Glycolysis, Link Reaction and Citric Acid Cycle

These flashcards cover key terms and concepts related to glycolysis, the link reaction, and the citric acid cycle, emphasizing their regulation, key enzymes, and energy production.

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.

Acetyl-CoA

• produced from pyruvate in aerobic conditions

• thioester

• high energy of hydrolysis

• contains adenine, ribose and phosphates

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.

phosphorylated form of pyruvate kinase

• protein kinase at ser12

• enzyme becomes les active

pyruvate decarboxylation

• 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

citric acid entire cycle

1. acetylcoa joins with oxaloacetate

2. citrate → isocitrate

3. isocitrate decarboxylated → alpha-ketoglutarate NADH IS FORMED, CO2 RELEASED

4. alpha-ketoglutarate decarboxylated → succinyl-CoA NADH IS FORMED, CO2 RELEASED

5. succinyl-CoA → succinate , GTP or ATP FORMED

6. dehydrogenation of succinate → fumarate FADH2 FORMED

7. hydration and dehydrogenation of fumarate → malate ANOTHER NADH IS FORMED

8. oxalacetate is formed and cycle continues

for each acetyl coa which enters the cycle what are the products

• two molecules of co2 are released

• 3 NADH

• FAD

• GTP/ATP