chp 19 the tricarboxylic acid cycle

the citric acid cycle is the central pathway for

oxidizing all metabolic fuels

most of the energy yield from the oxidation of substrates in the TCA is stored in what

reduced electron carriers (NADH and FADH2)

what are the 3 stages of respiration

• Step 1: glycolysis and making acetyl Coa

• Step 2: TCA cycle

• Step 3: electron transport chain and oxidative phosphorylation

The electrons from glucose oxidation feed into the electron transport pathway, driving synthesis of ATP.

where does glycolysis occur (in cell)

cytoplasm of the cell

where does the citric acid cycle occur in cell

mitochondrial matrix

exception for part of TCA cycle that does not occur in mitochondrial matric

succinate dehydrogenase, which is located in the inner membrane

• is part of complex 2 in the ETC

where does oxidative phosphorylation occur

inner membrane of mitochondria

tricarboxylate cycle (TCA) aka (2)

Krebs cycle and citric acid cycle

pyruvate from glycolysis is (what reaction and into what)

• oxidatively decarboxylated to acetate

  • Acetate is acetyl-Coa without the Coa

acetate is then made into what

degraded into 2 CO2 in the TCA cycle

what is produced in the TCA cycle

2 CO2

1 GTP

3 NADH

1 FADH2

THIS IS PER ONE PYRUVATE

in TCA cycle, what is the product of the 8th reaction and also the reactant of the frist reaction

oxaloacetate (is recycled)

sequence of events in TCA cycle (9)

• Prep: Convert Pyruvate to Actyl-CoA

• Step 1: C-C bond formation to make citrate

• Step 2: Isomerization via dehydration/rehydration

• Steps 3–4: Oxidative decarboxylations to give 2 NADH

• Step 5: Substrate-level phosphorylation to give GTP

• Step 6: Dehydrogenation to give reduced FADH2

• Step 7: Hydration

• Step 8: Dehydrogenation to give NADH

TCA cycle steps (substrate, products, and enzymes)

1. acetyl CoA + OAA—> citrate (citrate synthase)

2. citrate—> isocitrate (aconitase)

3. isocitrate —> alpha-ketoglutarate (isocitrate dehydrogenase)

4. alpha-ketoglutarate —> succinyl Coa (alpha ketoglutarate dehydrogenase)

5. succinyl Coa —> succinate (succinyl-Coa synthetase)

6. Succinate —> fumarate (succinate dehydrogenase)

7. Fumarate —> malate (fumarase)

8. malate —> oxaloacetate (malate dehydrogenase)

in which reactions is NADH made?

isocitrate —> alpha ketoglutarate by isocitrate dehydrogenase (3)

alpha ketoglutarate—> succinyl Coa by alpha ketoglutarate dehydrogenase (4)

malate—> oxaloacetate by malate dehydrogenase (8)

which reactions make FADH2

succinate —> fumarate by succinate dehydrogenase (6)

which reaction makes GTP

succinyl CoA —> succinate by succinyl CoA synthetase

what must pyruvate do to acess the TCA cycle

enter the mitochondria

how does pyruvate enter the mitochondria

by being converted into Acetyl-Coa

pyruvate —> acetyl CoA how (enzymes)

pyruvate dehydrogenase complex (PDC)

• 3 enzymes and 5 coenzymes

what occurs to pyrvate (what changes does it occur)

oxidative decarboxylase (so it has CO2 removed and a CoA added)

what are the fates for acetyl CoA (what can it do)

• enter TCA cycle and produce energy

• can be used to synthesize storage molecules

the pyruvate dehydrogenase complex (PDC) is what

a large multienzyme complex

the PDC consist of what

3 enzymes and 5 cofactors

what are the cofactors for the pyruvate dehydrogenase complex

• Thiamin Pyrophosphate (TPP) – thiamin

• Flavin adenine dinucleotide (FAD) – riboflavin

• Coenzyme A (CoA) – pantothenic acid

• Nicotinamide adenine dinucleotide (NAD) – niacin

• Lipoate (aka lipoic acid)

thiamin pyruphosphate (TPP) comes from

thiamin

flavin adenosine dinucleotide (FAD) comes from

riboflavin

coenzyme A (CoA) comes from

pantothenic acid

Nicotinamide adenine dinucleotide (NAD) comes from

Niacin

what are the advantages of a multienzyme complex (4)

• Short distance between catalytic sites allows channeling of substrates from one catalytic site to another

• Substrate channeling minimizes side reactions

• Regulation of activity of one subunit affects the entire complex

• Activity of the complex is subject to regulation (ATP)

Pyruvate dehydrogenase complex is the prototype for (2)

• α-ketoglutarate dehydrogenase

• Branched chain α-keto acid dehydrogenase

These are practically the same in coenzymes and technique but differ in active sites as deal with different molecules

