Biochemistry Exam 4

glycogen + important intermediate

• glycogen is glucose storage for only animals (but not all animals)

• glucose —> glucose 6 phosphate —> glucose 1 phosphate —> glycogen

• animals need more regulation bc they need tight control of blood glucose

  • vs plants dont have same tight control

• regulate blood glucose + form glycogen to regulate glucose by removing it from the bloodstream and storing it in a way that wont impact blood concentration

liver vs muscles for glycogen

• 10% of mass of liver is glycogen- major store of glucose in the body + regulates blood glucose

  • releases stored glucose into bloodstream to maintain constant concentration in the blood

• 2% of mass of muscle can be glycogen bc when muscles are actively working they may need supply of readily avail. glucose to continue to work

glucose 6 phosphate

• central metabolite

• can funnel many pathways to making this intermediate then branch out and form other types of biosynthetic material needed in the cell

glucose —> glucose 6 phosphate

• via hexokinase

• but G6P can go to either glycolysis, PPP, or glycogen storage depending on what cell needs

step one of glycogen building

glucose 6 phosphate —> glucose 1 phosphate

• phosphoglucomutase

  • prepare glucose pool to separate it from G6P and tag it specifcally for glycogen formation

  • by converting G6P —> G1P

  • phosphoglucomutase is not an isomerase- it has phosphorylated serine in AS which donates its group to C1, then the dephosphorylated serine takes phosphate from C6 and keeps it to regenerate phosphoserine. the regenerated is ready for next round of catalysis

  • the phosphate on C1 is different than the one that was on C6

step two of glycogen building

glucose 1 phosphate + UTP —> UDP-glucose + PPi

• UDP-Glucose Pyrophosphorylase

  • G1P has an O- that does nucleorphilic attack on alpha phosphate of UTP

    • UTP has 3 P groups and a uracil (specific for glycogen)

  • the end two phosphates on UTP get released as PPi and UDP-glucose forms with the G1P + rest of UTP (1 P group + uracil)

    • the two Ps together create a high energy bond that fuels the next rxn

  • deltaG = 0, so reaction is driven forward by fact that PPi is rapidly broken down into inorganic phosphate in cytosol (2 Pi)

    • removes one of the substrates needed for reversible reaction making it irreversible. Forces formation of UDP-glucose

step 3 of glycogen building

UDP-glucose + glycogen —> glycogen + UDP

• glycogen synthase aka glycosyl transferase

  • add the glucose part of UDP-glucose to the growing glycogen chain on the C4 non-reducing end

    • grows unidirectionally, builds from non-reducing C4 end (as opposed to reducing C1) via alpha 1,4 glycosidic bonds

    • E of this reaction is still stored in that high E bond

  • UDP (uracil + 2P groups) is released and converts/regenerates to UTP

    • ATP —> ADP; Heavy E investment process, generation of UTP req one ATP molecule

      • for every glucose you join to glycogen chain, need 1 ATP molecule, so can only generate glycogen when excess or high amt of ATP in cell

UDP glucose for synthesizing vs breaking down glycogen

• for synthesizing, is used to energize the glucose to join in to the chain

• for breaking down glycogen, it is not needed/involved

reversible vc irreversible steps

• hexokinase (irreversible)

• phosphoglucomutase (reversible depending on if G1P or G6P avail)

• UDP-glucose pyrophosphorylase (irreversible)

• glycogen synthase/glycocyl transferase (irreversible)

glycogenin dimer

• need to store glycogen in a structure that makes it easy to access that glucose, so structure is built around glycogen dimer anchor

  • dimer, two monomers

• catalyzes initiation of the glycogen structure

• stays in center as anchor and final glycogen structure is built (30-50,000 glucose molecules)

• only 2 points of contact with glycogen chains- Tyr residue where you start to grow chain to initiate generation

glycogen branching 2 reasons

1) creates non reducing ends available for breakdown- much easier than an indivisual strand. for glycogen breakdown, remove one glycogen from end of chain w each catalyzed reaction. the more ends the more efficient release of glucose

