Introduction

Many processes are going on within each living cells all the time. Some of these processes require energy while others generate it. There should exist some carriers or molecules for the storage, transfer and easy delivery of this energy to the site where it is needed. 

ATP is such an energy carrier and storage molecule. It is a nucleotide that acts as energy currency within the cells. All the energy transfers within a cell take place in the form of ATP. In this article, we will study in detail the structure, synthesis, and functions of ATP. So, keep reading. 

Structure

Adenosine triphosphate (ATP) is a nucleotide with three phosphate groups attached. Like other nucleotides, it is made up of the following three parts;

• Pentose Sugar
• Nitrogenous base
• Phosphate Groups

Pentose Sugar

The pentose sugar in the case of ATP is ribose. It is present in the form of a five-cornered ring or pentagon called Ribofuranose. The first carbon is attached to oxygen and is a part of the ring. The fifth carbon is outside the ring and is directed above the plane of the pentagon. The hydroxyl groups attached to the carbon atoms are directed below the plane of the ring. 

Nitrogenous Base

The nitrogenous base in the case of ATP is adenine. It is a purine base having two rings. The larger ring is hexagonal while the smaller ring is pentagonal. It is joined to the first carbon of the ribose sugar via an N-glycosidic bond. The amino group at the third position of adenine reacts with the hydroxyl group at the first carbon of ribose to form this N-glycosidic linkage. 

The resultant compound is a nucleoside called adenosine. 

Phosphate groups

ATP is a triphosphate compound having three phosphates attached to the fifth carbon of ribose. The first phosphate group is linked via an ester bond formed by the reaction of the hydroxyl group and the acidic phosphate group. The other two phosphate groups are attached via phosphoanhydride bonds. 

Synthesis

ATP can be synthesized from various processes. The electron transport chain in both animals and plants is the most important source of ATP within the cells. In this section, we will discuss various ways utilized by living cells to produce ATP.

AMP and ADP Synthesis

Adenosine monophosphate (AMP) is the precursor of both ADP and ATP. It is synthesized in the cells during the process of nucleoside synthesis. On a molecule of ribose-5-phosphate, purine ring is made using carbon dioxide and various amino acids. The immediate product is inosine monophosphate (IMP). 

This IMP molecule is converted to AMP in two stipe;

• In the first step, IMP is condensed with aspartate to form adenylosuccinate using energy provided by GTP. This reaction is catalyzed by adenylosuccinate synthetase enzyme. 
• In the second step, adenylosuccinate loses fumarate to form adenosine monophosphate. This reaction is catalyzed by adenylosuccinase enzyme.  

Once the AMP has been made, it is phosphorylated to form ADP by adenylate kinase enzyme. ADP acts as the immediate precursor of ATP. 

Cells have a large pool of ADP that can be converted to ATP by ATP synthase enzyme. This phosphorylation process requires energy that can be provided by various processes. The two most important processes are;

• Oxidative phosphorylation
• Z-reaction  

Oxidative Phosphorylation

It is the phosphorylation of ADP to ATP coupled with the electron transport chain. The oxidative phosphorylation is the major pathway by which energy is obtained in the form of ATP using various fuels.  

Electron transport chain (ETC) is a series of enzyme complexes that organized in an order of decreasing reducing power. It is present on the inner mitochondrial membrane and is a major source of ATP synthesis in animal cells. Plants cells use the mitochondrial source to generate energy from glucose and other fuels at night. 

The electrons derived from various energy fuels in the cell-like glucose, fatty acids, etc. are carried to the electron transport chain by carriers like NADH and FADH₂. As the electrons pass down the electron transport chain, they liberate a large amount of energy. This energy is used to pump the protons (H⁺) from to mitochondrial matrix to the intermembranous space (a space between inner and outer mitochondrial membranes) 

The protons in the mitochondrial intermembranous space are already in higher concentration. They tend to move down the concentration gradient into the mitochondrial matrix. They do so by passing through a proton channel. 

The proton channel is a part of the ATP synthase enzyme. As a proton passes through this channel, it liberates energy. This energy is used to phosphorylate ADP to form ATP. 

Z-Reaction

It is the other name of light reactions of photosynthesis. In this reaction, the energy for phosphorylation is provided by sunlight. It takes place on the thylakoid membranes of chloroplasts. 