In reaction 1 of TCA cycle what happens (substrate—> product, enzyme)

oxaloacetate + Acetyl Coa —> citrate by citrate synthase

In reaction 1 of TCA cycle what kind of reaction occurs

condensation reaction

Reaction 1 of TCA cycle creates what (only place this occurs)

reaction with C-C bond formation

Reaction 1 of TCA cycle is considered the

 limiting step of CAC, activity depends on [OAA]

• because its activity is tightly regulated by the availability of substrates, the cell’s energy status, and feedback inhibition from its products

• once reaction occurs, the cycle must occur

how is reaction 1 of TCA cycle inhibited

NADH and succinyl CoA are allosteric inhibitor

• f we have high NADH, then that means we have lots oh high energy electron carriers that will provides lots of ATP. So if we have a lot we don’t need more

• The accumulation of succinyl-CoA indicates that the cycle is operating and intermediates are being processed.

is reaction 1 of TCA reversible?

no!!!

which is why it is rate limiting, as once this reaction occurs it is committed to the cycle

reaction 2 of TCA cycle (substrate, product, enzyme)

citrate —> isocitrate by aconitase

what occurs at reaction 2 of TCA cycle

• (A) Elimination of H2O from citrate gives a cis C=C bond

• (B) Addition of H2O to cis-aconitate is stereospecific

so H2O is removed and then added back to switch the OH and H location to the opposite

isocitrate is good substrate for what type of reaction

oxidation

• will be able to take OH group from a alcohol to a ketone

aconitase is involved is what (not to do with TCA)

iron status regulation!!!!

iron regulation

reaction 3 of TCA cycle (substrate, enzyme, product)

isocitrate —→ alpha ketoglutarate by isocitrate dehydrogenase

reaction 3 is what reaction (like what type of reaction)

oxidative decarboxylation

isocitrate dehydrogenase does what in reaction 3

oxidative decarboxylation so a CO2 is removed from isocitrate

the alcohol group is oxidized into a ketone which gives NAD+ an H to form NADH

the CO2 removed from isocitrate comes what?

the OAA part of it

reaction 3 of TCA cycle makes what? (not substrate)

NADH

reaction 3 of TCA is reversible?

no

• Irreversible because it is very exergonic so would require A LOT of energy to reverse with is not favorable

what is isocitrate dehydrogenase regulated by

product inhibition (alpha-ketoglutarate) and ATP

• if lots of alpha-ketoglutarate, means we have a lot

• if lots of ATP why make more

reaction 4 of TCA cycle (substrate, product, enzyme)

alpha ketoglutarate —> succinyl CoA by alpha ketoglutarate dehydrogenase

in reaction 4 by alpha-ketoglutarate dehydrogenase what has been made, released?

NADH is made

CO2 is released

reaction 4 of TCA is the last

oxidative carboxylation reaction (release of last CO2)

after reaction 4, it is considered that there is a what of glucose

net full oxidation (after 2 turns)

carbon lost at reaction 4 came from what original source

oxaloacetate

succinyl CoA is a (think energy)

high energy thioster bond (important for next reaction as will help form energy)

• Hint of substrate level phosphorylation in the cycle

reaction 4 is reversible?

no, it is regulated by product inhibition

alpha ketoglutarate dehydrogenase is a complex similar to

pyruvate dehydrogenase (same coenzymes and identical mechanisms but differ in active sites to accommodate different sized substrates)

both the CO2 released from the citric acid cycle were derived from what molecule

oxaloacetate

reaction 5 (substrate, product, enzyme)

Succinyl CoA—> succinate by succinyl CoA synthase

reaction 5 makes what

GTP (which can then be converted to ATP)

reaction 5 is what kind of reaction

substrate level phosphorylation

• Energy of thioester allows for incorporation of inorganic phosphate

• Goes through a phospho-enzyme intermediate

reaction 6 (substrate, product, enzyme)

succinate —> fumarate by succinate dehydrogenase

reaction 6 makes what (not product)

FADH2

succinate dehydrogenase is found where

• Bound to mitochondrial inner membrane

  • Part of Complex II in the electron-transport chain!!!!!

reduction of succinate to fumerate requires

FADH2 (which then has its electrons passed to Coenzyme Q)

FAD relationship with succinate dehydrogenase

• FAD is covalently bound to succinate dehydrogenase, unusual

  • So electrons immediately get fed into ETC at complex 2

reaction 6 (succinate dehydrogenase) is inhibited by

malonate (structural analog of succinate)

• it is a competitive inhibitor that that will prevent the formation of fumarate and lead to buildup of succinate

reaction 7 (substrate, enzyme, product)

fumarate —> malate by fumarase

what occurs at reaction 7

• Stereospecific (so makes one specific stereoisomer though others could also be made)

  • Addition of water is always trans and forms L-malate

  • OH- adds to fumarate

  • Then H+ adds to the carbanion

reaction 8 (substrate, enzyme, product)

malate —> oxaloacetate by malate dehydrogenase

in reaction 8, we regenerate what product

oxaloacetate for citrate synthase to use

reaction 8 creates what (not product)

