FinalExamFALL2007Solutions

FinalExamFALL2007Solutions - INTRODUCTORY BIOCHEMISTRY...

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Unformatted text preview: INTRODUCTORY BIOCHEMISTRY FINAL EXAM FALL 2007 SEMESTER PAGE - 9 ANSWERS TO FALL 2007 FINAL EXAM SECTION A True/False Questions 1. True. Enzymes are biochemical catalysts to speed up the biochemical reactions by lowering the activation energy (Ea). 2. False. Phosphatidylglycerol is a phospholipid constituent for biological membrane bilayer structure. T riacylglycerols (TAG) are the major “long-term ” energy storage form found in animals stored in the adipose tissue. 3. True. An aldehyde (!CHO) is o xidized to carboxylic acid (!COOH, more O atoms) by an oxidant. 4. False. T hymine is o nly found in DNA associated with deoxyribose sugar component; whereas u racil is o nly found in RNA associated with ribose sugar. 5. True. The synthesis of the two strands of the DNA double helix is from 5’ phosphate end to 3’ !OH end in opposite direction, so- called a nti-parallel. The nucleotides are covalently joined by a nionic 3 ’÷5’ phosphodiester linkages. 6. True. G lucose is an aldohexose with a nomeric carbon at C-1 (i.e. h emiacetal), which can undergo mutarotation (inter-conversion of "- and $-anomers via open-chain aldehyde functional group). 7. False. In the citric acid cycle, only citric acid and isocitrate (6C) are tricarboxylic acids (so-called TCA cycle); whereas succinic acid (4C) is a dicarboxylic acid. 8. False. F ritz Lipmann is the Nobel Prize laureate first studying the biological importance of A TP in bioenergetics. 9. False. Both h exokinase (Step 1) and p hosphofructokinase (Step 3) require the input of one molecule of ATP. Two substrate-level phosphorylation steps involve the output of ATP: phosphoglycerate kinase (Step 7) and pyruvate kinase (Step 10). However, there are three irreversible steps w ith highly negative )G°’: hexokinase (Step 1), phosphofructokinase (Step 3) and pyruvate kinase (Step 10). 10. True. G lucose is a highly polar molecule and cannot cross the membrane by passive diffusion. G lucose from the blood stream is transported into the cell v ia GLUT transmembrane protein residing in the plasma membrane. G lucose is phosphorylated to anionic g lucose-6-phosphate catalyzed by enzyme hexokinase immediately entering into the cell, so that glycolysis can take place o nly in the cytoplasm. It is because the remaining metabolites are phosphorylated until pyruvate (non-phosphate 3C molecule) is reached. 1 1. False. O xidative phosphorylation c an take place in both eukaryotic cells and bacterial cells. In contrast to the general similarity in structure and function of the electron transport chains in eukaryotes, bacteria possess a large variety of electron-transfer enzymes. These use an equally wide set of chemicals as substrates. In common with eukaryotes, prokaryotic electron transport uses the energy released from the oxidation of a substrate to pump ions across a membrane and generate an electrochemical gradient. INTRODUCTORY BIOCHEMISTRY FINAL EXAM FALL 2007 SEMESTER PAGE - 10 SECTION B Multiple Choice Questions Detailed Solutions to Multiple Choice Questions 12. (A) A TP is a “high-energy” phosphate compound with phosphoanhydride bond. The (-phosphate group in ATP can transfer to substrate to form an “energy-rich ” phosphate compound. For example, glutamine synthetase catalyzes the conversion of glutamate to glutamine via p hosphorylation by ATP to form a “high-rich ” phosphate intermediate, glutamate-phosphate intermediate . Step Î: glutamate + ATP ÷ g lutamate-phosphate intermediate + ADP Step Ï: glutamate-phosphate intermediate + NH 3 ÷ g lutamine + P i Overall: glutamate + NH 3 + ATP ÷ g lutamine + ADP + Pi 13. (D) A mino acids (e.g. serine, threonine, tyrosine), simple sugars (e.g. glucose, fructose), glycerol all contain a lcohol (!OH) functional group, which can react with phosphate to form “low-energy” p hosphoester b ond. 14. (C) P hosphatidylglycerol (PG ) has the structure of three-carbon (3C) g lycerol backbone e ster-linked to two fatty acid chains (R1 and R2 groups) in C-1 and C-2, and p hosphoester-linked in C-3 to another neutral glycerol. Thus, P G carries !1 charge due to the anionic phosphate headgroup. Upon hydrolysis, P G releases two moles of fatty acids, one mole of inorganic phosphate and two moles of glycerol (one mole from the headgroup glycerol and one mole from the glycerol backbone). 