In this tutorial, we have discussed ‘what is ATP in biology‘, ‘ATP structure‘, ‘synthesis of ATP (phosphorylation), ‘function of ATP‘. 

(01). WHAT IS ATP

The chief energy currency all cells use is a molecule called adenosine triphosphate (ATP). Cells use their supply of ATP to power almost every energy-requiring process they carry out, from making sugars, to supplying activation energy for chemical reactions, to actively transporting substances across membranes, to moving through their environment and growing.

► DEFINITION

ATP (adenosine triphosphate) is an energy-storage compound containing adenine, ribose (five-carbon sugar), and three phosphate groups. When it is formed from ADP, useful energy is stored; when it is broken down (to ADP or AMP), energy is released to drive endergonic reactions.

ATP was discovered by Karl Lohmann in 1929. Its functioning through build up and hydrolysis of high energy phosphate bond was discovered by Fritz Lipmann (1941). Lipmann is called the father of ATP cycle.

► ATP STRUCTURE

Each ATP molecule is a nucleotide composed of three smaller components.

(i) The first component is a five-carbon sugar, ribose, which serves as the backbone to which the other two subunits are attached.  

(ii) The second component is adenine, an organic molecule composed of two carbon-nitrogen rings. Each of the nitrogen atoms in the ring has an unshared pair of electrons and weakly attracts hydrogen ions. Adenine, therefore, acts chemically as a base and is usually referred to as a nitrogenous base (it is one of the four nitrogenous bases found in DNA and RNA).

(iii) The third component of ATP is a chain of three phosphate groups are attached to ribose. The last two phosphate groups are attached by bonds of high transfer potential or energy rich bonds. These bonds are indicated by the squiggle sign (~) proposed by Lipmann (1941). These bonds are unstable and allow the phosphate groups to be transferred to other molecules, making these molecules more reactive. 

The bond between second and third phosphate groups possesses an energy equivalent of 7.3 Kcal/mole (30.5 kJ/mol) while the bond linking the second phosphate group with the first one has an energy equivalent of 6.5 Kcal/mole.

The compound thus formed from adenine and ribose is called adenosine

.

The easily available form of energy present in high energy bonds of ATP (and other energy carriers like GTP, UTP, or CTP) is known as biologically useful energy. Hence ATP can function as the energy currency of the living cells.

► HYDROLYSIS OF ATP TO RELEASE ENERGY

Phosphate groups are highly negatively charged, so they repel one another strongly. Because of the electrostatic repulsion between the charged phosphate groups, the two covalent bonds joining the phosphates are unstable. The ATP molecule is often referred to as a “coiled spring,” the phosphates straining away from one another.

The unstable bonds holding the phosphates together in the ATP molecule have a low activation energy and are easily broken by hydrolysis. This reaction is exergonic, releasing a considerable amount of free energy (negative ΔG) which is equal to -7.3 kcal/mole (–30.5 kJ/mol) under standard laboratory conditions. However, under cellular conditions, the value can be as much as –14 kcal/mol. Both values are correct, but in different conditions. A kilocalorie (kcal) of energy raises 1 kilogram (kg) of water 1°C. 

In most reactions involving ATP, only the outermost high-energy phosphate bond is hydrolyzed, cleaving off the phosphate group at the end. When this happens, ATP becomes adenosine diphosphate (ADP) and the phosphate group is released as inorganic phosphate (P_i). The energy it releases can be used to perform work. For this reason, we often refer to these as “high-energy” bonds, referring to this release of energy, not to the actual bond energy,

ATP + H₂O → ADP + P_i + free energy [∆G = – 30.5 kJ/mol (–7.3 kcal/mol)]

Two characteristics of ATP account for the free energy released by the loss of one or two of its phosphate groups:

➢ The free energy of the P~O bond between phosphate groups (called a phosphoric acid anhydride bond) is much higher than the energy of the O—H bond that forms after hydrolysis. So some usable energy is released by hydrolysis. 

