Janet Macaulay; Week 2 MED1011; Biochemistry
Metabolic pathways occur in small steps catalysed by specific enzymes, often compartmentalised and highly regulated. As a material is oxidised the electrons are transferred to another molecule which are therefore reduced. These are redox reactions (oxidation-reduction) and transfer large amounts of energy. NAD is a coenzyme that carries electrons from redox reactions. It has an oxidised (NAD) and a reduced (NADH) form. Oxidation/reduction can also be in terms of gain or loss of protons. Both forms participate in biological redox reactions and act as electron carriers.
Glycolysis operates in the presence of absence of O2. It is a pathway of 10 enzyme catalysed reactions located in the cytoplasm and provides starting materials for both cellular respiration and fermentation (pyruvate). It produces 2 ATP and 2 NADH. Glycolysis contains an endergonic initial phase and secondary exergonic phase. Endergonic consumes 2 molecules of ATP, and the payoff phase produces 4 molecules of ATP and 2 molecules of NADH. Pyruvate can be converted to acetyl CoA by pyruvate oxidation, which can enter the citric acid cycle. The citric acid cycle can produce NADH and FADH2. These two molecules enter the electron transfer chain. Some of this energy created by pyruvate oxidation is captured when NAD is converted to NADH and H.
- Pyruvate + NAD + CoA -> acetyl CoA + NADH + H + CO2
The citric acid cycle is one of the most important energy producing pathways. It gives off 2 CO2, 3 NADH, 1 FADH and one ATP for each acetyl CoA. There are two acetyl CoA released per glucose molecule. It requires thiamine to function. FADH is from succinate to fumarate. GTP is from succinyl CoA to succinate.
In eukaryotes, glycolysis and fermentation occur outside of mitochondria, in the matrix of mitochondria is the citric acid cycle and pyruvate oxidation, in the inner membrane is the citric acid cycle. In prokaryotes glycolysis, the citric acid cycle and fermentation occur in the cytoplasm, and pyruvate oxidation and the respiratory chain occur on the plasma membrane.
In some cells under anaerobic conditions pyruvate can be reduced by NADH to form lactate and regenerate the NAD needed for glycolysis. Fermentation reactions anaerobically oxidise NADH and H produced in glycolysis, and reduce pyruvate to lactic acid. NAD is regenerated.