5.10: Catabolism
- Describe the importance of ATP as a source of energy in living organisms.
- To understand the stages of catabolism.
ATP
Adenosine triphosphate (ATP), is a nucleotide composed of adenine, ribose, and three phosphate groups. It is the most important of the energy-rich compounds in a cell.
Energy-rich compounds are substances having particular structural features that lead to a release of energy after hydrolysis. As a result, these compounds are able to supply energy for biochemical processes that require energy. The structural feature important in ATP is the phosphoric acid anhydride linkage. This linkage is hydrolyzed when ATP is converted to adenosine diphosphate (ADP). In this hydrolysis reaction, the products contain less energy than the reactants; there is a release of energy (ΔH = - 7.4 kcal). The general equation for ATP hydrolysis is as follows:
ATP + H 2 O → ADP + P i + 7.4 kcal/mol
Guanosine triphosphate (GTP), is another nucleotide composed of guanine, ribose, and three phosphate groups. It is addressed as ATP equivalent (ATP eq ).
Overview of Metabolism
Animals obtain chemical energy from the food they eat. The food they eat are carbohydrates, fats, and proteins. The energy from food is derived through reactions defined collectively as catabolism . We can think of catabolism as occurring in three stages (Figure \(\PageIndex{1}\)). In stage I, carbohydrates, fats, and proteins are broken down into their individual monomer units: carbohydrates into simple sugars (monosaccharides), fats into fatty acids and glycerol, and proteins into amino acids. Stage I of catabolism is the breakdown of food molecules by hydrolysis reactions into the individual monomer units. This occurs in the mouth, stomach, and small intestine. It is referred to as digestion.
In stage II , these monomer units (or building blocks) are further broken down through different reaction pathways. They produces energy-rich molecules such as NADH, FADH 2 , and ATP or GTP as common end products. NADH and FADH 2 can then be used in stage III to produce even more ATP. Fatty acids undergo fatty acid catabolism and is converted to Acetyl-CoA. Monosaccharides such as glucose undergoes glycolysis to pyruvate, which then may be converted to Acetyl-CoA. There a 20 amino acids obtained from the hydrolysis of proteins. Some of them are converted to pyruvate, others to Acetyl-CoA, which the rest are converted to one of the intermediates in the citric acid cycle or the glycolysis pathway.
Acetyl-CoA is a two-carbon molecule that enters in the citric acid cycle in stage III. This cycle is understood in 8 steps and it happens in the mitochondria of the cell. Energy rich molecules such as NADH, FADH 2 , and GTP are produced during this cycle. The reduced coenzymes such as NADH and FADH 2 enter the electron transport chain (ETC) in stage III and are converted to ATP.
Summary
The conversion of food into cellular energy (as ATP) occurs in three stages. Catabolism of monosaccharides, fatty acids, and amino acids released in stage I of catabolism occurs in stages II and III of catabolism. Through Acetyl-CoA these steps converge into the citric acid cycle.