15. (B) E pimers b elong to Class 5: Isomerase, in which the isomers are different in configuration of o nly one asymmetric carbon . The two sugars are e pimers o f each other at C-3. Five-carbon sugar with aldehyde functional group at C-1 position is called aldopentose. 16. (D) A nomeric carbon is found in cyclic ring system (i.e. H aworth projection): C-1 in aldose forming hemiacetal; or C-2 in ketose forming hemiketal. Both "- and $configurations can exist. The structure of sedoheptulose shows seven carbons with C -2 as the a nomeric carbon (i.e. k etoheptose). It is an $-anomer with !OH at C-2 anomeric carbon pointing up, whereas !CH 2OH group at C-1 pointing down. 17. (B) “Galactose "(1 ÷6) glucose” indicates the C-1 a nomeric carbon of the first sugar galactose (an aldose) is joined to the C-6 atom of the second sugar glucose via "-glycosidic linkage (pointing down). 18. (B) T he 1 ’C p osition of (deoxy)ribose is $-glycosidic-linked to the nitrogenous bases (to N -1 in pyrimidines C, T and U; and to N -9 in purines G and A). The 3 ’C position of (deoxy)ribose is phosphodiester-linked to 5 ’C p osition of next (deoxy)ribose unit (formation of 3’÷5’ phosphodiester linkage). INTRODUCTORY BIOCHEMISTRY FINAL EXAM FALL 2007 SEMESTER PAGE - 11 19. (E) A TP is a “high-energy” phosphate or a nucleotide cofactor, which consists of two “high-energy” p hosphoanhydride linkages (between " and $ p hosphate groups and between $ and ( p hosphate groups) and one “low-energy” p hosphoester b ond (between " p hosphate group and C5’ of ribose sugar). 20. (C) A ccording to the equation: )G = )H ! T )S , positive enthalpy change (i.e. )H = r, endothermic) is unfavorable; and positive entropy (i.e. )S = r, more disorder) is favorable. A spontaneous reaction is d ependent on the temperature, in which h igh temperature w ill favor the forward reaction. 21. (C) T he metabolic pathway is commonly regulated by feedback inhibition , in which the final p roduct (metabolite E ) acts as modulator to control the initial enzyme (D) to commit the pathway. Low concentration of metabolite E a ctivates e nzyme D to go through the pathway to produce (replenish) more E ; whereas high concentration of metabolite E inhibits e nzyme D to stop the pathway as there are already plenty of metabolite E , and do not need to produce more. 22. (A) T he stepwise calculation is shown below: Î For the uncoupled reaction, at equilibrium, K eq = [B] / [A] = 10 G3 Ï For the ATP-coupled reaction: K eq = { [B] [ADP] [P i] } / { [A] [ATP] } = 200 Ð Since [ATP] / ( [ADP] [P i] ) = 5 × 10 3 is given, Ñ Take reciprocal yielding: ( [ADP] [P i] ) / [ATP] = 1 / 5 × 10 3 Ò Substitute Ñ into Ï, { [B] / [A] } { [ADP] [P i] ) / [ATP] } = 200 { [B] / [A] } { 1 / 5 × 10 3 } = 200 ˆ [B] / [A] = (200) × (5 × 10 3) = 1 × 10 6 Ó The ratio of [B] / [A] has been increased from 10 G3 to 10 6, ˆ 10 6 / 10 G3 = 1 0 9-fold increase 23. (D) D ue to the presence of three phosphate groups (", $, (), ATP has a net charge of !4 at physiological pH. Without interacting with two divalent cation M g 2+, ATP is thermodynamically unstable due to internal repulsion, and readily undergoes hydrolysis to release ADP +Pi or AMP + PPi. Mg 2+ is preferably than other cations due to the close proximity interaction with the anionic phosphate groups. 