2) increases solubility in aqueous environment of cytosol bc all chains/areas where H2O can come in + additional OHs on ends gives easy access

glycogenin dual activity

1) glycocyl transferase (attaching the first glucose unit to Tyr on glycogenin)

2) synthase activity to build

glycogenin and making primer + glycogen synthase

• glycogenin core protein extends from C4 end adding glucose units until 7 total (7-11)

  • septasaccharide primer

• glycogen synthase recognizes the 7 unit primer + continues to add units (total ~14)

  • from glucose —> G6P —> G1P —> UDP-glucose —> adding a glucose unit to glycogen

branching process

• branching enzyme (called glycogen-branching enzyme) takes the last 7 units of glycogen chain, cuts them, and moves them up to form alpha 1,6 glycosidic branch with the first monomer of the chain (the one attached to glycogenin core)

• glycogen synthase adds more glucose units to both ends via non reducing end

  • but as strand gets to be 14 units, cut and move the last 7 again + continue adding again

• branching is every 7-11 units (and no less than 4)

• 1 glycogen molecule has ~200 branches

• as glycog

purpose of branching

• to increase complexity of generating branches

• the new branch point is the new non-reducing end that can be extended

• original where cut from is also non-reducing end that can be extended

• more optimal storage of glucose molecules

breaking down glycogen to glucose- 2 enzymes

glycogen phosphorylase

• slides along glycogen chain and removes indiv. glucose molecules to G1P

• uses a phosphate to do so

• uses phosphorolysis

glycogen debranching enzymes

• breaks down complexity of branches

• removes branches that phosphorylase can’t get past to allow it to function and breakdown to release indiv. G1P

glycogen phosphorylase + mech

• possessive enzymes, slides across chain releasing one glucose molecule at a time

  • doesn’t bind/release/bind/release, so is faster rate of rxn AND is rate limiting step

    • has regulation/control over breakdown + synthesis of glycogen

• is a nitrogen ring with a PLP co-enzyme covalently attached to AS (cant leave)

  • removes individual glucose molecules by phosphorylysis

• has a P group that donates a proton to an O inorganic phosphate in the AS

  • another O- on inorganic phosphate acts as nucleophile attack glycosidic bond (the C4 electrophile end between two glucose molecules) of glycogen chain and break down a chain to G1P

    • the inorganic phosphate donates a proton to the glycosidic C4 oxygen

• can only work up until 4 units away from a branch point

regulating step

breaking of the glycosidic bond acts as rate limiting step and is where control is.

some energy is preserved in the formation of phosphate ester bond

can be regulated

debranching enzyme

transferase

• glycogen phosphorylase leaves 4 unit strand by a branch point so debranching enzyme cuts and moves the 4 units down to the regular chain

• no longer branched

• resolves alpha 1,6 link (bc stops phosphorylase activity) so glucosidase activity can occur (glucose released)

liver specific glycogen degredation and release of glucose

• glycogen or pyruvate converts to glucose 6-phosphate

• T1 transporter on endoplasmic reticulum brings G6P into lumen

• membrane bound G6Pase faces into ER lumen and converts G6P —> glucose + Pi

  • T3 transporter exports glucose to cytosol (where it can undergo processes like glycolysis) or then GLUT2 transports it out further to bloodstream

  • T2 transporter exports Pi (phosphate) to cytosol to be released for other cell activities

• specific mechanism to liver for purposes of controlling blood sugar

sequestering and building a pool

• T1 transporter brings G6P into ER lumen and G6Pase converts it to glucose

• builds up glucose concentration inside lumen

• sequestering or putting away glucose in the ER lumen, creating a pool of glucose away from cytosol and glycolysis

• bc hexokinase and G6Pase are not reciprocally regulated, instead to sequestering of glucose to keep it aside from glycolysis in cytosol, which would lead to futile cycle

