14 CAC

what is the major purpose of aerobic catabolism

to “capture” the energy in biological fuel molecules and use it to make ATP.

• needs oxygen

what are some general features of the CAC

• Occurs in the matrix - you have to bring pyruvate from cytosol to matrix

• Oxidizes the acetate in acetyl-CoA, happens twice- 2 carbon atoms converted to 2 CO2

• Cyclic process - starts and ends with oxaloacetate (OAA)

• Final stage in the aerobic catabolism of carbohydrates, fatty acids and amino acids

• Endless metabolic connections - involved in synthesis for a bunch of things, why it has to be in steady state

what type of pathway is the CAC

its an amphibolic pathway

• Used in both catabolic and anabolic processes

• This depends on cellular conditions

  • Oxidation of fuel molecules

  • Synthesis of metabolic intermediates

what are the main things that happen in the CAC

• acetyl-CoA (C2) condenses with oxaloacetate (C4) to make citrate (C6)

• two carbons are “lost” as CO2

• oxaloacetate is regenerated

• 3 NADH (2.5), 1 FADH2 (1.5) and 1 GTP are made - from one cycle you make 10 ATP’s for one acetyl CoA

• theres 5 energy capture steps

what are the reaction in the CAC and the enzymes involved

Regulated reactions - 134 bc of arrow

How many energy capture steps? 34568

ENZYMES IN RED MEMORIZE - know structure of products and reactants

why do we need to add water as the first step in CAC

Add water to ensure its a net exergonic rxn and couple it to the energy of the thioester bond

- exergonic and anabolic

what reaction is coupled in the first step

Step 1: OAA + A-CoA → Citroyl-CoA

Step 2: Citroyl-CoA + H2O → Citrate + HSCoA

• step 2 pulls reaction forward

• Hydrolysis of thioester makes the citrate synthase reaction exergonic

• irriversible

what is the second step of CAC

• the isomerization of citrate a tertiary alcohol to isocitrate a secondary alcohol

• two part process where citrate loses H20 and its added back differently

• reversible

what is the third step in the CAC

• with the enzyme isocitrate dehydrogenase

• its the first oxidative decarboxylation (also seen in the PDC), its regulated, irriversible, and an energy capture stage because NAD+ is reduced to NADH

• carbon dioxide is also produced as well as alpha-ketoglutarate with a carbonyl ketone

• takes isocitrate and NAD+ to make NADK, CO2, and alpha-ketoglutarate

what is the fourth reaction of the CAC

• regulated with enzyme alpha-ketoglutarate dehydrogenase complex

• the second oxidative decarboxylation step, transfer of C4 to CoA and forms a thioester high energy bond

• now all tthe carbons from glycolysis are fully oxidized

• take alpha-ketoglutarate, NAD+ and CoASH (coenzyme A that has acyl group and the sulfur part is reactive) to make NADH, CO2, and succinyl-CoA

• energy capture step, irriverisble

what is the difference of pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase

The only major difference is seen with regulation of the two complexes.

PD has its own protein kinase and phosphytase but KD is allosterically regulated

what is the fifth reaction of CAC

• regulated by enzyme succinyl-CoA synthetase

• the energy stores in the thioester bond drives the formation of GTP

• its a substrate level phosphorylation, and an energy capture step, reversible

• takes succinyl-CoA, GDP, and inorganic phosphate to make succinate, GTP, and CoASH

what are the intermediate in reaction 5

what is the difference between an enzyme synthase and synthetase

• synthetase means that theres an NTP involved

what do the last three reactions in the CAC do

The last three reactions convert a methylene group of succinate to a carbonyl group in OAA

• rxn 6 = succinate and FAD turns into FADH2 and fumarate with the enzyme succinate dehydrogenase complex (oxidation by FAD), energy capture, reversible

• rxn 7 = fumarate goes through hydrating and makes malate, water added accross double bond, reversible