24. (C) C arnitine p lays an essential role in the transport of long-chain fatty acids, in the form of fatty acyl ester, across the outer and inner mitochondrial membranes into the matrix of mitochondria for $-oxidation. Carnitine acyl transferase I acts at the outer mitochondrial membrane and converts the activated fatty acyl-CoA thioester into a fatty acyl carnitine ester. A transport protein moves this ester across the inner mitochondrial membrane, which is a highly impermeable membrane to most solutes. Once inside the matrix, the fatty acyl carnitine ester is reconverted to activated fatty acyl-CoA thioester catalyzed by carnitine acyltransferase II. INTRODUCTORY BIOCHEMISTRY FINAL EXAM FALL 2007 SEMESTER PAGE - 12 25. (A) A cyl-CoA dehydrogenase is the first e nzyme found in the $-oxidation of fatty acid, which catalyzes the following reaction: acyl-CoA + FAD ÷ enoyl-CoA + FADH 2 FAD acts as oxidizing agent to o xidize acyl-CoA by removing 2 H atoms and 2 eG into the product enoyl-CoA via d ehydrogenation. Since acyl-CoA dehydrogenase is associated with FAD, thus it is a membrane-bound enzyme found in the inner mitochondrial membrane. 26. (A) A rachidic acid is a saturated fatty acid with 20 carbon atoms (C20:0 ). Remember the mnemonic: “LMPSA ”, with L auric acid as C12:0, M yristic acid as C14:0, P almitic acid C16:0, and S tearic acid as C18:0. A rachidic acid (C20:0) undergoes n ine turns of $-oxidation to produce t en molecules o f acetyl-CoA (2C). Each acetyl-CoA is further o xidized in citric acid cycle to produce 10 ATP. Thus, ten rounds o f citric acid cycle are required for 1 0 molecules o f 2C acetyl-CoA. 27. (E) P yruvate kinase is the last e nzyme involved in anaerobic glycolysis, which takes place in the cytoplasm . P yruvate dehydrogenase c atalyzes the o xidative decarboxylation o f pyruvate (a 3C "-keto acid) to produce CO 2 + 2C acetyl-CoA, which is the starting material of citric acid cycle. S uccinate dehydrogenase is found in citric acid cycle and is bound to the inner mitochondrial membrane (also known as Complex II in ETC). E noyl-CoA hydratase is the second enzyme in $-oxidation of fatty acid to carry out hydration. It is known that both citric acid cycle and $-oxidation occur in the matrix of mitochondria . C arnitine acyltransferase is associated with the transport of long-chain fatty acid across mitochondrial membranes. 28. (D) B oth enoyl-CoA hydratase found in $-oxidation and fumarase found in citric acid cycle undergo similar chemical reaction, which is h ydration (water H 2O incorporation into the molecule). Both enzymes belong to Class 4: Lyase. Both m alate dehydrogenase and "-ketoglutarate dehydrogenase b elong to Class 1: Oxidoreductase. A conitase c atalyzes the isomerization of citrate to isocitrate, which belongs to Class 5: Isomerase. C itrate synthase is a condensing enzyme to combine 2C acetyl-CoA and 4C oxaloacetate to form 6C citrate, thus belongs to Class 4: Lyase. 29. (E) A ll the reactions has the same number of carbon atoms o f reactant and product, except the conversion of isocitrate (6C ) into "-ketoglutarate (5C ) catalyzed by isocitrate dehydrogenase, which is an oxidative decarboxylation step found in citric acid cycle, with removal of one molecule of C O 2. isocitrate (6C ) + NAD + ÷ "-ketoglutarate (5C ) + NADH + H + + C O 2 "-ketoglutarate is a 5 C "-keto acid. Both glutamate and glutamine are 5 C amino acids. Conversion of glutamate ((-COO G) into glutamine (!CO-NH 2) is catalyzed by glutamine synthetase. glutamate + NH 3 + ATP ÷ g lutamine + ADP + Pi INTRODUCTORY BIOCHEMISTRY FINAL EXAM FALL 2007 SEMESTER PAGE - 13 Oxaloacetate is a 4 C "-keto acid and aspartate is a 4 C amino acid ($-COO G). Conversion of 5 C "-ketoglutarate into 5 C g lutamate; and 4 C o xaloacetate into 4 C aspartate can be achieved by the enzyme called transaminase. Citrate (6C ) is converted to isocitrate (6C ) by a conitase (not called citrate isomerase) via the dehydration and hydration of the intermediate aconitate. citrate (6C) ÷ (dehydration) ÷ aconitate (6C) ÷ ( hydration) ÷ isocitrate (6C) 30. (A) B oth electron transport chain and oxidative phosphorylation are located in the inner mitochondrial membrane. Both electron transport and ATP synthesis are c oupled because each process requires the other to function in intact m itochondria. In other words, neither process can occur without the other. It is related to the H + being pumped out of matrix b y ETC Complexes and return flow of H + b y ATP synthase. 