Glycolysis is taking place in Cytoplasm. So Pyruvate is generated in Cytoplasm. This is to be transports into mitochondria by a Pyruvate transporter. It is a simport, where the hydrogen ion is transporter. Here two different enzymes are involved based on the condition. They are Pyruvate dehydrogenase and Lactate dehydrogenase enzymes. There are 2 different conditions are monitor by the Pyruvate fate.
- Anaerobic Condition (Absence of Oxygen) → It converts into Lactate
- Aerobic Condition (Presence of Oxygen) → It converts into Acetyl~CoA
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In Myocytes in muscles, the Pyruvate is converted into Lactate. In the presence of the enzyme “Lactate dehydrogenase (LDH)”. In the reaction NADH+H+ is consumed. Lactate Dehydrogenase (LDH) is classical example for Isoenzyme (or) Isozyme. The molecular weight is 1, 40,000.
Pyruvate → Lactate + NAD+
In the presence of oxygen the Pyruvate is converted into Acetyl~coA molecule. This reaction is catalyzed by “Pyruvate dehydrogenase” complex (PDH complex).
Pyruvate + CoenzymeA → Acetyl~coA + NADH + H+ + CO2
About Pyruvate Dehydrogenase Eznyme:
- PDH is a multi enzyme complex
- The molecular weight of PDH complex in Escherichia coli is 48,00,000.
- PDH complex is located in the Matrix Space of Mitochondria of the erythrocytes in the cytoplasm of the prokaryotes.
- The Enzyme contains 3 enzymatic sub-units and 5 co-enzymes.
Enzymatic subunits of PDH Complex:
- Pyruvate dehydrogenase (or) Pyruvate decarboxylase [E1]
- Dihydrolipoyl transacetylase [E2]
- Dihydrolipoyl dehydrogenase [E3]
CoEnzymes of PDH complex:
- Thiamine PyrPhosphate (TPP)
- Lipoic acid
- Flavin Adenine Dinucleotide (FAD)
- Nicotinamide Adenine Dinucleotide (NAD+)
Mechanism of Pyruvate Dehydrogenase Complex:
There are 5 successive stages in the conversion of Pyruvate into Acetyl~coA. These are as follows.
Pyruvate loses its carboxyl group as it reacts with the bound TPP of Pyruvate of E1 subunit to form the hydroxyl Methyl derivative of Thiazole ring of TPP.
The hydrogen atom and acetyl group is transferred to the oxidized form of lipoyl group of Coenzyme E2 to form 6-acetyl thio-ester of the reduced lipoyl groups.
A molecular CoEnzymeA reacts with the acetyl derivative of E2 to produce acetyl~coA and the fully reduces form of lipoyl group. the Acetyl~coA is involved in TCA cycle.
The fully reduced form of E2 is acted upon by E3 which promotes transfer of hydrogen atom from reduced lipoyl groups to the FAD prosthetic group of E3.
In this last stage the reduced group of E3 transfer hydrogen to NAD+ forming NADH.