• rxn 8 = takes malate and NAD+ to make oxaloacetate, NADH, and H+ ions with the enzyme malate dehydrogenase, reversible, energy capture (oxidation by NAD+)

what is the relationship between GTP and ATP

GTP + ADP → GDP + ATP

• so 1GTP = 1ATP

• can also be done with nucleotides

what does the sequence of the oxidation of fatty acids look like

what is the review of all the reactions in the CAC

how does the CAC connect to the ETC

• in reaction 6 when succinate turns fumarate with succinate dehydrogenase and the succunate is oxidized and loses electrons for FAD to accept them and become FADH2 this happens in the matrix then the FADH2 is in the inner mitochondrial membrane to go through complex 2

➢FADH2 is reoxidized by donating electrons to Q a mobile electron carrier, (also called ubiqunione or Coenzyme Q)

➢QH2 is reoxidized by Complex III in the electron transport chain (ETC)

➢Succinate dehydrogenase (SD) a membrane bound enzyme and is part of complex II in the ETC

see slide 19

what are the different inhibition and activations in the CAC

• citrate product inhibits reaction 1

• NADH inhibits reaction 3 and 4

• ADP activates reaction 3 when ATP allosterically inhibits it

• succinyl-CoA product inhibits reaxtion 4 where Ca2+ allosterically activates the keto complex

match the reaction numbers to the questions

• the reaction that links glycolysis with the CAC - rxn 9

• the oxidative decarboxylation reactions - rxn 9, 3, 4

• the irreversible (exergonic) reactions - rxn 1, 3, 4, 9

• the “energy capture” steps; the reactions in which NADH, FADH2 or GTP are generated - rxn 3, 4, 5, 6, 8, 9

• the reactions in which CO2 is produced - rxn 9, 3, 4

• the regulated reactions - 9, 1, 3, 4

• substrate level phosphorylation reaction - 5

• oxidation reactions- 9, 3, 4, 6, 8

what reactions take place in the CAC and what do they do

Anaplerotic Reactions - to help keep in steady state

reactions that:

• Replenish citric acid cycle intermediates.

  • Intermediates may be consumed in other processes.

  • Must be adequate intermediates to continue CAC.

• Many reactions may be anaplerotic

  • Amino acid breakdown

  • Pyruvate carboxylase

what are two alternative fates for pyruvate

• they can become acetyl-CoA with the PDH complex, lost of acetyl reduces the activity of PDH bc it activates kinase and with product inhibition

• they can also become oxaloacetate and high amounts of acetyl-CoA activates this pathway

• use oxaloacetate to power the CAC and oxidize acetyl-CoA from fats

where do fats burn

• fats burn in the flames of carbohydrates

• cant primarily use fats for energy bc its so slow, so you need a balance of fatty acids and carbs

• you need to breakdown carbohydrates at the same time

what are the functions of the CAC

• Provide biosynthetic precursors

• An important step in the generation of ATP for cellular needs

• Each round of the CAC generates 10 ATP - theres 5 energy capture steps

  • complete aerobic oxidation of glucose yields a net of 32 ATP.

  • anaerobic glycolysis generates 2 ATP (net) from glucose and 3 ATP net from glycogen

what is the ATP yield from the completion oxidation of glucose under aerobic conditions

32 ATP

when is lactate used as a fuel

• its a metabolic fuel in aerobic metabolism

• oxidation of lactate generates 15 ATP

How much water is generated by the ETC from the oxidation of lactate?

6

The two e- from each NADH and FADH2 generates one water molecule via the ETC

• water is the primary product of the ETC

How much water is generated by the ETC from the oxidation of glucose?

12 - everything happens twice

what is the major driving force in the regulation of aerobic metabolism

the relative concentration of ADP in the matrix

what happens when ADP decreases in the matrix

what happens when ADP increases in the matrix

what increases in the presence of an uncoupler

• oxygen consumption

•This refers to situations when electron transport occurs without/ reduced ATP synthesis (and thus, also, when catabolism of fuel molecules occurs without/reduced ATP synthesis).

•The proton gradient is then dissipated faster, and the rate of electron transport increases (O2 consumption goes up). The rate of re-oxidation of reduced electron carriers increases, and the rate of reactions in the CAC increases!

•Catabolic pathways are active as cell needs Energy

what is gluconeogenesis

Gluconeogenesis is the “reverse” of glycolysis. Unlike glycolysis, which occurs in most tissues (ubiquitous), gluconeogenesis occurs primarily in the liver.

• is not endergonic