NADH

oxaloacetate is kept at what levels in cell, why

kept VERY LOW by citrate synthase

• Continuously used to make citrate

• Pulls the reaction forward instead of going backwards to malate as it is reversible step!!!

the carbon atoms of acetyl CoA have different

fates

what are the 2 carbons we are referring to

carboxyl carbon and methyl carbon of acetyl CoA

carbonyl C of acetyl CoA becomes CO2 when

The carboxyl carbon atom of acetate is released as CO2 in the second turn of the cycle (following entry of acetyl CoA)

methyl carbon of acetyl CoA is removed as CO2 when

Release of the methyl-C of acetate as CO2 requires multiple turns of the cycle (4 turns)

one turn of the TCA cycle makes what

• 3 NADH

• 1 FADH2

• 1 GTP —> ATP

• releases 2 CO2

because glucose is broken down into 2 pyruvates, how many TCA cycles does a glucose molecule go through

2

what reactions of the TCA are irreversible

acetyl CoA + OAA—> citrate by citrate synthase (1)

isocitrate—> α ketoglutarate by isocitrate dehydrogenase (3)

α ketoglutarate —> sucinnyl CoA by α-ketoglutarate dehydrogenase (4)

one TCA cycle has net oxidation of

2 carbons to CO2

• Equivalent to two carbons of acetyl-CoA

• but NOT the exact same carbons as it is the OAA carbons that are released

In TCA cycle, energy is captured by

electron transfer to 3 NADH and 1 FADH2

in 1 TCA cycle we make what that is then converted to ATP

GTP (through substrate level phosphorylation)

in TCA cycle, intermediates are not

depleted but constant being regenerated so that reaction can keep going

direct and indirect ATP yield (how much ATP, NADH, and FADH from complete oxidation of 1 glucose are made)

• 10 NADH

• 2 FADH2

• 4 ATP

the TCA cycle provides intermediates for what

intermediates for biosynthesis

• other pathways

what are anapleurotic processes

 (“filling up”) processes are required to restore these intermediates so the TCA can continue

-The TCA cycle (Citric Acid Cycle) is a central hub of cellular metabolism, and it involves a number of intermediates (such as citrate, α-ketoglutarate, succinate, malate, etc.) that are used in various biosynthetic processes. However, some of these intermediates may be drawn off the cycle for the synthesis of amino acids, nucleotides, or other biosynthetic molecules, potentially leading to a depletion of these intermediates. Anapleurotic reactions  help refill the TCA cycle with intermediates to ensure that it can continue functioning effectively. Without these replenishing reactions, the cycle could become "blocked" or inefficient, limiting energy production.

what is the most important antipleuritic reaction of the TCA cycle

OAA

Intermediates in the citric acid cycle can be used in

biosynthetic pathways (removed from cycle to be used for other anabolic pathways)

we must ____________ intermediates in order for cycle and central metabolism to continue

replenish

4-carbon intermediates are formed by carboxylation of

3-C precursors

transamination reaction is what, does what

amino acid—> regenerate/ fill up CAC intermediate

• Part of first step of amino acid metabolism

• We remove amino group from amino acid to a keto acid, and then the keto acid becomes amino acid and other one is now a keto acid

pancreas releases insulin as response to

high glucose in the bloodstream

Recent research has shown that anaplerotic enzymes feed alternative pathways that (based on insulin)

produce cytosolic signal molecules that also support insulin secretion

Pancreatic β-cells maintain high levels of

the anaplerotic enzyme pyruvate carboxylase

what is The pyruvate/malate cycle (where it occurs, what is the cycle, what is made that is of importance to insulin)

-in pancreatic β cells.

-cycle where Oxaloacetate produced by high levels of pyruvate carboxylase is converted to malate, then exported to the cytosol

-then it can be made into pyruvate, enter mitochondria and become OAA again

-The NADPH produced by turning Malate into pyruvate act as intracellular messengers that appear to be as significant as ATP in provoking insulin release

pyruvate carboxylase does what and how does it relate to insulin

• In these cells, half the pyruvate from glycolysis is converted by pyruvate carboxylase to oxaloacetate, much of which is converted to malate and exported to the cytosol, as part of a pyruvate/malate cycle

•  NADPH and other metabolites generated from the pyruvate/malate cycle act as intracellular messengers that appear to be as significant as ATP in provoking insulin release

what is pyruvate carboxylase

enzyme that turns pyruvate into oxaloacetate

regulation of TCA cycle occur by controlling what (what do we target- 2 things)

• Entry into the cycle (PDH-pyruvate dehydrogenase- and citrate synthase)

• key irreversible reactions (isocitrate dehydrogenase and α ketoglutarate dehydrogenase)

cycle lux is primarily controlled by what 3 things

• Allosteric activation of isocitrate dehydrogenase by ADP (energy state)

• NAD+/NADH ratio (redox state)

• Inhibition of relevant enzymes by acetyl-CoA and succinyl-CoA (availability of energy-rich compounds)