in anaerobic conditions by conversion of lactate to pyruvate by lactate dehydrogenase, or in aerobic conditions by a glycerol-3PDH shuttle that passes H2 to the mitochondria.
glycerol-3 phosphate shuttle
In aerobic conditions (esp in brain, skeletal muscle), regenerates NAD+ by passing the hydrogens onto dihydroacetone phosphate (DHAP) forming glycerol-3-phosphate, and then onto flavoproteins in the inner mitochondrial membrane.
malate-aspartate shuttle
Under aerobic conditions (esp in the cardiac muscle, liver and kidney) the hydrogens from glyceraldehyde 3-phosphate are transfered to NAD+ by G3PDH. NAD+ is regenerated by passing the hydrogens on to oxaloacetate forming malate, which enters the mitochondria and participates in the TCA cycle. Aerobic conditions in the cardiac muscle, liver and kidney.
What modulates glycolysis in liver vs muscle/RBCs?
LIVER: pyruvate kinase activates in presence of insulin, glucagon action phosphorylates PFK-2 preventing its product F-2,6-BP from activation PFK-1 (thus inhibiting glycolysis). MUSCLE/RBCs: ATP/(AMP+ADP) ratio; ATP binds to and inhibits PFK-1 and pyruvate kinase, AMP activates PFK-2.
How does glycogen synthesis occur?
Glucose –(glucokinase)–> Glu6P –(phosphoglucomutase)–> Glu1P + UTP —-> UDP-glucose + glycogenin –(glycogen synthase)–> elongated glycogen –(branching enzyme)–> branched/elongated glycogen
How does glycogen degradation occur?
Under the influence of glucagon, glycogen phosphorylase uses a phosphate (instead of water) to split glucose off glycogen, leaving Glu1P. WHEN THERE ARE 4 LEFT IN A BRANCH (ie “limit dextrin”): transferase moves 3 over to the straight chain, and an α-1,6-glucosidase removes the last 1. The free Glu1P is changed to Glu6P by phosphoglucomutase and then to glucose by Glu-6-phosphatase (expressed ONLY in the liver).
Outline the mechanism by which glucagon/epinephrine cause glycogen degredation.
Both glucagon and epinephrin activate adenylyl cyclase, which turns ATP to cAMP. cAMP causes the dissociation of the regulatory subunits of PKA from it’s catalytic subunits. PKA activates phosphorylase kinase, which activates phosphorylase. Phosphorylase uses a phosphate to separate Glu1P from the glycogen chain. PKA also inactivates glycogen synthase to avoid cycling.
What role does fat (fatty acids) have in gluconeogenesis?
Glucagon signals release of FFAs from adipose tissue (by hormone-sensitive lipase), which is broken down in the liver and provides ATP for gluconeogenesis.
The Cori Cycle
The recycling by the liver of the lactate produced in RBCs; lactate becomes the principle substrate for gluconeogenesis.
Walk through all the steps of gluconeogenesis, starting with lactate.
Lactate + NAD+ –(lactate dehydrogenase)–> pyruvate + NADH –(enters mitochondria)–> pyruvate –(pyruvate carboxylase, biotin)–> OAA –(transamination)–> Asp –(leaves mitochondria)–> Asp –(deamination)–> OAA –(phosphoenol-pyruvate carboxylase)–> PEP –(enolase)–> 2-phosphoglycerate –(phosphoglycerate transmutase)–> 3-phosphoglycerate –(phosphoglycerate kinase)–> 1,3-bisphosphoglycerate + NADH –(glyceraldehyde-3-phosphate dehydrogenase)–> glyceraldehyde-3-phosphate + NAD+ –(aldolase)–> fructose-1,6-bisphosphate –(fructose-1,6-bisphosphatase in the absence of F-2,6-P)–> fructose-6-phosphate –(phosphohexose isomerase)–> glucose-6-phosphate –(enters ER)–> glucose-6-phosphate –(glucose-6-phosphatase)–> GLUCOSE! :D take it to the blood!
Describe the process of phosphoenol pyruvate formation in the absence of lactate.
First we need to generate pyruvate (since liver doesn’t do glycolysis in fed state); in the absence of lactate, amino acids like alanine (but not leucine or lysine) are turned into pyruvate. Pyruvate enters mitochondria and is carboxylated to oxaloacetate (pyruvate carboxylase, biotin dependent). OAA is reduced to malate, leaves the mitochondria and is re-oxidized to OAA. OAA is then decarboxylated (phosphoenol-pyruvate carboxylase, GTP) to form PEP.
