By Dr Ananya Mandal, MD
Metabolism comprises of two major parts: anabolism and catabolism. Catabolism is the set of metabolic processes that break down large molecules. These more complex molecules are broken down to produce energy necessary for various functions of the body. The energy is utilized for building or anabolic processes.
Catabolism in different organisms
The exact nature of these catabolic reactions differs from organism to organism and organisms can be classified based on their sources of energy and carbon:
- in organotrophs, organic sources are used as a source of energy
- in lithotrophs, inorganic substrates are used
- in phototrophs, sunlight is used as chemical energy
The basic common reactions in catabolism include redox reactions that involve the transfer of electrons from reduced donor molecules such as organic molecules, water, ammonia, hydrogen sulfide or ferrous ions to acceptor molecules such as oxygen, nitrate or sulfate.
In humans and animals, redox reactions involve complex organic molecules being broken down to simpler molecules, such as carbon dioxide and water.
In photosynthetic organisms such as plants and cyanobacteria, these electron-transfer reactions do not release energy. These reactions just help store energy absorbed from sunlight.
Classification of organisms based on their metabolism
Stages of catabolism
Catabolism can be broken down into 3 main stages.
Stage 1 – Stage of Digestion
The large organic molecules like proteins, lipids and polysaccharides are digested into their smaller components outside cells. This stage acts on starch, cellulose or proteins that cannot be directly absorbed by the cells and need to be broken into their smaller units before they can be used in cell metabolism.
Digestive enzymes include glycoside hydrolases that digest polysaccharides into monosaccharides or simple sugars.
The primary enzyme involved in protein digestion is pepsin which catalyzes the nonspecific hydrolysis of peptide bonds at an optimal pH of 2. In the lumen of the small intestine, the pancreas secretes zymogens of trypsin, chymotrypsin, elastase etc. These proteolytic enzymes break the proteins down into free amino acids as well as dipeptides and tripeptides. The free amino acids as well as the di and tripeptides are absorbed by the intestinal mucosa cells which subsequently are released into the blood stream where they are absorbed by other tissues.
The amino acids and sugars are then pumped into cells by specific active transport proteins.
Stage 2 – Release of energy
Once broken down these molecules are taken up by cells and converted to yet smaller molecules, usually acetyl coenzyme A (acetyl-CoA), which releases some energy.
Stage 3 - The acetyl group on the CoA is oxidised to water and carbon dioxide in the citric acid cycle and electron transport chain, releasing the energy that is stored by reducing the coenzyme nicotinamide adenine dinucleotide (NAD+) into NADH.
When complex carbohydrates are broken they form simple sugars or monosaccharides. This is taken up by the cells. Once inside these sugars undergo glycolysis, where sugars such as glucose and fructose are converted into pyruvate and some ATP is generated. Pyruvate is an intermediate in several metabolic pathways, but the majority is converted to acetyl-CoA and fed into the citric acid cycle or the Kreb’s cycle.
Within the citric acid cycle more ATP is generated by the monosaccharides. The most important product is NADH, which is made from NAD+ as the acetyl-CoA is oxidized. This oxidation releases carbon dioxide as a waste product.
When there is no oxygen, glycolysis produces lactate, through the enzyme lactate dehydrogenase, re-oxidizing NADH to NAD+ for re-use in glycolysis.
Glucose can also be broken down by pentose phosphate pathway, which reduces the coenzyme NADPH and produces pentose sugars such as ribose, the sugar component of nucleic acids.
Amino acid breakdown
Proteins are broken down into amino acids. Amino acids are either used to synthesize proteins and other biomolecules, or oxidized to urea and carbon dioxide as a source of energy.
In the process of oxidation, first the amino group is removed by a transaminase. The amino group is fed into the urea cycle, leaving a deaminated carbon skeleton in the form of a keto acid.
These keto acids enter the citric acid cycle. Glutamate, for example, forms α-ketoglutarate. Some of the amines may also be converted into glucose, through gluconeogenesis.
Some proteins are incredibly stable, others are very short lived. The short lived proteins usually play important metabolic roles. The short life times of these proteins allow the cell to rapidly adjust to changes in the metabolic state of the cell.
Fats are catabolised by hydrolysis to free fatty acids and glycerol. The glycerol enters glycolysis and the fatty acids are broken down by beta oxidation to release acetyl-CoA. This acetyl co-A reaches the citric acid cycle next. Fatty acids release more energy upon oxidation than carbohydrates because carbohydrates contain more oxygen in their structures.
Calories obtained by complete oxidation
- Carbohydrates yield 4 kcal/g.
- Carbohydrates have to be stored with water and each 1g of glycogen is hydrated with 2g water. Hydrated carbohydrates: 1.3 kcal/g
- Fat: 9 kcal/g (fats are not hydrated)
- Proteins: 4 kcal/g
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)