  • bc sequestering prevents glycolysis and glycogen synthesis from happening at same time (by not releasing glucose to cytosol when glycolysis is running)

  • tagging the glucose as G6P for internal use

• glucose can easily be released to circulatory system to adjust to blood glucose outside of cell

glucose release in liver vs muscle purpose

• if glycogen is broken down and glucose is released in muscle it used for muscle use only

• if glycogen is broken down in the liver, it helps regulate blood sugar and release glucose into the bloodstream as needed

where is G6P converted to glucose

liver

• so glucose can be released into circulatory system to do metabolic processes

cori cycle purpose

• way to recycle lactate —> glucose when its built up in anaerobic fermentation

  • bc build up of lactate in muscles causes cramps

• move it as lactose in skeletal muscle to glucose in the liver

cori cycle mech

• glycolysis in smooth muscle/skeletal muscle converts glucose —> pyruvate, and pyruvate —> lactate via lactate dehydrogenase in anaerobic fermentation

  • glucose —> pyruvate release ATP

  • pyruvate —> lactate release NAD+

    • transports to circulatory system to liver

• liver runs lactate dehydrogenase in reverse, converting lactate —> pyruvate, then pyruvate —> glucose via gluconeogenesis to make glucose to release to the circulatory system then back to muscle

  • lactate —> pyruvate release NADH

  • pyruvate —> glucose invest ATP

glycogen storage diseases

• Tarui

• McArdle

tarui disease

• deficiency in PFK-1

  • in muscle only

• causes increased storage of glycogen in muscle only

  • leads to weak muscles bc cant break down sm glycogen + inflammation

  • PFK defect means glycolysis cannot run, and there is buildup of G6P, and since this is in the muscle, it releases G6P and goes to glycogen store- excess

• exercise tolerance is most maintained with this disease (prod less lactic acid than control, but more than mcardle)

  • since glycolysis isnt running, no ATP release, less muscle contraction, weakened activity

mcardle disease

• glycogen phosphorylase deficiency

  • in muscle only

• causes increased storage of glycogen

  • defect in enzyme that breaks down glycogen, but since synthase is still working, there is just lots of building of glycogen but no breakdown

• exercise tolerance is least maintained with this disease (prod least amt of lactic acid than control + tarui)

  • less tolerance bc no access or breaking down on glycogen

  • indirect flux through glycolysis decreases bc glucose is being funneled very quickly to glycogen/isnt being released so glycolysis will run out of fuel —> even less ATP and energy

futile cycle meaning + problems

• futile cycle is if glycogen synthesis and breakdown occurs at the same time

  • no net gain or release of glucose

  • needs tighter regulation on what cell needs at a particular time to make sure this doesn’t happen

  • regulate glycogen phosphorylase

glycogen phosphorylase characteristics

• dimer

• has an active site on each monomer

• phosphorylation occurs on serine 14

• switches between b (inactive) and a (active) in covalent regulation (via phosphorylation)

• switches between T (inactive) and R (active) in allosteric regulation (via negative and pos effectors)

• glycogen breakdown activity is regulation by internal and external signals

2 types of regulation of glycogen phosphorylase

• covalent regulation

  • phosphorylation by phosphorylase kinase: b —> a (more active)

  • dephosphorylation by phosphoprotein phosphatase: a —> b (less active)

    • hormonal signals- ex: epinephrine, insulin, glucagon

    • tells cell about outside signals and how to regulate glycogen or glucose release

• allosteric regulation

  • negative effector binds to GPa: R —> T state

  • positive effector binds to GPb: T —> R state

    • internal signals- ex: ATP, glucose

    • tell whats happening inside the cell and how to regulate glycogen or glucose release

glycogen phosphorylase allosteric regulation in liver

glycogen phosphorylase a (phosphorylated, active covalently)

(R state, active) —> (T state, inactive)