31. (E) C omplex I reduces Coenzyme Q (CoQ) to r educed C oQH 2 while re-oxidizing NADH to o xidized N AD +. Thus, r educed C oQH 2, not cytochrome c, carries 2 e G between Complexes I and III. Similarly, Complex II (succinate dehydrogenase) reduces CoQ to r educed C oQH 2 while re-oxidizing FADH 2 to o xidized F AD. Complex III reduces o xidized cytochrome c (heme-Fe3+) to r educed cytochrome c (heme-Fe2+) while re-oxidizing CoQH 2; Complex IV reduces ½ O 2 to H 2O while re-oxidizing cytochrome c (heme- Fe2+). In conclusion, the order of the electron cofactor carriers in the ETC based on the inhibitor studies is: NADH ÷ C omplex I ÷ C oQ ÷ C omplex III ÷ c ytochrome c ÷ O 2 FADH 2 ÷ C omplex II ÷ C oQ ÷ C omplex III ÷ c ytochrome c ÷ O 2 Coenzyme Q (CoQ ; Q stands for Q uinone) is an o xidizing agent, which accepts 2 eG + 2H + (= 2 H atoms), similar to FAD, to become r educed h ydroquinone. C oQ b ears a long hydrophobic “isoprene” tail that keeps the molecule inserted into the membrane. 32. (D) A s the ( s ubunit rotates with respect to the ("$)3 complex in the F 1 portion of ATP synthase in the matrix of mitochondria, the three "$ p airs undergo three distinct conformations in a cyclic m anner: Î “Open ” state - no substrates bound; Ï “Loose” state - binds ADP and Pi; and Ð “Tight” state - ATP is formed in the $-subunit and is tightly b ound to the enzyme. Energy generated from the p roton motive force ()G = )µ + F )R = 2 .3 R T )pH + F )R; where )µ is the chemical potential difference and )R is the electrical potential difference) via electrochemical gradient in ETC is used to release ATP from ATP synthase. 33. (A) W olfgang Jünge hypothesized that the “a ” subunit consists of two discrete “half-channels” through which protons (H +) flow back across the inner mitochondrial membrane, down their chemical potential gradient. H + enter into one half-channel from the inter-membrane space side o f the membrane into the “a ” subunit; H + then jump from the “a ” subunit onto the adjacent “c” subunit; and exit from the other half-channel from inside of the “a” subunit to the matrix. INTRODUCTORY BIOCHEMISTRY FINAL EXAM FALL 2007 SEMESTER PAGE - 14 34. (B) G lucose is a p olyhydroxyl-aldehyde (i.e. a ldose), which is a highly polar molecule, and cannot enter cells by passive diffusion across the membrane. G LUT s (GLUcose Transporters) residing in the plasma membrane can import glucose into the cell for cytoplasmic g lycolysis to take place under a naerobic condition. Once glucose is transported into the cell, glucose is rapidly p hosphorylated by hexokinase. The remaining metabolites in glycolytic pathway stay phosphorylated and are trapped inside the cell due to a nionic n ature of the phosphate group, until pyruvate (CH 3-C(O)-COO G, a non-phosphate "-keto acid) is produced. 35. (B) A naerobic yeast fermentation involves two enzyme-catalyzed steps of the reduction of pyruvate (CH 3-C(O)-COO G) to produce ethanol (CH 3-CH 2OH). Î pyruvate decarboxylase: pyruvate ÷ acetaldehyde + CO 2 Ï alcohol dehydrogenase: acetaldehyde + NADH + H + ÷ ethanol + NAD + In the presence of O 2, the product ethanol (an aldehyde) is o xidized to acetic acid (a carboxylic acid ), which gives a sour vinegar taste and is undrinkable, and is almost alcohol-free. 36. (C) G lyceraldehyde-3-dehydrogenase b elongs to Class 1: Oxidoreductase, which catalyzes the o xidation o f glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, a “super-high ” energy phosphate, in association with o xidized cofactor N AD +. g lyceraldehyde-3-phosphate + NAD + + P i ÷ 1 ,3-bisphosphoglycerate + NADH + H + Two biological importance of this enzyme-catalyzed step: Î production of NADH : (i) Under anaerobic condition (low O 2 demand), N ADH is required for lactate dehydrogenase. The re-oxidation of NADH produces