KREBS CYCLE

Question 1

where does krebs cycle occur?

Answer 1

inside the mitochondrial matrix under aerobic conditions

Question 2

what are the two stages of krebs cycle?

Answer 2

stage 1 :
- pyruvate is converted to Acetyl-CoA
Stage 2 :
- the acetyl group of Acetyl-CoA is oxidized via krebs cycle

Question 3

why is krebs cycle known as hub of aerobic metabolism?

Answer 3

meeting point of the catabolism of carbohydrates, lipids and amino acids
- intermediates of krebs cycle are also starting points for many biosynthetic pathways

Question 4

why is krebs cycle amphibolic ?

Answer 4

it involves both catabolic and anabolic reactions

Question 5

how many ATP, NADH and FADH2 are produced in one spin of krebs cycle?

Answer 5

1 ATP
4 NADH
1 FADH2
3 CO2

Question 6

what are the 8 steps of krebs cycle?

Answer 6

1 ) Pyruvate is converted to Acetyl-CoA with the help of pyruvate dehydrogenase complex and NAD+ = NADH
2 ) Acetyl-CoA + oxaloacetate is converted to citrate with the help of citrate synthase in which water is used and CoA group will exit
3 ) Citrate will be converted to Isocitrate with the help of aconitase
4 ) Isocitrate will be converted to alpha-ketoglutarate with the help of isocitrate dehydrogenase in which NAD+ will enter and NADH will exit
5 ) alpha-ketoglutarate is converted to succinate with the help of alpha-ketoglutarate dehydrogenase complex in which H2O, ADP and NAD+ will enter and ATP and NADH will exit
6 ) succinate will be converted to fumarate with the help of succinate dehydrogenase in which FAD will enter and FADH2 will exit
7 ) fumarate will be converted to malate with the help of fumarase in which H20 will be added
8 ) malate will be converted to oxaloacetate with the help of malate dehydrogenase in which NAD+ enters and NADH exit

Question 7

what are the regulating enzymes?

Answer 7

1 ) PYRUVATE DEHYDROGENASE COMPLEX
- substrates of PDC activate the complex and the products inhibit it (feedback inhibition)
- mammalian PDC are further regulated by covalent modifications in which PD kinase inactivates it by phosphorylating it whereas PD phosphate activates it by adding water and dephosphorylating it
2 ) ISOCITRATE DEHYDROGENASE
- inhibitor : NADH
- activators : Ca++ and ADP from muscle contraction
3 ) ALPHA-KETOGLUTARATE DEHYDROGENASE COMPLEX
- inhibitors : succinyl-CoA and NADH
- activators : Ca++ from muscle contraction

Question 8

how is pyruvate transported inside the mitochondrial matrix during stage 1 ?

Answer 8

via pyruvate translocase located on the inner mitochondrial membrane
- within the mitochondrial matrix, pyruvate is converted to Acetyl-CoA which enters the krebs cycle

Question 9

what is pyruvate oxidation?

Answer 9

pyruvate is converted to Acetyl-CoA in which NAD+ and coenzyme A enters and NADH and CO2 exits

Question 10

when is coenzyme A released?

Answer 10

when Acetyl-CoA combines with oxaloacetate to form citrate