• negative regulation by internal glucose, binds to allosteric site

  • if glucose is high in the cell, it would be futile to continue breaking it down, so acts as negative effector

  • because of G6Pase in liver and sequestering of glucose, if glucose is high in cytosol, cell doesn’t need to breakdown anymore

glycogen phosphorylase allosteric regulation in muscle

glycogen phosphorylase b (dephosphorylated, inactive covalently)

(R state, active) —> (T state, inactive)

• negative regulation by ATP, G6P

  • high energy levels or high G6P intermediate levels means don’t need to break down glycogen bc have enough stored for later when glucose is low

(T state, inactive) —> (R state, active)

• positive regulation by AMP

  • AMP means low E, need more flux through glycolysis, increase phosphorylase activity to breakdown/release more glyucose

isozymes

same enzyme function but coded by different genes and expressed in different tissue types

• ex liver vs muscle glycogen phosphorylase

glycogen phosphorylase covalent regulation in muscle

(R state, active) —> (T state, inactive)

• insulin (hormone) stimulates protein phosphatase-1 (H2O —> Pi) which dephosphorylates + inactivates the 2 Serine-14 in active site

(T state, inactive) —> (R state, active)

• epinephrine (hormone via signaling, fight/flight response) stimulates phosphorylase kinase (2ATP —> 2ADP) to phosphoryalse + activate the 2 Serine-14 in the active site

when unphosphorylated (high insulin):

T —> R via allosteric (+ by AMP) or R —> T via allosteric (- by ATP, G6P) will still bind to inhibit or activate the enzyme based on INTERNAL conditions

• even if external insulin is high and switches enzyme to the T state, if internal AMP is high, this wins out and switches enzyme back to R state so glycogen breakdown can still occur to release glucose

glycogen phosphorylase covalent regulation in liver

(T state, inactive) —> (R state, active)

• glucagon and epinephrine (hormonal signals) activate phosphorylase kinase to phosphorylate enzyme to activate it so it breaks down glycogen releasing glucose when low blood sugar

(R state, active) —> (T state, inactive)

• insulin activates protein phosphatase-1 to dephosphorylate enzyme to T state, so glycogen breakdown and glucose release doesn’t occur (bc lots of sugar avail)

when phosphorylated (high epinephrine + glucagon):

R —> T state by glucose, glucose is bound allosterically

• so, even if epinephrine/glucagon high externall, INTERNAL glucose wins out if high internally, pushing enzyme to T state even tho phosphorylated

when unphosphorylated (high insulin):

T —> higher T state by insulin activating protein phosphatase-1 and removing both P groups and glucose

when does covalent vs allosteric regulation occur

• covalent (due to outside signals) always occurs

  • allosteric (AMP vs ATP,G6P) in muscle only occurs when in unphosphorylated/high insulin state

  • allosteric (glucose, insulin) in liver can occur in phosphorylated state to dephpshprylate or inactivate it

relationship between internal and external signals

even if external signals are telling the cell activate via phosphorylation or inactivate via dephosphorylation, the internal signals win out if they are high internally and can turn enzyme to R or T state to inhibit or activate glycogen breakdown

glycogenolysis

breaking down glucose molecules from glycogen and releasing them

signal transduction pathways for covalent hormonal regulators

epinephrine/glucagon (switches enzyme from T —> R, activating glycogen breakdown)

• binds to receptor on membrane

• GCPR converts GDP —> GTP

  • creates ATP —> cyclic AMP

• cyclic AMP activates protein kinase A

  • 1) phosphorylates phosphorylase kinase

    • activates it, allows for breakdown of glycogen, enzyme becomes more active

  • 2) phosphorylates glycogen synthase

    • inactivates it, tells it so stop making glycogen, enzyme becomes less active

hormonal/covalent regulation of glycogen synthase

(R state, active) —> (T state, inactive)

• epinephrine or glucagon

  • activate Ca2+ or protein kinase A or glycogen synthase kinase 3 which converts ATP —> ADP and phosphorylates and inactivates it

