2Pentose phosphate pathway alternate catabolism of glucose 6-Penergy channeled into reducing potential (high-energy e–s), not ATPcytosol of most cells, especially adipose tissue, liverfunctions: synthesis of pentoses, supply e–s for fat synthesiscoenzyme: NADPNAD with a phosphoryl group on a 2' ribosee– carrier used in reductive biosynthesis (e.g., fatty acid synthesis)stoichiometry varies with specific cell, situationstoichiometry of version that maximizes making NADPH:glc 6-P + 12 NADP+ + 7 H2O → 6 CO NADPH + 12 H+ + PiSimilar but different enzyme, NADP, a wide variety of version of this pathway, and this is just showing one, one glucose, 6 carbons converted to 6 CO2 and energy is being harnessed in this form, that can be used to make fat, something that is prominent when high carbs in the diet are converted to fat1
3Pentose phosphate pathway: first 2 steps, control Control of the pentose phosphate pathway: glc 6-P DHase rate controlled by [NADP+]6-P gluconate DHaseTwo of the steps of the pathway, two oxidations, how electrons end up as NADPH. One of the substrates regulates the pathway, the NADP+. The pathway gets its name because you get 5 carbon sugar which can be used to make ribose phosphate for nucleotide synthesis¯ ribose-P, other sugars2
4Overview of catabolism FATSPOLYSACCHARIDESPROTEINSStage 1glucose, other sugarsfatty acids, glycerolamino acidspyruvateStage 2acetyl CoACoAH2O O2Shows what we’ve covered here, now we have to take a look at Kreb’s cycleStage 3e– CO2Krebs cycleoxidative phosphorylationvvATP ADP + Piadapted from Fig
5The Krebs Cycleaka the citric acid cycle; the tricarboxylic acid (TCA) cycle; the final common pathway for fuel oxidationlocation: mitochondrial matrixfunction: acetyl group → 2 CO2 ATP productionaerobicO2 not used directlyNADH & FADH2 transfer e– pairs to e– transport chaintransfer required to regenerate e– carriersATP made by oxidative phosphorylationKrebs cycle takes place in mitochondrial matrix, generates most of the electrons in the form of NADH which eventually produces lots of ATP, AcetylCOA comes in, two decarboxylations, back to C4, and the other steps generate high energy phosphate, more oxidation that takes place. A lot of electrons are being transferred to these electron carries which go to electron transport chain and that’s how you make a lot of ATP3
6The Krebs Cycle (steps 1-4) step enzyme reaction type1 citrate synthase* condensation (Claisen)2,3 aconitase isomerization via dehydration-hydration4 isocitrate DHase† oxidative decarboxylation*inh by ATP†inh by NADH, ATP; activ by ADPRather than go through all the steps, the first step is the condensation, where acetyl COA is condensed, one of the places where the pathway is controlled, unidirectional arrow. One of the steps that goes in only one direction, makes the cycle as a whole unidirectional although some of the individual steps can go backwards. The other place where control is exerted is step 4, oxidative decarboxylation so isocitrate is both oxidized, electron carrier taking electron, and loses a carbon dioxide. In the second half of the pathway4
7The Krebs Cycle (steps 5-9) step enzyme reaction type5 a-ketoglutarate DHase oxid. decarb.6 succinyl thiokinase phosphorylation driven by thioester hydrolysis 7 succinate DHase oxid.-reduction (complex II)8 fumarase hydration9 malate DHase oxid.-reduction5 carbon intermediate goes under another decarboxylation. GTP is made and another oxidation, this time electrons go to FAD and this succinate dehydrogenase is complex II, embedded in the matrix side of mitochondrial inner membrane. Then the last couple of step are hydration and one final oxidation producing more NADPH. This table lists the types of reactions going on5
8Connection to electron transport chain most e–s enter e– chain via NADHtransferred from NADH via 3 complexes (I, III, IV) to O2end up in H2O via transfer to O2 (the final electron acceptor)stoichiometry: NADH + H+ + ½ O2 → NAD+ + H2O (2.5 ATP made)some e–s enter via FADH2 enzymes (enter e– chain at Q) stoichiometry: FADH2 + ½ O2 → FAD + H2O (1.5 ATP made)NADH → complx I → Q → complx III → cyt c → complx IVfrom ↑ ↓mal-asp shuttle FADH2 enzymes: O2pyr DHase succinate DHase (complex II)Krebs cycle DHases GOP DHasehydroxyacyl CoA DHase* acyl CoA DHase*others othersLehninger 3ed Fig 19-8Connect to electron transport chain. Arrows show the path of electrons. Various pathways and processes generated. Here are some of the named pathways that generate NADH and you’ve seen that the krebs cycle generates lots of NADH, eventually 2 ATP are made. Complex II generate electrons as FADH2, lower energetic level, get 1.5 ATP made for each pair of electrons. As you continue, several places where both of the electron carriers are generated* fatty acid catabolism (Section 7)6
