Kreb’s Cycle

Question 1

………..is a multienzyme complex located

in the mitochondrial matrix.

Answer 1

PYRUVATE DEHYDROGENASE COMPLEX

Question 2

…………..It converts pyruvate into acetyl CoA.

Answer 2

PYRUVATE DEHYDROGENASE COMPLEX

Question 3

the Oxidative decarboxylation of pyruvate is a reversible pathway.
T or F?

Answer 3

F

Irrversible

Question 4

PDH enzyme needs 5 coenzymes, mention them.

Answer 4

TPP
NAD+
FAD+ 
CoASH
Lipoic acid

Question 5

give a reason, acetyl CoA can not give glucose?

Answer 5

because pyruvate dehydrogenase complex makes pyruvate decarboxylation and gives acetyl CoA by an irreversible step.

Question 6

acetyl CoA is completely oxidized in the Krebs cycle

T or F?

Answer 6

T gives 2 CO2

Question 7

Synthesis of citrate from acetyl CoA and oxaloacetate (OAA) by citrate synthase named as ……………

Answer 7

condensation reaction

Question 8

give a reason, acetyl CoA is completely catabolized in the Krebs cycle?

Answer 8

• Kreb’s cycle is started by acetyl CoA (2 carbons) and oxaloacetate (4 carbons).
• It ends by oxaloacetate (4 carbons).
• The difference between the starting compounds (6 carbons) and the ending compound (4 carbons) is 2 carbons that are removed in the form of 2 CO2.
• These 2 carbons are derived from acetyl CoA.
• For this reason, acetyl CoA is completely catabolized and never gives glucose.

Question 9

illustrate Complete oxidation of one molecule of glucose under aerobic condition

Answer 9

1- Aerobic glycolysis = 8 ATP
2- Oxidative decarboxylation of 2 molecules of pyruvate = (2 × 3 ATP = 6 ATP). 3- Oxidation of two molecule of acetyl CoA in TCA cycle = (2 × 12 ATP = 24 ATP).
Energy produced from complete oxidation of one molecule of glucose = 8 + 6 + 24 = 38 ATP molecules.

Question 10

why the presence of O2 is important to the Krebs cycle?

Answer 10

The citric acid cycle needs oxygen to proceed (i.e. aerobic pathway). This is because in absence of oxygen, the respiratory chain is inhibited leading to increase NADH+H+
/NAD.
NADH+H+ will inhibit TCA cycle.

Question 11

give a reason, Arsenate causes toxicity and death?

Answer 11

It forms a stable complex with the (-SH) groups of lipoic acid, making that compound unavailable to serve
as a coenzyme. So, it inhibits the pyruvate DH complex and the α-KG DH complex. This particularly affects the brain, causing neurologic disturbances and death.

Question 12

The KREBS cycle is the major source of energy for all cells

T or F

Answer 12

F cells containing mitochondria.

Question 13

what are the Anabolic functions of Krebs function?

Answer 13

1) Formation of some amino acids:
• α-Ketoglutarate: by transamination → Glutamate.
• Oxaloacetate: by transamination → Aspartate.
2) Formation of Glucose: e.g.
- Oxaloacetate and malate: by Gluconeogenesis → Glucose.
3) Heme synthesis: from Succinyl CoA.
4) Fatty acid and cholesterol synthesis: from Citrate.