➢ Because phosphate groups are negatively charged and so repel each other, it takes energy to get phosphates near enough to each other to make the covalent bond that links them together (e.g., to add a phosphate to ADP to make ATP). Some of this energy is conserved when the third phosphate is attached.

A molecule of ATP is hydrolyzed to form adenosine diphosphate (ADP) and inorganic phosphate (HPO₄^2–; commonly abbreviated to P_i). ADP can be hydrolyzed further to adenosine monophosphate (AMP). AMP can be hydrolyzed to form adenosine. When one phosphate group of ATP is hydrolyzed, for each the following energy yields are obtained (as
observed by Bettelheim and March, 1984).

Cells use the energy released by ATP hydrolysis to fuel endergonic reactions (such as the biosynthesis of complex molecules), for active transport, and for movement. Another interesting example of the use of ATP involves converting its chemical energy into light energy.

► ATP/ADP CYCLE LINKS EXERGONIC AND ENDERGONIC REACTIONS

When a reaction occurs in more than one step and at least one of the steps is exergonic (energy-yielding), the energy released by that step of the reaction can drive all other steps forward to complete the reaction. Such reactions are called coupled reactions.

For example, The addition of phosphate group derived from the hydrolysis of ATP to glucose forms the molecule glucose 6-phosphate (in a reaction catalyzed by hexokinase). ATP hydrolysis is exergonic and the energy released drives the second reaction, which is endergonic.

Coupling reactions occur continuously in living cells, consuming a tremendous amount of ATP. How do cells generate that ATP? Cells make and use ATP cyclically: Cells use exergonic reactions to provide energy to synthesize ATP from ADP + P_i; they then use the hydrolysis of ATP to provide energy to drive endergonic processes. The energy for ATP synthesis comes from exergonic reactions that involve the breakdown of complex molecules that contain an abundance of free energy: carbohydrates, proteins, and fats in food. 

Thus, ATP occupies an intermediate position in the metabolism of the cell and is an important link between exergonic reactions, which are generally components of catabolic pathways, and endergonic reactions, which are generally part of anabolic pathways.

The continual hydrolysis and resynthesis of ATP is called the ATP/ADP cycle. Approximately 10 million ATP molecules are hydrolyzed and resynthesized each second in a typical cell, illustrating that this cycle operates at an astonishing rate. In fact, if ATP were not regenerated from ADP and Pi, the average human would use an estimated 75 kg of ATP per day.

— Definition: Phosphorylation is the chemical reaction resulting in the addition of a phosphate group to an organic molecule. Phosphorylation of ADP (adenosine diphosphate) yields ATP.  Many proteins are also activated or inactivated by phosphorylation. Phosphorylation is of three types—

● Substrate level phosphorylation.

● Oxidative phosphorylation and

● Photophosphorylation

(01) SUBSTRATE LEVEL PHOSPHORYLATION

— Definition: The enzyme-catalyzed transfer of phosphate groups from donor phosphorylated molecules to ADP to form ATP is called substrate-level phosphorylation. (Phosphorylation is the addition of a phosphate group to a molecule.) Substrate-level phosphorylation is distinct from oxidative phosphorylation, which is carried out by the respiratory chain and ATP synthase.

— In substrate­ level phosphorylation, ATP is formed by transferring a phosphate group directly to ADP from a phosphate-bearing intermediate, or substrate. It is directly linked to the liberation energy in chemical reactions of respiration. For example, 

(i) During glycolysis, one of the two phosphates of 1,3-biphosphoglycerate is linked by high energy bond. This phosphate reacts with ADP to form ATP and form 3-phosphoglycerate with the help of the enzyme is phosphoglycerate kinase

. This transfer of phosphate from a phosphorylated intermediate to ATP is referred to as substrate level phosphorylation.

(ii) Phosphoenolpyruvate (PEP), possesses a high-energy phosphate (P) bond similar to the bonds in ATP. At the last step of glycolysis, with the help of enzyme pyruvate kinase, PEP’s phosphate group is transferred enzymatically to ADP, the energy in the bond is conserved, and ATP is created. This is a substrate level phosphorylation reaction. 