(T state, inactive) —> (R state, active)

• insulin activates protein phosphatase-1 which dephosphorylates glycogen synthase, activating it

  • G6P is allosteric regulation- converts phosphorylated T state to phosphorylated R state when high concentration in the cell

    • increases flux and generates more G6P to continue building glycogen

pathway of low blood glucose levels in liver

need to switch from T —> R

• glucagon binds

• activates and phosphorylates protein kinase A

  • phosphorylates and inactivates glycogen synthase

  • phosphorylates and activates phosphorylase kinase

• active phosphorylase kinase —> phosphorylates glycogen phosphorylase —> R state —> glycogen degredation —> glucose release via GLUT2

pathway of high blood glucose levels in liver

need to switch from R —> T state

• insulin binds

• actiavtes protein phosphatase-1

  • dephosphorylates and inactivates glycogen phosphorylase

  • dephosphorylates and activates glycogen synthase

• active glycogen synthase —> glycogen synthesis —> glucose import into the liver via GLUT2 to store glucose as glycogen

PDH + CAC + glycogen breakdown effectors

(+) ADP (AMP for glycogen breakdown)

(-) ATP

gluconeogenesis effectors

(-) ADP/AMP, F26BP

(+) acetyl coA, citrate

glycolysis effectors

(-) ATP, citrate, alanine, G6P

(+) ADP/AMP, F26BP, F16BP

glycogenesis vs glycogenolysis

• glycogenesis

  • glucose —> glycogen

• glycogenolysis

  • glycogen —> glucose 1P

glycogenesis hormonal + allosteric effectors

glucose —> glycogen

hormonal:

• muscle (+) epinephrine

• muscle and liver (+) epinephrine and glucagon

• muscle and liver (-) insulin

allosteric

• (-) ATP, G6P

• (+) AMP

glycogenolysis effectors

glycogen —> glucose

hormonal:

• muscle and liver (-) epinephrine and glucagon

• muscle (-) epinephrine

• muscle and liver (+) insulin

allosteric

• (+) glucose 6P (transient)

the positive allosteric and hormonal effectors on glycogen phosphorylase (AMP, epinephrine, glucagon) and the negative effectors of glycogen synthase (epinephrine, glucagon)

when glucagon present

• gluconeogenesis running

• inhibits glycogenesis by being (-) effector for glycogen synthase

  • both increases glucose avail.

  • occurs primarily in liver

global carbohydrate regulation in liver- high blood glucose

high blood glucose

• inc in insulin

• inc PFK-2, F26BP, PFK-1 = inc glycolysis

• inc PKB, dec GSK-2 = inc glycogen synthesis

• inc protein kinase, inc PP1, dec phosphorylase kinase, dec glycogen phosphorylase = dec glygogen breakdown

  • overall: low glycogen breakdown but high glycolysis and glycogen synthesis

global carbohydrate regulation in liver- low blood glucose

low blood glucose

• inc in glucagon

• inc camp, inc PKA

• inc FBPase-2, dec PFK-2, dec F26BP, dec PFK-1 = dec glycolysis

• inc PKA, dec glycogen synthase = dec glycogen synthesis

• inc PKA, inc phosphorylase kinase, inc glycogen phosphorylase = inc glycogen breakdown

  • overall: high glycogen breakdown but low glycolysis and glycogen synthesis

mitochondria components

• outer membrane

• inner membrane

• intermembrane space between them

  • H+ gradient

• matrix (inside)

• ETC on inner membrane (4 complexes)

• ATP synthase also on inner membrane

• PDH and CAC occur here

mitochondria relation to glycolysis

• glucose —> pryuvate in cytosol

• pryuvate gets translocated to mitochondrial matrix to go through PDH —> acetyl coA

• then CAC from acetyl coA —> 6 NADH + 2 FADH2

• the electrons from NADH and FADH2 go to ETC and move through the complexes

• in the process, protons are pumped from matrix to intermembrane space, creating a proton gradient

