citric acid cycle and link reaction

malate aspartate shuttle

• nadh donates electrons to malate

• malate is transported into mitochondrial matrix

• malate turns into oxaloacetate reducing nad+

• oxaloacetate turns into alpha ketoglutarate

• alpha ketoglutarate transported into cytoplasm through antiporter

glycerol phosphate shuttle

• brown adipose

• each NADH converts into 1.5ATP

malate aspartate shuttle compared to glycerol phosphate shuttle

• more common

• more efficient

• 2.5 ATP for each NADH instead of 1.5

feedback inhibition of PDC and feed forward activation

• high levels of acetylCoA inhibits E2

• high levels of NADH inhibits E3

• high levels of NAD+ actviates

allosteric regulation of citric acid cycle

• pyruvate kinase is allosertically activated by fructose-1,6-bisphosphate

phosphorylation modulation of PDC

• active form is non phosphorylated

• phosphorylation is done by PDK pyruvate dehydrogenase kinase adding phosphate to E1

• pyruvate dehydrogenase phosphatase removes the phosphate

• done by hormone signalling

enzymes which are important for regulation of citric acid cycle

• isocitrate dehydrogenase

• alpha ketoglutarate dehydrogenase

  • for decarboxylation in eukaryotes

  • citrate synthase in bacteria

Hexokinase

An enzyme that catalyzes the phosphorylation of glucose to glucose-6-phosphate, inhibited by its product G6P.

glucokinase

• found in the liver

• catalyses phosphorylation of glucose to glucose-6-phosphate

• not inhibited by G6P

Phosphofructokinase (PFK-1), activated and inhibited by what

The main regulatory enzyme in glycolysis

inhibited by ATP and citrate

activated by AMP and fructose-2,6-bisP.

what is PFK-1 activated by

• low ATP/AMP ratio

• fructose-2,6-bisP - feedforward activation

what inhibits PFK-1

• high ATP - feedback inhibition

• citrate

• high H+ concentration

purpose of high H+ inhibiting PFK-1

• glycolysis inhibition prevents accumulation of lactic acid

PFK-1 allosteric inhibition mechanism

• tetramer PFK-1

• ATP binds to regulatory site

• slows reaction dramatically

Pyruvate Kinase

An enzyme that catalyzes the conversion of phosphoenolpyruvate to pyruvate, regulated by ATP and fructose-1,6-bisphosphate.

Feedback Inhibition

A method of metabolic control whereby the end product of a metabolic pathway inhibits an earlier step.

Citric Acid Cycle

A series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA.

Acetyl-CoA

• produced from pyruvate in aerobic conditions

• thioester

• high energy of hydrolysis

• contains adenine, ribose and phosphates

NADH

A coenzyme that stores energy through reduction and is used in the electron transport chain to produce ATP.

FADH2

A coenzyme similar to NADH that carries electrons to the electron transport chain, generating ATP.

GTP/ATP production

Energy-carrying molecules produced during the citric acid cycle, derived from substrate-level phosphorylation.

Allosteric Enzyme

An enzyme with multiple binding sites that can be regulated by molecules that bind to sites other than the active site.

Covalent Modification

A process involving the addition or removal of a molecule, such as phosphate, to regulate enzyme activity.

Anaerobic Conditions

Conditions in which the absence of oxygen causes pyruvate to be converted either to ethanol or lactate.

Oxidative Phosphorylation

The metabolic pathway through which cells use enzymatic reactions to oxidize nutrients, releasing energy to form ATP.

Fructose-2,6-bisphosphate

A potent regulator of PFK-1, increasing the enzyme's activity during glycolysis.

Lactic Acid

A product of anaerobic respiration in muscles; accumulation can occur from inhibited glycolysis.

phosphorylated form of pyruvate kinase

• protein kinase at ser12

• enzyme becomes les active

pyruvate decarboxylationq

• acetyl is converted to coa to create coenzyme coa (factor)

• catalysed by pyruvate dehydrogenase complex

• NADH is formed (2 electrons from pyruvate are transferred)

• one co2 is released

what enzyme is used in pyruvate decarboxylation

• pyruvate dehydrogenase complex

• E1 - in charge of decarboxylation

• E2 - role is to transfer acetyl group to CoA

• E3 - in charge of cofactor regeneration

where is pyruvate dehydrogenase found in eukaryotes

• mitochondrial matrix

• pyruvate/H+ symporter allows transportation of pyruvate through inner membrane

first stage of citric acid cycle

Two carbon atoms (Acetyl-CoA) enter the cycle

Two different carbon atoms are oxidized

Two molecules of CO2 are released

Two molecules of NAD+ are reduced to NADH

Succinyl-CoA is formed (energy rich thioester)

second stage of citric acid cycle

Succinyl-CoA is hydrolyzed

Succinate is oxidized in 3 steps to Oxaloacetate

One molecule of GTP or ATP is formed

One CH2 group is oxidized to a C=O group

One molecule of FAD is reduced to FADH2

One molecule of NAD+ is reduced to NADH

breakdown of energy per nadh, fadh and per glucose

Each NADH ≃ 2.5 ATP

Each FADH2 ≃ 1.5 ATP

• for each glucose 30-32 ATP produced

advantage of glucose-6-phosphate

cannot pass through the membrane to the extracellular side

what ion does hexokinase require

mg 2+

• divalent cation is needed to form complex with ATP