ATP, or adenosine triphosphate, is a molecule that plays a vital role in the energy metabolism of cells. It is the immediate source of energy for many biological processes, such as muscle contraction, nerve transmission, and macromolecule synthesis. It is constantly cycling between hydrolysis and synthesis and is reused over and over again in metabolism. Therefor ATP is also called energy currency or coin.
Structure of ATP
ATP is a nucleotide. It is similar to a building block of nucleic acids like DNA and RNA. ATP consists of three main components: a nitrogenous base, a sugar, and a phosphate group. The nitrogenous base in ATP is adenine, which is one of the four bases found in DNA and RNA. The sugar in ATP is ribose, a five-carbon sugar that forms part of the backbone of RNA. The phosphate group in ATP is actually a chain of three phosphate groups. The three phosphate groups in ATP have negative charges that repel each other. When they are close to each other, they are like a compressed spring, creating a lot of potential energy. The hydrolysis of these bonds releases more than twice the energy of other chemical bonds. Every mole ATP can release 30.54 kj energy. Not all of this energy is consumed by organisms. Some of the energy will be dissipated as heat.
Hydrolysis and synthesis of ATP in Biochemistry, ATP cycle
ATP contains a lot of energy and is therefore an unstable compound. They are easily broken down by enzyme catalysis. The hydrolysis of ATP is the process by which ATP is broken down into ADP and an inorganic phosphate group (Pi). This reaction releases energy that can be used for various cellular activities. The hydrolysis of ATP can be represented by the following chemical equation:
ATP + H₂O → ADP + Pi + Energy
The hydrolysis of ATP is reversible, meaning that ADP and Pi can be recombined to form ATP again with the help of ATPase. However, this reaction requires energy input from another source, such as cellular respiration or photosynthesis. Not all the enery of respiration is used to synthesize ATP and 60% is dissipated as thermal enery. The same is photosynthesis, some enery is stored in polysaccharide. The synthesis of ATP from ADP and Pi can be represented by the following chemical equation:
ADP + Pi + Energy → ATP + H₂O
ATP and ADP are found in very small amounts in living things. They are recycled to ensure that biochemical reactions take place. For example, when a muscle cell contracts, it uses ATP to power the movement. This converts ATP into ADP and releases energy. Then, the ADP is recharged with energy from respiration to form ATP again. This cycle repeats constantly in all living cells. If ATP could not be regenerated by the phosphorylation of ADP, humans would use up nearly their body weight in ATP each day.
How ATP provides energy that performs Chemical Reactions
Covalent Bond Formation: When ATP is hydrolyzed, the phosphate groups is released to form covalent bonds with reactant. The negatively charged group alters the charge distribution of the reactant to change their conformation. This process is called phosphorylation. The energy in ATP is transferred to unstable intermediate which can easily be converted into product and phosphate group with the help of enzymes. For instance, transport proteins are phosphorylated in active transport. Glutamine is formed from Glutamic acid and ammonia.
Non-Covalent Binding: ATP can also directly bind non-covalently to reactants without involving group transfer. During muscle contraction, ATP non-covalently binds to myosin to maintain its conformation. When myosin catalyzes ATP hydrolysis, ADP and Pi detach from myosin to change its shape. Myosin return to original conformation until another ATP molecule binds.
In practice, ATP rarely provides energy through direct hydrolysis. Instead, it typically involves phosphate group transfers and changes in reactant conformation to release energy.
ATP acts as a signal molecule.
In biological negative feedback, ATP acts as a “stop” signal for cellular respiration. High levels of ATP inhibit PFK-1 to slow down the glycolysis, and then the aerobic respiration. It indicates that the cell has enough energy and does not need to produce more through cellular respiration. A low level of ATP lifts the restriction on glycolysis, and thus speeds up aerobic respiration to make more energy and ATP.