You step into a bustling marketplace and discover that you happen to have no cash, so you tell the hawker that you have a villa near Central Park in Manhattan and some Microsoft stocks. Hawkers only give you a shrug and the deal is failed. It is similar in biology. Although abundant energy is stored in organic matter such as fats and sugars that is like valuable real estate or stocks, it cannot be directly used by cells. What cells need is ATP, a high energy compound that circulates as quickly as cash to facilitate all transactions in the “biological marketplace.” For example, adding glucose to a test tube containing luciferin and enzymes does not produce light, but a faint glow is emitted when ATP is added. Therefore, ATP or adenosine triphosphate is also known as the energy currency of cells 💰.
Structure: How does ATP store energy?
ATP is a nucleotide derivative composed of three smaller components. The first component is a five-carbon ribose that acts as scaffold for adsorption of other two subunits. The second component is adenine, an organic molecule made of two carbon-nitrogen rings that is common in DNA and RNA. The third component is the most important part of ATP, and it is a chain of three negatively charged phosphates. Considerable chemical energy is required to overcome the strong electrostatic repulsion and join them together. They are like a compressed spring that ready to stretch and release potential energy at any moment. Thus, ATP is an unstable, high-energy compound. Its low activation energy makes it easy to hydrolyze into ADP and phosphate (Pi). The decomposition of ATP releases more energy in living organisms than under standard conditions. Living organisms harvest 35 kJ of energy per mole of ATP hydrolyzed.
ATP cycles continuously in cells: hydrolysis and synthesis
Exergonic reactions require an input of activation energy to overcome barriers. Endergonic reactions also do not occur spontaneously because products have more free energy than reactants (∆G > 0). However, almost all biochemical reactions in cells consume less energy than the energy released by ATP hydrolysis. When they are coupled together, the reduced total free energy always drives them to proceed spontaneously (∆G < 0). ATP hydrolysis provides energy for all cellular activities, and a portion of energy dissipated as heat.
Most biochemical reactions only involve hydrolysis of the outermost high-energy phosphate because it is more easily broken due to the repulsion by the other two phosphates. In some cases, the second phosphate group is removed to release additional energy, leaving adenosine monophosphate (AMP). AMP has no other phosphate groups to provide electrostatic repulsion, so it does not have high-energy bonds.
ATP + H₂O ↔ ADP + Pi + Energy
ADP + H₂O ↔ AMP + Pi + Energy
Hydrolysis is reversible. Most of the ATP is produced by ATP synthase, which is embedded in the membranes of chloroplasts or mitochondria and works like a mini hydro generator. When protons flow through this micro-molecular machine, phosphate is squeezed into ADP to synthesize ATP. In plant cells and some prokaryotes, photosynthesis stores solar energy in ATP. They will be transferred to potential energy in the carbon-hydrogen bonds of polysaccharide. All organisms also break down organic matter in aerobic or anaerobic respiration to store chemical energy in ATP. Most of the energy is wasted as thermal motion because their efficiency is not high. The efficiency of respiration is about 40%, while the efficiency of photosynthesis is less than 4%.
Humans use up a large quantity of ATP that is almost equivalent to their body weight each day. Obviously, this far exceeds the storage capacity of cell. Instead, all ATP is consumed by cells within seconds. Thus, ATP is regenerated from ADP and Pi to form a balanced cycle by which all the cell activities are supported.
How does ATP provide energy to drive chemical reactions?
Covalent binding: When ATP is hydrolyzed, a phosphate group is released and combine substrate covalently. Negatively charged phosphate alters the charge distribution of substrate to result in a conformational change. This process is called phosphorylation. Energy in ATP is transferred to an unstable intermediate. Then, the intermediate rapidly decomposes into products and phosphate under the catalysis of enzymes. For example, various pumps in active transport, such as Na⁺/K⁺-ATPase, Ca²⁺-ATPase, and H⁺-ATPase.
Non-covalent binding: Its principle is similar to phosphorylation, but substrate accepts the intact ATP molecule to cause its distortion. Then ATP is broken down into ADP and Pi. This mode occurs in myosin responsible for muscle contraction and ABC proteins responsible for the active transport of small molecules across bio-membranes.