Anaerobic respiration is the process of creating energy without the presence of oxygen. Sometimes the body can’t supply the muscles with the oxygen it needs to create energy – such as in a sprinting situation. Without the process of anaerobic respiration there may be no energy supplied to muscles in times of high demand.
This article will consider the process of anaerobic respiration and its clinical significance.
Process of Anaerobic Respiration
Without the presence of oxygen, the electron transport chain (ETC) cannot continue as there is no terminal electron acceptor. Therefore the usual number of ATP molecules cannot be created. Cessation of the ETC leads to reduced activity of the reactions before this step, such as the Krebs cycle and glycolysis. The anaerobic pathway utilises pyruvate, the final product of glycolysis.
Without the functioning ETC there are an excess of NADH and pyruvate. Pyruvate is subsequently reduced to lactate (lactic acid) by NADH, leaving NAD+ after the reduction. This reaction is catalyzed by the enzyme lactate dehydrogenase. This essentially leads to the recycling of NAD+.
By recycling NAD+ the process of glycolysis is able to continue as the NAD+ ‘stock’ has been replenished. The glycolysis pathway produces 2 net ATP molecules which can be used for energy to drive muscular contraction etc. The 2 ATP molecules is much less than would be produced by aerobic respiration, it is necessary as without anaerobic respiration there would be no other method of ATP production.
This may happen in conditions of ischaemia. Glycolysis will happen faster and will produce lactic acid. This is necessary in situations such as exercise where the oxygen demand of muscles increases above the supply, in ischaemic heart disease or when a malignant tumour outgrows its blood supply.
Anaerobic glycolysis happens faster than aerobic because less energy is produced for every glucose broken down (2ATP cf. 32ATP), so more must be broken down at a faster rate to meet demands. This may lead to lactic acidosis.
Removal of lactate
The lactate produced as a result of anaerobic respiration must be removed as it is acidic. There are two main way to do this;
- The lactate is transported to metabolically active cells, such as the heart and brain. Here it is converted back to pyruvate by the above reaction. The pyruvate is then utilised in the Krebs cycle.
- Lactate is transported to the liver and converted to pyruvate by the above reaction. Pyruvate is then used in the process of gluconeogenesis to create more glucose
Clinical Relevance – Lactic acidosis
In some cases excessive production of lactate can lead to a condition known as lactic acidosis, a sub-type of metabolic acidosis. This is whereby the pH of the blood has become more acidic due to rising levels of lactate within the body. There are a number of causes for lactic acidosis but broadly it is caused by the body being unable to respire aerobically. Some causes include:
- Diabetes mellitus
- Enzyme deficiencies – for example pyruvate dehydrogenase deficiency
- Drugs – for example, metformin and isoniazid
- Mitochondrial disorders
Symptoms are typical of metabolic acidosis and include nausea, vomiting, muscle weakness and rapid breathing. Treatment is difficult, as there is little evidence to support the use of sodium bicarbonate solutions (to balance the pH) or direct removal of lactate (via haemofiltration). Overall the treatment is supportive and would depend on the cause; if medication is the cause it may need to be withdrawn and certain mitochondrial disorders may require adapted diets.