glycolysis: fate of pyruvate under aerobic and anaerobic conditions

1. Chirantan mondal
topic – Glycolysis: Fate of pyruvate under aerobic and anaerobic
conditions
by

2.  Pyruvate is the end-product of glycolysis and is eventually transported into
mitochondria as a major energy and participates in the TCA cycle. Pyruvate are
mainly converted into ethanol, lactic acid, or carbon dioxide.
 In the glycolysis, glucose is converted into two molecules of pyruvate with the
generation of ATP. However, if reactions stops at pyruvate, due to imbalance
redox, it would not proceed for long.
 Pyruvate, a key molecule in metabolism of eukaryotic and human and its fate
differs depending upon presence and absence of oxygen.
a) If oxygen is present, pyruvate will continue on through the process of aerobic
cellular respiration and become oxidized and form Acetyl CoA.
b) If oxygen is not present or if the cell lacks mitochondria, fermentation will
occur.. There are two types of fermentation:
a)Lactic acid (Lactate) fermentation
b)Alcohol (Ethanol) fermentation

3.  In aerobic condition pyruvate become acetyl Coenzyme A (acetyl CoA) The
conversion of pyruvate to acetyl CoA is a three-step process.
Step 1. A carboxyl group is removed from pyruvate, releasing a molecule of
carbon dioxide into the surrounding medium. The result of this step is a two-
carbon hydroxyethyl group bound to the enzyme pyruvate dehydrogenase; This
step proceeds twice for every molecule of glucose metabolized (remember:
there are two pyruvate molecules produced at the end of glycolysis); thus, two
of the six carbons will have been removed at the end of both of these steps.
Step 2. The hydroxyethyl group is oxidized to an acetyl group, and the
electrons are picked up by NAD+, forming NADH (the reduced form of NAD+).
Step 3. The enzyme-bound acetyl group is transferred to CoA, producing a
molecule of acetyl CoA. This molecule of acetyl CoA is then further converted
to be used in the next pathway of metabolism, the citric acid cycle.

4.  LACTATE FERMENTATION:
 Lactic acid fermentation occurs in many microbes leading to from lactate from pyruvate
 In this process, lactate is formed from pyruvate in the reaction catalyzed by Lactate
dehydrogenase.
CH3-CO-COOH + NADH + H+ CH3-CHOH-COOH + NAD+
 It is an important step to restore the supply of NAD+ in order to ensure that glycolysis long
lasts.
 In this process, NADH is oxidized to NAD+ , which assists the reduction of pyruvate to
lactate.
 Due to lack of mitochondria in RBC, site for the TCA cycle, pyruvate is converted Lactate.
This step also helps to regenerate NAD+ and will further enter into the glycolytic pathway,
the site of ATP synthesis for RBC.
 Lactate builds up causing a drop in pH which inactivates glycolytic enzymes. End result is
energy deprivation and cell death; the symptoms being pain and fatigue of the muscle.

5.  ETHANOL FERMENTATION:
 In another process, under anaerobic condition pyruvate is further metabolized to
ethanol with regeneration of NAD+ and formation of carbon dioxide.
 In a simple eukaryotic cell like yeast, pyruvate is converted to ethanol with a
liberation of carbon dioxide in a two-step fermentation process.
Step 1. In a first step is catalyzed Pyruvate decarboxylase in which pyruvate is
decarboxylated and converted to acetaldehyde. This is a irreversible reaction.
CH3-CO-COOH CH3-CHO + CO2
Step 2. in the second step Acetaldehyde is reduced by alcohol dehydrogenase and form
ethanol . This is a reversible reaction.
CH3-CHO + NADH + H+ CH3CH2OH + NAD+
 Pyruvate decarboxylase requires Thiamine Pyro Phosphate (TPP) and Mg2+ as
coenzyme and a cofactor respectively. Thiamine (vitamin B1) contains a thiazolium
ring and serve as sources of TPP