• H+ returns to matrix during ATP synthase, and ADP —> ATP generated

  • big E payoff in cell

4 complexes of ETC

• complex 1: Ubiquinone Oxidoreductase

• complex 2:

complex I

• ubiquinone oxidoreductase

• one of biggest macromolecular assemblies in mammalian cells

• more than 40 diff polypeptide chains, encoded by nuclear and mitochondrial DNA

NADH relation to ETC

NADH is generated in step 6 glycolysis

• NADH feeds into complex I from matrix and is oxidized to NAD+ + H+ (H+ released to matrix), which can then be used in catabolic processes

• electrons are first fed to FMN in complex I

  • has isoalloxazine ring (so can hold 1 or 2 e-) and only 1 nucleotide

• then the 2e- moved through 7 Fe-S centers mobile carriers

• the Fe-S centers and FMN transfers the 2e- to Q

  • also gets 2 H+ from matrix

• mobile carrier Q accepts the protons and is reduced to QH2

• QH2 moves to complex III

  • Energy from moving the electrons is conserved and used to pump 4 protons from matrix to IMS, contributing to proton gradient

iron-sulfur centers

• Fe+ with 4 S-

• electrons bind reversible to Fe

• causes conformational change when bound, Fe becomes more negative as sulfurs move closer in, inducing e- to move onto the next Fe-S center

• acts as springs to pop electron along chain until gets to mobile carrier Q

complex I net reaction

NADH + Q + 5 H_N+ —> NAD+ + QH2 + 4 H_P+

• 5 H from the matrix in beginning

• 4 H pumped to IMS

complex II

• succinate dehydrogenase

• links CAC with ETC

  • still has mobile carrier Q moving electrons to complex III, has mobile carrier cyt c moving electrons to complex IV, still have terminal elecrron acceptor O2

    • qualities of NADH, FADH, FADH2

FADH2 feeds into complex II

• complex A: substrate binding site

  • succinate —> fumarate

  • FAD —> FADH2

    • prosthetic group so can’t leave, so electrons are directly moved to Q

    • H+ released to matrix

• complex B: ubiqionone binding site

  • 3 iron-sulfur center clusters transports electrons

• complex C —> complex D

  • still transporting the electrons down

  • Heme group between C and D

  • Q takes electrons from complex D and converts to QH2 (also picks up 2 addintional H+ from matrix)

heme group between C and D

heme is involved with reactive oxygen species protection (ROS)

• captures any loose electrons so they aren’t inadvertently released from ETC

• important because need E from moving electrons through complexes to pump H+ across membrane to generate gradient

  • if any electrons escape on the way, it’ll cause reactive oxygen species (like H2O2 of O2- and cause damage)

complex II net reaction

FADH2 + Q + 2 H_N+ —> FAD + QH2 2 H_N+

• 2 H+ picked up on matrix side and used in mobile carrier

• 2 H+ released from FADH2 to matrix

  • no H+ pumped to IMS, so complex II does not contribute to proton gradient

Q structure/properties

• Q structure:

  • can hold 2 electrons, one at each carbonyl O on ring

  • QH2 = 2e- = ubiquinol

  • Q* = 1e- = semiquinone

  • Q = 0e- = ubiqio

• Q is central molecule- mobile carrier common to all electrons feeding into ETC

  • can give other electrons directly to Q from other FAD proteins with FAD prosthetic groups

• Q is mobile carrier but stays in membrane and reduces to Q* or QH2 depending on how many electrosn given

• can be reduced to either Q* or QH2, but even though it can have the semiquionone version, it still needs to wait and carry a second electron before moving onto complex III

direct transfer of electrons to Q, forming QH2 (3 examples)

1) complex I donates electrons to Q, forming QH2

2) glycerol 3-phosphate dehydrogenase (cytosolic)

• since FAD not mobile carrier, this enzyme has to dock with membrane and donate electrons from binding glycerol 3 phosphate directly to Q, forming QH2

