Describe in detail three different fates of pyruvate that are known to exist in
ID: 179181 • Letter: D
Question
Describe in detail three different fates of pyruvate that are known to exist in normal catabolism of glucose in different organisms and under various conditions. DESCRIBE THE PURPOSE BEHIND THESE SCHEMES! WHAT DID EACH SCHEME ACCOMPLISH, AND THEREFORE WHAT WAS ITS PURPOSE?? Anabolism and catabolism are coordinately regulated. If this regulation breaks down, a futile cycle can result. Give an example of where a potential futile cycle can exist in glycolysis. Be sure to include enzyme names, structures of the substrates and names of any cofactors. Draw the NET reaction of this futile cycle and EXPLAIN why the cycle is not a good thing.Explanation / Answer
answer 21
Different fates of pyruvate :
The fate of pyruvate varies in different organisms. It may get converted into either ethanol. Lactic acid or carbon dioxide.
1. Under aerobic conditions pyruvate enters the citric acid cycle and electron transport chain in the form of acetyl coenzyme A (acetyl CoA). Acetyl CoA is formed by oxidative decarboxylation of pyruvate. After entering the citric acid cycle, it is oxidized to CO2 and H2O. During aerobic (mitochondrial) respiration, NADH is reoxidized to NAD+ by passing its electrons to oxygen.
2. In yeast and other microorganisms, pyruvate get converted into ethanol by the following steps. This is also known as alcoholic fermentation.
Step 1 : Decarboxylation of pyruvate by the enzyme ‘pyruvate decarboxylase’ in presence of thiamine pyrophosphate as a coenzyme.
Step 2 : Acetaldehyde gets reduced to ethanol by NADH, in a reaction catalysed by alcohol dehydrogenase. NAD+ gets regenerated in this step.
In a few microorganisms, erythrocytes and cells of higher organisms with limited oxygen (muscle cells during intense activity), lactate is formed from pyruvate in a process known as lactic acid fermentation.
Pyruvate is reduced by NADH to form lactate with the aid of enzyme – lactate dehydrogenase. Here since oxygen is limiting, NADH generated by dehydrogenation of glyceraldehyde-3-phosphate cannot be re-oxidized. In case of failure to regenerate NAD+, the energy yielding reactions of glycolysis may cease to occur. Hence the organisms have developed an ability to regenerate NAD+ by transferring electrons from NADH to for a reduced end product – ethanol or lactate.
The scheme for these differnt fates - for regeneration of NAD+ and continuation of glycolysis.
3. In the mitochondria, pyruvate is converted to oxaloacetate by pyruvate carboxylase where biotin acts as a coenzyme. Oxaloacetate is used further for synthesis of Aspartate, phosphoenol pyruvate or in the TCA cycle.
4. Pyruvate may be converted to alanine (non-essential amino acid) by transamination reaction.
answer 22 )
Futile cycles are called so because they do not essentially carry out any overall conversions but rather couple two reactions or metabolic pathways which operate is opposite directions yielding no net result but only dissipation of energy in the form of ATP.
Examples :
UTP--àUDP + Pi
To prevent such futile cycles, glycogen synthase and glycogen phosphorylase are reciprocally regulated by allostearic effectors and covalent modifications.
Fructose 1,6-biphosphate
Fructose -6-phosphate + ATP ß-------à Fructose 1,6 – biphosphate + ADP
Sum of two reactions :
ATP + H2O ß-------à ADP + Pi + heat