o xidized N AD +, which replenishes the reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase, so the cytoplasmic g lycolysis will not cease. (ii) Under aerobic condition (high O 2 demand), N ADH is transported into mitochondria via m alate-aspartate shuttle in heart, liver and kidney; or via g lycerol-3-phosphate shuttle in muscle and brain, in association with aerobic E TC and OP to produce 2.5 ATP and 1.5 ATP, respectively. Ï requirement of inorganic phosphate (Pi): Incorporation of P i into glyceraldehyde-3-phosphate produces a “super-high ” energy 1,3-bisphosphoglycerate, which then undergoes the first s ubstrate-level phosphorylation in glycolysis via p hosphoglycerate kinase to produce A TP . Fructose-1,6-bisphosphate u ndergoes reverse aldol condensation catalyzed by aldolase to cleave into 2× trioses. D ihydroxyacetone phosphate (a ketotriose) is converted into glyceraldehyde-3-phosphate (an a ldotriose) by triose phosphate isomerase. Phosphoenolpyruvate, a “super-high ” energy phosphate, phosphorylates ADP to form ATP in the second substrate-level phosphorylation in glycolysis via p yruvate kinase. Glucose-6-phosphate (an a ldohexose) is converted into fructose-6-phosphate (a ketohexose) catalyzed by phosphohexose isomerase. INTRODUCTORY BIOCHEMISTRY FINAL EXAM SECTION C 1. FALL 2007 SEMESTER PAGE - 15 Short Answers (20 marks total) Enzyme-catalyzed catabolism (5 marks) substrate enzyme name fructose-1,6-bisphosphate aldolase 3 -phosphoglycerate phosphoglycerate mutase 2 -phosphoglycerate enolase m alate citrate 2. malate dehydrogenase aconitase “Pas de deux” Naming of substrate by catabolic enzymes (4 marks) enzyme substrate 1 substrate 2 h exokinase p yruvate kinase ADP fumarase fumarate H 2O (water) thiolase CoA-SH $-ketoacyl-CoA lactate dehydrogenase 3. glucose ATP pyruvate NADH (+ H +) PEP (phosphoenolpyruvate) “Sweet success”. Bioenergetics for ATP yield of aerobic glucose oxidation (6 marks) Metabolic process Immediate product ATP yield glycolysis (glucose to pyruvate) malate-aspartate shuttle 2 N ADH 5 2 A TP 2 pyruvate oxidation 2 NADH 5 acetyl-CoA oxidation 6 NADH 15 2 F ADH 2 3 2 A TP 2 S UM (TOTAL NET YIELD OF ATP) = 32 INTRODUCTORY BIOCHEMISTRY FINAL EXAM 4. FALL 2007 SEMESTER PAGE - 16 “Bits and pieces”. Matching of chemical components and biomolecules (5 marks) component letter biomolecule d eoxyribose D A: trehalose p orphyrin C B : coenzyme Q p antothenic acid E C : cytochrome c g lucose A D : DNA N ,N,N-triethylethanolamine F E : coenzyme A isoprenoid side chain B F : phosphatidylcholine SECTION D 1. Structures and Written Answers (Answer any three o f the four questions. Each question is worth 8 marks; 24 marks total.) N AD + (oxidized form) (Nicotinamide Adenine Dinucleotide) consists of the following components: Î first nucleotide (oxidized n icotinamide ring + ribose + P) Ï second nucleotide (AMP) (adenine + ribose + phosphate) Ð via 5’÷5’ d iphosphate linkage in two ribose sugars Ñ adenine is a bicyclic purine base Ò C-1’ is $-glycosylic-linked to N-9 in adenine base INTRODUCTORY BIOCHEMISTRY FINAL EXAM 2. FALL 2007 SEMESTER PAGE - 17 There are three different types of lipid aggregates when they are dispersed in water: (a) Micelle (i) roughly spherical; polar groups on surface; usually small (approx. 3-10 nm diameter) but may be up to 100 nm (ii) fatty acids; soaps (salts of fatty acids) and detergents (iii) soap; casein (milk micelles); emulsification of lipids in the G.I. tract (b) Bilayer (i) planar; polar headgroups on surface; hydrophobic interior; approximately 3 nm thickness, But surface area may be very large (ii) many membrane lipids (e.g. PC, same as for vesicles) (iii) the lipid component of all biological membranes (c) Vesicle (or L iposome) (i) roughly spherical; encloses aqueous core; 100 nm to microns (µm) diameter; may be unilamellar bilayer or or multilamellar bilayer (concentric “onion-skins”) (ii) many membrane lipids (e.g. PC) (iii) “artificial cell” (model system); drug delivery vehicle INTRODUCTORY