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Bioenergetics

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Bioenergetics Bioenergetics Presentation Transcript

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  • Bioenergetics
    • Living cells are in a dynamic state maintained by metabolism
    • catabolism is to supply energy while anabolism is for energy storage
    • purpose of catabolic pathways is to convert the chemical energy in food to molecules of ATP
    • the mitochondria are the sites of catabolic pathways which yield ATP
    • it is made of 2 membranes
    • outer = permeable to small molecules and ions
    • = no transporting membrane proteins
    • = not folded
    • inner = resistant to penetration of any ions and most uncharged
    • molecules
    • = transport membrane proteins abound for transfer of materials
    • = highly folded
    • All mitochondrial enzymes are synthesized in the cytosol
    • translocator outer membrane (TOM) channels
      • where enzymes cross into the intermembrane space
    • chaperone-like translocator inner membrane (TIM) complexes
      • accepts and inserts enzymes into the inner membrane
    • enzymes are located only inside the inner membrane, thus, substrates must pass the 2 membranes ----- products leave the same way
    • matrix is the inner nonmembranous portion of the mitochondrion where enzymes for the citric acid cycle are located
    • the cristae (infoldings) project into the matrix and is the locale of enzymes involved in the oxidative phosphorylation
  • THE COMMON CATABOLIC PATHWAY
    • Has 2 parts
    • 1. Citric acid cycle (TCA cycle or Krebs cycle)
    • 2. Oxidative phosphorylation (electron transport chain, phosphorylation)
    • A. Agents for storage of energy and transfer of phosphate groups
    • AMP --- contain heterocyclic amine adenine and D-ribose
    • ADP --- sugar joined together by  N-glycosidic bond
    • ATP --- to form adenosine; further linked to Pi
    • when one phosphate group is hydrolyzed from each of these…
    • ATP = 7.3 kcal/mol
    • ADP = 7.3 kcal/mol
    • AMP = 3.4 kcal/mole
    • ATP molecules in the cells do not normally last longer than about 1 minute, thus, a high turnover rate (40 kg ATP/day is manufactured and degraded
    • B. Agents for transfer of electrons in biological redox reactions
      • coenzymes, NAD + and FAD, both contain an ADP core in the structure which is the link of the coenzyme to the apoenzyme
    • NAD + = +charge is due to the nitrogen
    • =operative part is the nicotinamide part which gets reduced
    • FAD=operative part is the flavin part which gets reduced
    • reduced forms are NADH AND FADH 2
    H + and e - transporting molecules
    • C. Agent for transfer of acetyl groups
      • CoA is the acetyl-transporting molecules linked via a thioester bond (high energy bond) 7.51 kcal/mol
      • CoA contain ADP linked to pantothenic acid and mercaptoethylamine
      • active part is mercaptoethylamine
      • o
      • CH 3 - c - s - CoA
  • Citric acid cycle
    • Common catabolism of carbohydrates and lipids begins when they are broken down into 2-carbon products (acetyl units)
    • transported by CoA as acetylCoA
    • Step 1
    • acetylCoA enters the cycle by combining with a C 4 compound called oxaloacetate to produce citrate
    • addition of - CH 3 group of acetylCoA to the
    • C=O of the oxaloacetate
    • hydrolysis of the thioester to produce the C 6
    • compound citrate
    • enzyme used is citrate synthase
    C 6 CO 2 C 5 C 4 C 4 C 2 C 2 Carbon balance
    • Step 2
    • citrate ion is dehydrated to cis-aconitate
    • cis-aconitate is hydrated back to isocitrate
    • enzyme used is aconitase
    • Step 3
    • isocitrate is oxidized to produce oxalosuccinate and decarboxylated at the same time to produce a C 5  -ketoglutarate (can be made into glutamic acid)
    • enzyme used is ICD
    • required NAD +
    • Steps 4 and 5
    • removal of another CO 2 from  -KG to produce succinate (C 4 )
    • uses a complex enzyme system
    • production of a high energy compound, GTP
    • Step 6
    • succinate is oxidized by FAD to produce fumarate (by removal of 2 hydrogen)
    • fumarate has a trans-double bond
    • enzyme used is succinate dehydrogenase
    • Step 7
    • fumarate is hydrated to give the malate ion (C 4 )
    • enzyme used is fumarase
    • Step 8
    • final step is the oxidation of malate by NAD + to give oxaloacetate
    • enzyme used is malate dehydrogenase
    • An acetyl unit enters the TCA cycle and 2 CO 2 molecules are given off
    • How does the TCA cycle produce energy?