9Stoichiometries ATP Krebs cycle (steps 1-9): yield 2FAD + 6NAD+ + 2acetyl CoA + 6H2O → FADH2 + 6NADH + 6H+ + 2CoA + 4COoxidative phosphorylation:2FADH2 + 6NADH + 6H+ + 4O2 → 2FAD + 6NAD+ + 8H2O 18stage III: 2acetyl CoA + 4O2 → 4CO2 + 2H2O + 2CoA 20add in stages I & II: glucose + 2O2 + 2CoA → 2acetyl CoA + 2CO2 + 4H2Ocomplete oxidation of glc:glucose + 6 O2 → 6 CO2 + 6 H2O9 steps, 2 ATP or GTP, ATP equivalents, feed electrons to carriers, hgue number of ATP generated. This is where if you start from glucose about 2/3 of the ATP from glucose comes from this final common pathway, this Krebs cycle. Stage three adds up in this fashion, so this sums of up Krebs cycle itself plus the transfer of electrons. Now we add stage 1 and 2 and find that we get the complete oxidation of one glucose producing this number of ATP10-1230-327
10Replenishing (anaplerotic) reactions cycle intermediates used to make other biomoleculese.g., succinyl CoA → heme oxaloacetate → aspartatecycle itself results in no net change of [intermediates] (slide 7: Stage III)other reactions needed to ↑ [intermediates]example of a replenishing reaction:CH3COCOO– + HCO3– → –OOCCH2COCOO–pyruvate oxaloacetatedriven by being coupled to ATP hydrolysisenzyme: pyruvate carboxylase (coenzyme: biotin)allosterically activated by acetyl CoAsame reaction as gluconeogenesis: step 10'a (S6L4,slide4)Krebs doesn't produce or consume those intermediates, some used for other purposes. Reaction to replenish oxaloaccetate. Cant elaborate any further, Krebs cycle intermediates need to be maintained at minimal level. )occasionally when there is not enough oxygen, krebs cycle slows , and Acetyl co A is used to make ATP. Generates acetic acid.8
11Krebs cycle: anaerobic conditions Krebs cycle is the same in microorganisms as in eukaryotescycle is aerobic (linked to electron transport chain)to regenerate e– carriers (e.g., NAD+), e– transfer to O2 must occurunder anaerobic conditions, some microorganisms produce acids from cycle “backup”examples: acetyl CoA + ADP + Pi → ATP + acetic acid + CoA succinate → CO2 + propionic acid (CH3CH2COOH)these acid products are membrane-permeantby acidifying local regions, products can damage tissues e.g., in caries & periodontal disease, they are among the numerous substances that cause damageOther acids produced that contribute to carries and other dental problems. One final mention of control of all these pathways feedback inhibition as one general mechanism, ATP is by fat the most important feedback inhibitor of catabolic pathways. Feed forward activation AMP and ADP, signals that ATP is depleted, signals for the activation of catabolic pathways, and this is what this looks like. ..9
12Control of metabolic pathways feedback inhibition usually an early step (committed step) of a pathway is inhibited by a pathway productexample: a pathway functioning to produce F:A → B → C → D → E → FF will often allosterically inhibit step A → B or B → Cin catabolism, main product is ATP, so ATP common allosteric inhibitorfeedforward activationusually a precursor: of the pathway’s product or of a related pathway’s productexample: AMP & ADP as precursors of ATP10
13Control of carbohydrate catabolism glycogenAMP, ADP glucose 6-Pfructose 6-PAMP, ADP fructose 1,6 bisPpyruvateAMP acetylCoAoxaloacetate citrateisocitrateADP -ketoglutarate ATPNADH ↑ox phosADP + PiGLYCOGEN- OLYSIS×pentoseP ? pathwayGLYCOLYSISEach of these steps has either AMP or ADP as an activator, see also that just about all of these pathways have ATP as inhibitors, or was to put the metabolic breaks on the pathway. One other time not here, pentose phosphate pathway, what it is that regulates its activity.KREBS CYCLE11
14Dental aspects of carb metabolism: summary sucrosesource of fermentable monosaccharidesan activated precursor of plaque polysaccharidesplaque polysaccharides (mutans, dextrans, levans)synthesis catalyzed by bacteria-secreted sucrasesadhesion, fuel, anaerobic conditions for microorganismsanaerobic conditions (fermentation)glycolysis lactic acidKrebs cycle acetic acid, propionic acid, otherslow pHsolubilizes enamel hydroxyapatite (caries)damages supporting tissue proteins, cells (gingivitis, periodontal disease, pulpitis)Not going to say much about this because I already said it, puts in all the dental or oral aspects and how they are affecting12
15Web links Stryer site: Chapter 19 (Glycolysis) see also Chapters 18, 20, 22 at that siteCarbohydrate Structure and Metabolism from the University of Kansas Medical Biochemistry Center. This site is essentially an online course in carbohydrates and many biochemical pathways.
16Next section: 7. Lipid Metabolism Next exam (#6): Monday, Nov. 7 at 8 a.m.