Glycolysis And TCA Cycle

Question 1

Give an overview of glycolysis including general process and the products of the reaction pathway

Answer 1

1. Glycolysis is a process that oxidises one molecule of glucose into two molecules of pyruvate
2. Two NAD reduced to NADH per molecule of glucose
3. 2 ATP used in the production of fructose 1,6-bisphosphate and 4 ATP produced by dephosphorylation of 1,3-BPG and Phosphoenolpyruvate, so net gain of 2 ATP
4. Equation: GLucose + 2Pi + 2ADP + 2NAD+ —> 2Pyruvate + 2 ATP + 2NADH + 2H+ , with deltaG = -197 kJ mol-1

Question 2

How is glycolysis controlled

Answer 2

1. The main control point is the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate
2. The enzyme, PFK, is negatively regulated by ATP, Citrate and H+, whereas AMP and F-2,6-bisphosphate are positive regulators of this irreversible reaction
3. The final irreversible step producing pyruvate is negatively regulated by ATP and Alanine, whereas feed forward positive regulation by F-1,6,-Bisphosphate prevents ‘backing up’ in the pathway

Question 3

How are fructose and galactose introduced into glycolysis

Answer 3

1. Galactose is converted into GLucose-6-phosphate
2. Fructose from adipose tissue is converted to Fructose-6-Phosphate
3. Fructose from liver is converted to Dihydroxyacetonephosphate (DHAP) or Glyceraldehyde-3-Phosphate

Question 4

What happens when someone is lactose intolerant

Answer 4

1. They lack lactase
2. Lactose is not digested or absorbed so it ends up in the colon
3. Microorganisms in the colon ferment lactose into lactic acid generated methane and hydrogen gasses

Question 5

What happens when anaerobic respiration occurs in humans

Answer 5

1. Lactate is produced from pyruvate
2. Lactate dehydrogenase interconverts pyruvate and lactate, and allows for energy generation in limited O2 supply
3. Lactate is transported to liver for conversion back to pyruvate via the Cori cycle by LDH
4. NADH usually oxidises to NAD+ and reduces hydrogen carriers on the ETC, however in lack of O2, LDH catalyses reaction of NADH with pyruvate producing lactic acid and NAD+. Without LDH NADH buildup will mean not enough NAD+ to continue glycolysis
5. In yeast conversion of pyruvate to ethanol requires alcohol decarboxylase and alcohol dyhydrogenase in absence of O2

Question 6

Link of cancer to glycolysis

Answer 6

1. Cancer cells display the Warburg effect, when glycolysis is the main source of ATP even when oxygen is plentiful
2. This produces lots of lactate and cells experience hypoxia
3. This leads to hypoxia-inducible transcription factor activation leading to vascularisation
4. So this inefficient ATP producing process leads to more agressive tumours

Question 7

Overall results of glycolysis and the equation, commenting on deltaG

Answer 7

1. One molecule of glucose oxidised to two molecules of pyruvate
2. Per molecule of glucose 2 NAD+ are reduced to two NADH
3. Two molecules of ATP used in the production of fructose 1,6-bisphosphate and four molecules of ATP produced total. Net gain of 2 ATP per molecule of ATP

Glucose + 2Pi + 2ADP + 2NAD+ —> 2Pyruvate + 2ATP + 2NADH + 2H+
DeltaG = -197 kJ mol-1

4. NADH inhibits glyceraldehyde-3-phosphate dehydrogenase which can halt glycolysis if NADH not used up

Question 8

Name all the irreversible steps and controls of glycolysis

Answer 8

1. Irreversible step (large -deltaG) is the phosphorylation of fructose-6-ph to fructose 1,6-bisph
2. AMP and Ca2+ are positive regulators of phosphorylase (enzyme forming glucose-1 ph), and negative regulators are ATP and glucose-6-ph
3. Hexokinase is positively regulated by glucose and AMP, whereas it’s negatively regulators are glucose 6-ph and ATP
4. Irreversible step to convert phosphoenolpyruvate to pyruvate by pyruvate kinase is positively regulated by fructose-1,6-bisph to prevent the backing up of metabolites within the glycolytic pathway. ATP is its negative regulator