      • Production of GTP
      • most of the energy is produced via reactions that convert NAD + to NADH and FAD to FADH 2
      • NADH and FADH 2 carries the e - and H + that will produce ATP in mitochondrion
    • Stepwise degradation and oxidation of acetate in the TCA cycle for most efficient extraction of energy
    • Other advantages of the TCA cycle
      • 1. By-products provide raw materials for amino acid synthesis as per need
      • 2. The many-component cycle provides an excellent method for regulating the speed of catabolic reactions
    • In summary, the overall reactions in the TCA cycle :
    • GDP + Pi + CH 3 - CO - S - CoA + 2H 2 O + 3NAD + + FAD
    • (exhaled)
    • CoA + GTD + 2CO 2 + 3NADH + FADH 2 + 3H +
    • feedback mechanism occurs when
    • NADH + H + accumulates - inhibit steps 1, 3 and 4
    • ATP accumulates - inhibit steps 1, 3 and 4
    • acetylCoA is in abundance - cycle accelerates
    • presence of ADP and NAD + - stimulates ICD
  • ELECTRON and H + TRANSPORT
    • The reduced coenzymes, NADH and FADH 2 , are end products of the TCA cycle
    • they carry H + and e - , thus, have the potential to yield energy when these combine with oxygen to form water
    • EXO 4 H + + 4e - + O 2 2H 2 O + energy
    • involves a number of enzymes embedded in the inner membrane of mitochondria arranged in an (assembly line) increasing affinity for e -
  • The sequence of the electron - carrying enzyme systems starts with
    • Complex I
    • largest complex
    • some 40 subunits, among them a flavoprotein and several FeS clusters
    • CoQ or ubiquinone is associated with complex I
    • oxidizes the NADH produced in the citric acid cycle and reduces the CoQ
    • NADH + H + + CoQ --  NAD + + CoQH 2
    • some of the energy released in this reaction is used to move 2H + across the membrane (matrix to intermembrane space)
    Soluble in lipid, thus, can move laterally within the membrane
    • Complex II
    • also catalyzes the transfer of e - to CoQ from the oxidation of succinate in the TCA cycle, producing FADH 2
    • energy derived from this is not enough to pump two protons across the membrane nor a channel for such transfer is possible
    • Complex III
    • an integral membrane complex contains 11 subunits, including cytochrome b, cytochrome C 1 and FeS clusters
    • delivers the e - from CoQH 2 to cytochrome c
    • the complex has 2 channels through which two H + are pumped from CoQH 2 into the intermembrane space
    • since each cyt c can pick up only electron, 2 cytochrome c’s are needed:
    • CoQH 2 + 2 cyt c (reduced)
    • CoQ + 2H + + 2 cytochrome c (oxid)
    • each cytochrome has an iron-ion-containing heme center embedded in its own protein and the letters used to designate them were given in order of their discovery
    • Complex IV
    • known as cytochrome oxidase, contains 13 subunits-most importantly, cyt  3 , a heme that has an associated copper center
    • an integral membrane protein complex
    • e - moves from cyt c  cyt a  cyt  3  cleavage of O-O bond
    • oxidized form of the enzyme takes up two H + from the matrix for each oxygen atom forming H 2 O which is released into the matrix
    • 1/2 O 2 + 2H + + 2e - -- H 2 O
    • during this process, two more H + are pumped out of the matrix and into the intermembrane space (energy driving this process comes from the energy of water formation)
    • this final pumping into the intermembrane space makes a total of 6H + /NADH + H + and 4H + /FADH 2 molecules
  • PHOSPHORYLATION AND THE CHEMIOSOMOTIC PUMP
    • CHEMIOSMOTIC THEORY by Mitchell
    • proposed that the electron transport is accompanied by an accumulation of protons in the intermembrane space of the mitochondrion, which in turn,
    • creates an osmotic pressure
    • protons driven back to mitochondrion under this pressure generate ATP
    • How do the e - and H + transports produce the chemical energy of ATP?