Outline the oxidative pentose phosphate pathway.
Glucose-6-phosphate + NADP+ –(glucose-6-phosphate dehydrogenase)–> NADPH+ a lactone structure that is acted on by another enzyme and NADP+ —-> NADPH + CO2 + ribulose-5-phosphate (NOT RIBOSE-5-phosphate). So, a six carbon sugar has been oxidized to 2 NADPH and a 5 carbon sugar.
Explain how macrophages generate bacteriocidal ROS.
MAcrophages take the NADPH from the PPP: NADPH + O2 –(NADPH oxidase)–> O2- + NADP+ –(superoxide dismutase)–> H2O2, which can be dumped on bacteria or –(myeloperoxidase)–> HOCl.
Outline the conversion of pyruvate to acetyl-CoA.
1) E1*TPP displaces CO2 from pyruvate and donates an H+. 2) The hydroxyethyl group is transfered to oxidized (S-S) lipoyllisine on E2, reducing the lipoyllysine and converting the hydroxyethyl to an acetyl group. 3) E2 then attaches CoA-SH to the acyl lipoyllysine forming acetyl-CoA, which leaves. 4) Lipoyllysine is still reduced, so E3 uses FAD to oxidize it. 5) NAD+ then oxidizes the FADH2.
Describe the regulation of PDH
ACTIVATED: allosterically via AMP, CoA and NAD+, Ca++ (ie low ATP + nececssary substrates) and covalently via dephosphorylation. INHIBITION: allosterically via ATP, acetyl-CoA, and NADH, and covalently via autophosphorylation of E1.
Outline the “important” steps of TCA (according to Dory’s slides).
You add acetyl-CoA (2 carbons) to OAA and make citrate. [In the liver, citrate leaves for the cytoplasm and is reconverted to acetyl-CoA]. The other important things to remember are that the first CO2 comes off via isocitrate dehydrogenase (yielding 1 NADH), the next comes of via α-ketoglutarate dehydrogenase (analogous to PDH; yields 1 NADH), then there’s substrate-level phosphorylation producing GTP and succinate. Succinate dehyrogenase (a member of the e-transport chain) uses FAD to oxidize succinate, then fumarate is converted to malate which is oxidized by malate dehydrogenase (yields 1 NADH) to OAA. So: we add 2 carbons to the cycle and we get 3 NADH, 1 FADH2, and some substrate-level phosphorylation.
How does alcohol inhibit gluconeogenesis?
Cyctosolic alcohol dehydrogenase turns all the NAD to NADH, which always drives pyruvate to lactate, inhibiting gluconeogenesis. (alcohol also enters the mitochondria and turns into acetaldehyde and the acetate in a process that uses up mitochondrial NAD as well).
Outline the general steps of the e- transport system.
NADH dehydrogenase in Complex I oxidizes NADH to NAD+, simultaneously pumping an H+ across the membrane. Complex I (from NADH from wherever), Complex II (from FADH2 from TCA), Glycerol 3-phosphate dehydrogenase (from FADH2 glycerol 3-phosphate shuttle in glycolysis), and ETF (from β-oxidation) dump e-s onto CoQ. CoQ shuttles e-s to Complex III (another H+ pump) then to cytochrome C then to Complex IV (another pump; this is where O2 is needed + H+ —-> H20). The H+ ion gradient drives Complex V (ATP synthase).
How are ROS generated in the mitochondria? How do mitochondria handle the ROS?
In hypoxic conditions the transfer of e-s slows down enough that O2 can react with either Complex I or III to form superoxide. O2- –(superoxide dismutase)–> H2O2 –(glutathione peroxidase)–> H2O. Glutathione peroxidase also leaves glutathione dimers in the oxidized state, and NADPH regenerates them.
How do we get Pi and ADP into the mitochondrial matrix to make ATP?
Pi enters using H+ symport (thanks to the pumps). ADP enters using ATP/ADP antiport.
Outline the process of lypolysis.
Perilipin is phosphorylated by PKA causing it to open up and give HSL access to the triacylglycerol inside. HSL removes the first FFA from TG, other lipases release the other 2.
Outline the steps of the carnitine cycle.
The FA is brought into the intermembrane space where it is converted to Acyl-CoA. Acyl-CoA + carnitine –(carnitine-palmitoyl acyltransferase 1)–> CoA + acyl-carnitine. Acyl-carnitine can cross the other lipid layer and is reconverted to acyl-CoA by CPT2, and it can then undergo β-oxidation.