(iii) In Krebs cycle, Succinyl coenzyme A is converted to succinate, and substrate level phosphorylation takes place. Bond attaching coenzyme A to succinate (~S) is unstable. Breakdown of succinyl coenzyme A is coupled to phosphorylation of GDP to form GTP (compound similar to ATP). 

GTP transfers its phosphate to ADP, yielding ATP. 

 

(02) OXIDATIVE PHOSPHORYLATION

— Definition: oxidative phosphorylation is ATP formation in the mitochondrion, associated with the flow of electrons through the respiratory chain.

Oxidative phosphorylation is linked to terminal oxidation of reduced coenzymes (NADH and FADH₂) in respiration. Two components of the process can be distinguished:

(i). Electron transport:  The coenzymes release H⁺ ions and electrons (e^–). The electrons pass through the respiratory chain, a series of membrane associated electron carriers. The flow of electrons along this pathway results in the active transport of protons out of the mitochondrial matrix and across the inner mitochondrial membrane, creating a proton concentration gradient. Together, the proton concentration gradient and the electrical charge difference constitute a source of potential energy called the proton-motive force (PMF). This force tends to drive the protons back into the mitochondrial matrix across the inner mitochondrial membrane from mitochondrial intermembrane space.

(ii). Chemiosmosis: The protons diffuse back into the mitochondrial matrix through a specific proton channel, ATP synthase, which couples this diffusion to the synthesis of ATP from ADP and Pi. The inner mitochondrial membrane is otherwise impermeable to protons, so the only way for them to follow their concentration gradient is through the channel. This coupling of proton-motive force and ATP synthesis is called chemiosmosis.

(03) PHOTOPHOSPHORYLATION

— Definition: photophosphorylation is the production of ATP by chemiosmosis during the light reactions of photosynthesis.

Photophosphorylation occurs on the thylakoids of chloroplasts. In the primary photochemical reaction, an electron is extruded by chlorophyll a on the receipt of radiation energy. The electron passes over a transport chain of carriers. Sufficient energy is released when the electron passes between cytochrome b

and cytochrome f (cyclic photophosphorylation) or plastoquinone to cytochrome f (noncyclic photophosphorylation). The electron transport chain pumps protons (H⁺) into the thylakoid space (lumen). It creates a proton concentration gradient. Together, the proton concentration gradient and the electrical charge difference constitute a source of potential energy called the proton-motive force (PMF). This force tends to drive the protons back into stroma from the thylakoid lumen through a specific proton channel called ATP synthase, an enzyme complex in the thylakoid membrane, powering the synthesis of ATP.

(i) It can store small packets of energy as soon as the energy becomes available so that wastage of energy is minimized.

(ii) ATP makes energy available at a spot away from the area of release of energy.

(iii) By its accumulation at a spot, it makes available a large and continuous supply of energy for carrying out heavy work.

(iv) ATP releases a small amount of energy required for building new chemical bonds during anabolism.

(v) It helps in driving energetically unfavourable processes like absorption of inorganic solutes.

(vi) ATP acts as a phosphorylating agent for activating certain metabolites like sugars.

(vii) It maintains bio-electric potential of cellular membranes.

(viii) ATP energizes the membrane carriers for influx and efflux of biochemicals.

(ix) It energizes the enzyme luciferase in bioluminescent organisms for the liberation of light.

(i) There is a step-wise release of chemical bond energy which is very easily trapped in forming ATP molecules.

(ii) Cellular temperature is not allowed to rise.

(iii) Wastage of energy is reduced.

(iv) There are several intermediates that can be used in the production of a number of biochemicals.

(v) Through their metabolic intermediates different substances can undergo respiratory catabolism.

(vi) Each step of respiration is controlled by its own enzyme. The activity of different enzymes can be enhanced or inhibited by specific compounds. This helps in controlling the rate of respiration and the amount of energy liberated by it.