BIOCHEMISTRY FINAL EXAM 3. FALL 2007 SEMESTER PAGE - 18 (a) Pyruvate dehydrogenase catalyzes metabolism of pyruvate to acety-CoA. When the enzyme activity is diminished, sugar catabolism (e.g. in the muscles) leads to pyruvate accumulation. Some of the excess pyruvate is r educed b y NADH to yield lactate, catalyzed by lactate dehydrogenase. Tissue lactate is exported to the liver via the blood. L actic acid (CH 3-CH(OH)-COO H ) causes lowering of blood pH . Î p yruvate + NADH + H + ÷ lactic acid + NAD + Ï lactic acid (CH 3-CH(OH)-COO H ) ÷ lactate (CH 3-CH(OH)-COO G) + H + (b) Alanine ( HOOC-CH(CH 3)-NH 3+) is the "-amino acid with the same carbon skeleton (3C ) as the "-keto acid pyruvate (3C , CH 3-C(O)-COO G). Some of the excess pyruvate is converted to alanine by transamination . pyruvate (3C "-amino acid) ÷ alanine (3C "-amino acid) (c) The b rain solely relies on g lucose for energy supply. D iminished p yruvate dehydrogenase activity lowers production of acetyl-CoA, so the ATP yield of glucose catabolism is greatly lowered (effectively, from the a erobic processes (citric acid cycle and ETC) producing ~ 30 ATP via g lycerol-3-phosphate shuttle to the a naerobic glycolysis yielding 2 A TP ). The b rain is starved of energy and does not develop normally. (d) Most metabolic energy is derived from a cetate / acetyl-CoA catabolism (e.g. Krebs cycle, oxidative phosphorylation). Pyruvate dehydrogenase deficiency prevents acetyl-CoA production from s ugars (via g lycolysis producing pyruvate), but it does not affect acetyl-CoA production from fatty acids (via $-oxidation), so fats are the preferable energy sources. INTRODUCTORY BIOCHEMISTRY FINAL EXAM 4. FALL 2007 SEMESTER PAGE - 19 The molecular “rotary mechanism ” of the ATP synthase F o m otor (c 10 ring) is probed by a subunit via H + g radient across IMM. (Refer to the right d iagram above.) This mechanism was proposed by Professor Wolfgang Jünge. Î aspH residues in c1 0 ring (Positions 3 to 10) are p rotonated a nd associated with the h ydrophobic IMM. N egatively charged a sp G residues in c10 ring (Positions 1 and 2) are ionic-attracted to the matrix side and inter-membrane space side in a subunit (via arg + residue) in IMM, respectively. Ï Proton e nters from the inter-membrane space side half-channel in a subunit, and jumps onto the closest c subunit (Position 2 a sp G ), becoming a spH . Ð The p rotonated a spH b reaks an ionic attraction between the c subunit aspartate and a conserved a rginine residue in the a s ubunit, and this protonation event sets the c10 ring free to turn. Ñ Based on the right d iagram above, the c10 ring rotates “anti-clockwise”, so that Position 2 p rotonated a spH m ove away from the a subunit into the h ydrophobic m embrane; and Position 1 a sp G m ove to interact with the inter-membrane space side in a s ubunit stabilized by arg + residue. Note: Position 1 a sp G is unfavourable to move “clockwise or up” direction because the d eprotonated negatively charged a sp G is thermodynamically unstable to be situated in the h ydrophobic IMM environment. INTRODUCTORY BIOCHEMISTRY FINAL EXAM FALL 2007 SEMESTER PAGE - 20 Ò Simultaneously, another c subunit (Position 10 protonated a spH ) is forced into contact with the matrix side half-channel in the a subunit, and releasing a proton into the matrix. Position 10 now becomes deprotonated a sp G . Ó After “anti-clockwise rotation ”, Position 1 a sp G o f c subunit, now in the inter-membrane space side, is ready to pick up another H + from the intermembrane space side half-channel in a subunit. Ô In conclusion, for a c10 ring rotor, ten p rotonation/deprotonation events are required to complete one revolution. Õ Consequently, two factors dictate the d irection of rotation of the c 10 ring : (a) the asymmetric orientation of the a subunit half-channels (i.e. intermembrane space side and the matrix side) w .r.t. the IMM; and (b) the directional proton gradient (i.e. the proton-motive force). ...
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