3) fatty acid/beta-oxidation (B-ox)

• series of enzymes with bound FAD prosthetic groups that dock with each other and donate electrons and finally ETF-Q donates electrons directly to Q

flow of electrons through ETC

• electrons from complex I or II reduce Q to QH2 or CoQ

• QH2 transfers electrons to complex III

  • 1) pumps H+ to generate gradient in IMS

  • 2) transition of electrons from a carrier with 2e- to a carrier with 1e-

    • docking station between the 2 carriers- QH2 binds and transfers electrons one at a time to cyt c

• transfers electrons to cytochrome c

  • mobile carrier, can only carry 1e-

• transfers electrons to complex IV

• O2 binds and neutralizes electrons to H2O

complex III

• cytochrome c oxidoreductase

• 11 subunits

• Fe-S clusters

• alpha helices in transmembrane domain

• cytochromes B and C are heme-like and coordinate with Fe to hold electrons and move them down the chain

2 phase Q cycle of complex III

phase 1)

• QH2 enters and binds into position 1

  • one electron is transferred to oxidized cyt c, reducing it (moves via Fe-S clusters)

  • another electron is transferred to Q in position 2. becomes semiquinone (Q*) and stays here

    • this transfer occurs through cytochrome B and C

    • Q leaves AS?

  • movement of electrons through these diff ways results in pumping 2 H+ from matrix —> IMS

phase 2)

• another QH2 enters and binds into position 1

  • one electron is transferred to a new oxidized cyt c, reducing it (moves via Fe-S clusters)

  • another electron is transferred to the Q* that’s still in position 2, forming QH2 which can now leave the complex

    • leaves AS after taking 2 H+ from matrix to form QH2

    • Q leaves AS?

  • movement of electrons through these diff ways results in pumping 2 H+ from matrix —> IMS

total 2 cytochrome c reduced, each by one electron

complex III net reaction

phase 1)

QH2 + Q + cyt c (ox) —> Q + Q* + 2 H+ + cyt c (red)

• QH2 transfers e to Q and cyt c

• Q leaves AS

• Q* stays in AS

• 2H+ pumped into IMS

phase 2)

QH2 + Q* + 2 H_N+ + cyt c (ox) —> Q + 2 H_P+ + QH2 + cyt c (red)

• QH2 transfers e to Q* and cyt c

• Q leaves AS

• QH2 leaves AS

• QH2 takes 2 H+ from matrix to leave

• 2 H+ pumped into IMS

cytochrome c mobile carrier structure

• heme group: coordinated Fe with 4 N-, catalyzes transfer of 1e- from complex III to IV

• is a protein in IMS

• Cys-S on exterior

complex IV

• cytochrome c oxidase

• passes electrons to terminal acceptor O2

• binuclear copper group (heme a Cu-Fe)

• heme a3 (Cu-Fe) where O2 also binds

cytochrome c interaction with complex IV

• need 4e- to generate 2H2O, so since each cyt c carries one electron, need 4 to bind- moved in pairs

• one cyt c drops off an electron, then another docks and drops another electron

• once have 2e-, can move them to Heme_a with coordinated Fe center

• then move electrons to Heme_a3 (in conjunction with second Fe-Cu center)

• O2 binds to Heme a3 and generates 2 H2O when interacts with the electrons (uses 4H+ from matrix as well)

  • a diff 4 H+ are pumped from matrix to IMS, contributing to proton gradient

2 electrons are moved to center a, then next 2 cytochromes come and transfer 2 electrons to heme a3

once have all 4 electrons, can form H2O

complex IV net reaction

4 cyt c (red) + 8 H_N+ + O2 —> 4 cyt c (ox) + 4 H_P+ + 2H2O

• need 4 cyt c for a total of 4e-

• 4 H+ to pump to IMS, 4 H+ to generate H2O = 8 H+

• 4 H+ moved to IMS