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Unformatted text preview: BIology 1A Professor Doudna 1 Pmentel 02/10/10 8am Lecture #10: Aerobic metabolism • Reading: Chapter 9, pp. 170‐177 • Lecture outline: – Citric acid cycle – Oxida>ve phosphoryla>on The citric acid cycle completes the energy‐ yielding oxida=on of organic molecules • In the presence of O2, pyruvate enters the mitochondrion • Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis acetyl Coenzyme A
2 per every glucose Fig. 9‐10 Done by pyruvate dehydrogenase (PDH) high energy bond, allowing it to be uphill and then go downhill to gather energy CYTOSOL NAD+ 2 MITOCHONDRION NADH + H+ 1 Pyruvate Transport protein CO2 3 Coenzyme A Acetyl CoA active transport derived from a vitamin b, thyamine; deficiencies = beri beri, unable to make coenzyme A • The citric acid cycle, also called the Krebs cycle or the tricarboxylic acid (TCA) cycle, takes place within the mitochondrial matrix • The cycle oxidizes organic fuel derived from pyruvate, genera>ng 1 ATP, 3 NADH, and 1 FADH2 per turn cofactor similar to NADH Fig. 9‐11 Pyruvate NAD+ NADH + H+ Acetyl CoA CoA CoA CO2 CoA enzymes are embedded in the mitochondrial matrix. the mitochondria is the site of citric acid cycle and oxidative phosphorylation. in photosynethic organisms, chloroplasts are responsible in the light. plasma membrane of bacteria has the enzymes that undergo oxidative phosphorylation. Citric acid cycle FADH2 FAD ADP + P i ATP 2 CO2 3 NAD+ 3 NADH + 3 H+ specific enzymes or enzyme complexs some steps of cellular respiration are not exergonic nor spontaneous • The citric acid cycle has eight steps, each catalyzed by a speciﬁc enzyme • The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate • The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle • The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain Fig. 9‐12‐1 Acetyl CoA CoA—SH 1 Oxaloacetate Citrate Citric acid cycle Fig. 9‐12‐2 Acetyl CoA CoA—SH not favorable rxn but equilibrium drives it forward
H2O 1 Oxaloacetate Citrate Citric acid cycle 2 Isocitrate Fig. 9‐12‐3 Acetyl CoA CoA—SH 1 H2O Oxaloacetate Citrate Citric acid cycle 2 Isocitrate 3 isocitrate dehydrogenase NAD+ NADH + H+ CO2 α‐Keto‐ glutarate Fig. 9‐12‐4 Acetyl CoA CoA—SH 1 H2O Oxaloacetate Citrate Citric acid cycle 2 Isocitrate 3 NAD+ NADH + H+ CO2 CoA—SH 4 α‐Keto‐ glutarate NAD+ NADH + H+ CO2 Succinyl CoA Fig. 9‐12‐5 Acetyl CoA CoA—SH 1 H2O Oxaloacetate Citrate Citric acid cycle 2 Isocitrate 3 NAD+ NADH + H+ CO2 CoA—SH CoA—SH 4 α‐Keto‐ glutarate 5 NAD+ CO2 Succinate GTP GDP ADP ATP P
i Succinyl CoA NADH + H+ high energy molecule which is coupled with phosphorylation of AT*; this is an example of substrate-level phosphorylation Fig. 9‐12‐6 Acetyl CoA CoA—SH 1 H2O Oxaloacetate Citrate Citric acid cycle Fumarate 2 Isocitrate 3 NAD+ NADH + H+ CO2 CoA—SH 6 CoA—SH 4 α‐Keto‐ glutarate FADH2 FAD 5 NAD+ CO2 Succinate GTP GDP ADP ATP P
i Succinyl CoA NADH + H+ Fig. 9‐12‐7 Acetyl CoA CoA—SH 1 H2O Oxaloacetate Malate Citric acid cycle Fumarate Citrate 2 Isocitrate 3 NAD+ NADH + H+ H2O 7 CO2 CoA—SH hydration reaction
6 CoA—SH 4 α‐Keto‐ glutarate FADH2 FAD 5 NAD+ CO2 Succinate GTP GDP ADP ATP P i Succinyl CoA NADH + H+ Fig. 9‐12‐8 - delta G = 29 kcal/ mol - not spontaneous but oxaloacetate is consumed quickly so equilibrium is pushed to the right Acetyl CoA CoA—SH NADH +H+ NAD+ 8 1 H2O Oxaloacetate Citrate Citric acid cycle 2 Malate Isocitrate 3 NAD+ NADH + H+ H2O 7 CO2 CoA—SH Fumarate 6 CoA—SH 4 α‐Keto‐ glutarate FADH2 FAD 5 NAD+ CO2 Succinate GTP GDP ADP ATP P i Succinyl CoA NADH + H+ During oxida=ve phosphoryla=on, chemiosmosis couples electron transport to ATP synthesis • Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food • These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxida>ve phosphoryla>on The Pathway of Electron Transport • The electron transport chain is in the cristae of the mitochondrion • Most of the chain’s components are proteins, which exist in mul>protein complexes • The carriers alternate reduced and oxidized states as they accept and donate electrons • Electrons drop in free energy as they go down the chain and are ﬁnally passed to O2, forming H2O most electronegative Fig. 9‐13 Free energy (G) rela=ve to O2 (kcal/mol) start at high energy and then gradually go down to lower energy - reduced molecules bind to molecules in the membrane and pass electron along - have prosthetic groups bound to them that do the electron accepting/ donating NADH 50 2 e– NAD+ FADH2 2 e– FAD Mul=protein complexes enters at a lower place because of less energy 40 FMN Fe•S Ι FAD Fe•S ΙΙ Q Cyt b Fe•S ΙΙΙ 30 Cyt c1 Cyt c Cyt a IV 20 Cyt a3 ubiquinone, not an enzyme but part of the chain. it can actually move around because it is small and is not embedded in the complex. 10 2 e– (from NADH or FADH2) cytochromes have heme, or iron groups, that carry electrons instead of O2 0 2 H+ + 1/2 O2 H2O • Electrons are transferred from NADH or FADH2 to the electron transport chain • Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O2 • The electron transport chain generates no ATP • The chain’s func>on is to break the large free‐ energy drop from food to O2 into smaller steps that release energy in manageable amounts H 2O Chemiosmosis: The Energy‐Coupling Mechanism • Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space • H+ then moves back across the membrane, passing "molecular motor" through channels in ATP synthase • ATP synthase uses the exergonic ﬂow of H+ to drive phosphoryla>on of ATP • This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work Fig. 9‐14 INTERMEMBRANE SPACE enter stator, bind to rotor to spin it, jumps off and sent into the mitochondrial matrix. while it leaves, it spins the rod and phosphorylates ADP into ATP H+ spins in one direction to synthesize ATP, spin in other direction to hydrolyze ATP Stator Rotor Internal rod Cata‐ ly=c knob ADP + P i ATP MITOCHONDRIAL MATRIX Fig. 9‐15a EXPERIMENT Magne=c bead Electromagnet Sample Internal rod Cataly=c knob Nickel plate Fig. 9‐15b RESULTS Rota=on in one direc=on Rota=on in opposite direc=on No rota=on 30 25 20 0 Sequen=al trials Number of photons detected (x 103) • The energy stored in a H+ gradient across a membrane couples the redox reac>ons of the electron transport chain to ATP synthesis • The H+ gradient is referred to asa proton‐ mo=ve force, emphasizing its capacity to do work PMF Fig. 9‐16 H+ H+ Protein complex of electron carriers Q Ι ΙΙ FADH2 NADH (carrying electrons from food) 1 Electron transport chain NAD+ FAD ΙΙΙ Cyt c H+ H+ ΙV ATP synthase 2 H+ + 1/2O2 H2O ADP + P i H+ 2 Chemiosmosis ATP Oxida=ve phosphoryla=on An Accoun>ng of ATP Produc>on by Cellular Respira>on • During cellular respira>on, most energy ﬂows in this sequence: glucose → NADH → electron transport chain → proton‐mo>ve force → ATP • About 40% of the energy in a glucose molecule is transferred to ATP during cellular respira>on, making about 38 ATP molecules ADP + Pi -> ATP 7.3kcal/mol
cars get less that 25% efficiency Fig. 9‐17 CYTOSOL Electron shueles span membrane 2 NADH or 2 FADH2 MITOCHONDRION 2 NADH 2 NADH 6 NADH 2 FADH2 Glycolysis Glucose 2 Pyruvate 2 Acetyl CoA Citric acid cycle Oxida=ve phosphoryla=on: electron transport and chemiosmosis + 2 ATP + 2 ATP + about 32 or 34 ATP Maximum per glucose: About 36‐38 ATP Bacterial cells have plasma membrane surrounded by outer membrane think that mitochondria are from gram negative bacteria ...
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