    • The energy in the e - transfer chain creates a proton gradient
    Spontaneous flow of ions from a region of high concentration to a region of low concentration results in a driving force that propels the protons back to the mitochondrion through the proton translocating ATPase in the inner membrane of mitochondrion catalyzing ATPase ADP + Pi ATP + H 2 O A continuous variation in the H + conc along a given region  H + conc in intermembrane space than matrix
    • Proton translocating ATPase is a complex “rotor engine” made of 16 different proteins
      • has F o sector, embedded in the membrane, contains the proton channel
      • the proton channel composed of 12 subunits rotate every time a proton passes from the cytoplasmic side (intermembrane) to the matrix side of the mitochondrion
      • rotation is transmitted to the F 1 sector “rotor”
    - F 1 sector contains 5 kinds of polypeptides - the F 1 catalytic unit converts the mechanical energy of the rotor to chemical energy of the ATP molecule
    • Rotor ( γ &  subunits)
    • catalytic unit (  &  subunits) surrounds the rotor & makes the ATP
    • stator unit (  ) for stability of the whole complex
  • INNER INTERMEMBRANE SPACE OUTER Accumulated H + Pump H + out A molecule of ATP synthesized / pair of translocated H + storage of electrical energy (due to flow of charges) in the form of chemical energy Hydrolysis of ATP
    • ONLY when the two parts of the proton translocating ATPase, F 1 and F o , are linked is energy production possible
    • disruption of the interaction between F 1 and F o is disrupted –
    • energy transduction is lost
    • Protons that enter a mitochondrion combine with the electrons transported through the electron transport chain and with oxygen to form water
    • the net result of the two processes
    • The oxygen has two functions
    • 1. Oxidize NADH to NAD + and FADH 2 to FAD so that these molecules can go back and participate in the TCA cycle
    • 2. Provide energy for the conversion of ADP to ATP which is indirectly accomplished
    Electron/H + transport ATP formed Each O 2 molecule we breathe in 2H 2 O Combines with 4H + ions & 4 e - Coming from the NADH and FADH 2 molecules (TCA cycle ATP formation is driven by the entrance of H + into the mitochondrion HOH from O 2 -------  increase in H 2 O depleted the H + conc O 2 is not utilized but is needed for cell’s survival !
    • The overall reactions in oxidative phosphorylation is
    • NADH + 3ADP + 1/2 O 2 + 3Pi + H + NAD + + 3ATP + H 2 O
    • FADH 2 + 2ADP + 1/2 O 2 + 2Pi FAD + 2ATP + H 2 O
  • THE ENERGY YIELD
    • The energy released during electron transport is now finally built into the ATP molecule
    • each pair of protons entering a mitochondrion results in the production of one ATP molecule
    • for each NADH molecule, we get 3 ATP molecules
    • for each FADH 2 molecule, only 4 protons are pumped out of the mitochondrion, thus, only 2 ATP molecules are produced for each FADH 2
    • combining the TCA cycle and oxidative phosphorylation:
      • for each c 2 fragment entering the TCA cycle
      • A. we obtain 3NADH x 3ATP/NADH = 9ATP
      • 1FADH 2 x 2ATP/FADH 2 = 2ATP
      • 1GTP = 12ATP
      • B. uses up to 2O 2 molecules
      • one c 2 fragment is oxidized with two molecules of O 2 to produce two molecules
      • c 2 + 2O 2 + 12 ADP + 12 Pi 12 ATP + 2CO 2
  • COMPARISON OF CHEMICAL ENERGY TO OTHER FORMS OF ENERGY
    • Activity of many enzymes is controlled and regulated by phosphorylation
    Phosphorylase b Phosphorylase (seryl-PO 4 ) ATP ADP Glycogen glucose
      • Body maintains a high conc of K + inside the cells, low outside the cells
      • the reverse is true for Na +
      • special transport proteins in the cell membranes constantly pump K + into and Na + out of the cells
      • pumping requires energy via hydrolysis of ATP to ADP
      • with this pumping, the charges in and out of the cell are unequal which generates electrical potential
      • chemical energy of ATP is transformed into electrical energy which operates in neurotransmission
    • ATP is the immediate source of energy in muscle contraction
      • as ATP binds to myosin the actin-myosin complex (contracted muscle) dissociates and the muscle relaxes
      • when myosin hydrolyses ATP, it interacts with actin once more, and new contraction occurs
    • a molecule of ATP upon hydrolysis to ADP yields 7.3 kcal/mol
    • = some of this energy is released as heat and used to maintain body temperature.
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