Citric Acid Cycle

Question 1

Citric Acid Cycle other names

Answer 1

krebs Cycle, Tricarboxylic acid (TCA) cycle

Question 2

What is the citric acid common for?

Answer 2

Common metabolic pathway for all “fuel” molecules (carbs/FA/AA)

Question 3

Where does the CAC occur

Answer 3

Mitochondrial matrix

Question 4

What happens in the citric acid cycle in terms of energy

Answer 4

yealis mich energy that is passed on to other biochemical system (ETC) which produces large amounts of ATP - part of catabolic processes

Question 5

Give some key facts/points about the CAC

Answer 5

• gateway to aerobic metabolism of any molecule that can be transformed into an acetyl group or component of the cycle
• Cycle doesn’t produce ATP directly (removes e- and passes them on to form NADH and FADH2
• Cycle in collaboration with oxidative phosphorylation
• Very effeicinet: cyclical, small no. of CA molecules make many NADH and FADH2

Question 6

how has respiration evolved

Answer 6

used to be glycolsis (but since NADH recycled no osidation of glucose occured). Thereforec can’t yeild all the Ep glucose contains, so only got 2ATP per Glc. Then evolved use of O2 in breakdown of high energy food moleules - evolutionary advantage

Question 7

How else can we look at the CAC in terms of its evolution - why has it evolved

Answer 7

To harvest e- that could then be used to completely oxidise food molecules to CO2 and H2O

Question 8

How is Acetyl CoA formed - brief

Answer 8

Pyruvate from glycolysis and/or FA are oxidised further to Acetyl CoA in the mitochondrial matrix

Question 9

Why is Acetyl so important and at the “centre of energy production” for the cell

Answer 9

Allows different intermediates into the main E producing pathway of the CAC

Question 10

What enzyme makes Acetyl CoA from pyruvate

Answer 10

pyruvate dehydrogenase

Question 11

Give formation of Acetyl CoA in more detail

Answer 11

Involves decarboxylation of pyruvate, then oxidation, followed by transfer of CoA complex - decarboxylation step releases 2 e- (in form of 2H- ions) which can pass to O2 to produce more TAP through NADH intermediates

Question 12

Pyruvate dehydrogenase

Answer 12

E catalysing pyruvate to acetyl CoA
Multiple copies of each od the 3 subunits each catalysing diff part of reaction:
E1: decarboxylation of pyruvate
E2: transfers acteyl group to coenzyme A
E3: process… passes e- to NAD+ making NADH

Question 13

What happens for each turn of the CAC?

Answer 13

2C’s enter (acetyl CoA) followed by removal of two different C’s as 2xCO2

Question 14

What is acetyl CoA oxidised to

Answer 14

CO2 and H2O

Question 15

Is CoA used again in the cycle

Answer 15

Yes: used in step 4 then removed step 5

Question 16

What is the a-ketoglutarate dehydrogenase reation similar to

Answer 16

pyruvate dehydrogenase

Question 17

How is entry into the CAC controlled?

Answer 17

Pyruvate dehydrogenase is regulated by it’s immediate products and the endo point of cellular respiration, ATP

Question 18

how is the CAC regulated - explain

Answer 18

Bases on the needs of the cell:
1. If has enough E: NADH and ATP negatively regulate PDH on it’s allosteric site, stopping conversion of pyruvate to acetyl CoA
2. If cell needs E: ADP and Pyruvate +vely regulate PDH to catalysise conversion of Pyruvate to Acetyl CoA

Question 19

Give 2 other non-reversible reactions (exergonic steps) and points of control of CAC

Answer 19

1. Isocitrate dehydrogenase - E allostreically controlled through ATP and NADH conc (-vely regulate) and ADP +ve regulate
2. a-ketoglutarate dehydrogenase - ATP and NADH -vely regulate, also succintly CoA -vely regulates

Question 20

What do the 2 other control points of the CAC allow?

Answer 20

Re-direction of cellular resources:
* blocking isocitrate dehydrogenase causes citrate build up which shuttles citrate into cytoplasm causing phosphofructokinase to stop glycolysis
* a-ketoglutarate builds up when a-ketoglutarate dehydrogenase is inactive, which switches it’s use to production of AA

Question 21

amphibilic pathway: definition and example

Answer 21

pathway which serves both catabolic and anabolic processes: CAC

Question 22

What other metabolic processes are each major molecule in the CAC involved in?

Answer 22

• Citrate: FA, steroids - fats
• A-ketogluterate: Glutamate… Diff AA/P - AA/purine bases
• Succinyl-CoA: porphyrins, heme (groups)- heam group, phorphyrin group
• Oxaloacetate: aspartate, asparagine –> pyrimidines - nucleotide bases - Glucose, AA, purine bases, pyrimidine bases

Question 23

What ahppens when energy neets are met through the CAC

Answer 23

it can then produce building blocks of nucleotide bases, heme groups and proteins
- however this depleties the cell of CAC intermediates

Question 24

If the cell is depleted of CAC intermediates and an exercising muscle cells requires ATP, what happens?

Answer 24

• Depleadted amount of oxaloacetate
• Pyruvate can be converted to oxaloacetate by enzyme pyruvate carboxylase
• Enzyme only active when acetyl CoA is present, so build up of acetyl CoA triggers this
• Know as anaplerotic reaction

Question 25

Which reactions involve electron carreirs, and which release CO2

Answer 25

SEE SHEET

Question 26

What regulates pyruvate dehydrogenase

Answer 26

• ATP
• NADH
• Acetyl CoA