It is a series of chemical reactions in which energy from light photons is used to move electrons to a higher reducing state. The electrons are obtained from water and their reducing ability is enhanced using light energy. These electrons are later channeled from the higher reducing state into the electron transport chain (ETC). 

As these electrons pass down the electron transport chain on the thylakoid membranes, they release energy that is used to pump protons from the stroma of chloroplasts into the thylakoid lumen, against their concentration gradient. 

These protons tend to move down the concentration gradient back into the stroma by passing through the proton channel of ATP synthase, also present in the thylakoid membrane of chloroplasts. As the protons move, they liberate the energy that is used to phosphorylate ADP to ATP. 

Other reactions

The oxidative phosphorylation and the Z-reaction are the two major energy sources for phosphorylation of ADP to ATP. 

In addition to these reactions, some exergonic reactions liberate energy enough for phosphorylation of ATP to ADP. Some examples of such reactions are given below.

Glycolysis

It is the process by which oxygen is oxidized to pyruvic acid.  The second phase of the glycolytic reaction yield enough energy to convert two ADP molecules to ATP.

Krebs Cycle

In each Krebs cycle, one molecule of GDP gets converted to GTP which in turn provides energy for phosphorylation of ADP to ATP.

It should be kept in mind that in addition to providing energy for direct phosphorylation, these reactions also generate electron carriers like NADH and FADH₂. These electron carriers cause ATP synthesis via electron transport chain as mentioned above. 

Hydrolysis of ATP

As mentioned earlier in this article. ATP acts as an energy carrier. The synthesis of ATP gives an idea of how energy present in different fuels used by the body is released and stored in the form of high energy phosphate bonds of ATP. This energy is made available for cellular processes by ATP hydrolysis.

The terminal two phosphate groups of ATP are linked to the rest of the molecule by high energy phosphate bonds. Each of these bonds releases 7.3 Kcal/mol of energy upon hydrolysis. 

The hydrolysis of ATP is a two-step process. In the first step, it is converted to ADP by breaking one terminal bond and 7.3 Kcal/mol energy is released. 

The ADP molecule thus formed can either be recycled to form of ATP or can be broken down to AMP by hydrolysis of the phosphate bond releasing 7.6 Kcal of energy along with an inorganic phosphate. 

Functions of ATP

The list of functions performed by ATP is never-ending. It is required for almost every process taking place in living cells. Some of the commonly known functions are discussed below.

1. Gluconeogenesis

It is the process of making glucose from non-carbohydrate sources. Gluconeogenesis provides glucose to the body when glycogen stores are depleted during starving. Four molecules of ATP are used for the synthesis of each glucose molecule. Severe hypoglycemia and other associated clinical conditions can result if gluconeogenesis fails to occur properly. 

2. Active transport

Active transport is a process in which substances move from an area of lower concentration to higher concentration. Energy is required for this transport against the concentration gradient that is provided by ATP. Active transport takes place via special carrier protein channels that require ATP for their functioning. Such proteins have an inherent ATPase activity to cleave ATP to ADP and use the released energy in transport. 

Examples of such transporter proteins are sodium-potassium pump, calcium pump, etc. 

3. Cellular signaling

ATP is necessary for the activation of molecules involved in signaling processes within the cell. Various kinases that are involved in signal transduction cascades are activated only upon phosphorylation by ATP. An example of such kinases is mitogen-activated protein kinase or MAP kinase. It is involved in a signaling cascade that regulates cell division. 

ATP also provides second messenger in some signaling pathways. Adenylate cyclase is an enzyme that acts on ATP upon activation by G-protein coupled receptors to form cyclic AMO (cAMP) that carries out various intracellular responses. 

4. Nucleic acid synthesis

DNA synthesis involves two steps;

• Synthesis of nucleotides
• Formation of polynucleotides 

DNA synthesis involves the additional step of polynucleotides assembly to form a double helix. All these processes are dependent on ATP. It is used not in the rate-limiting step of nucleotides synthesis but also the successive steps.

 Also, it acts as a monomer for nucleic acid synthesis like DNA and RNA.

5. Protein synthesis

Protein synthesis is an energy-dependent process that takes place on ribosomes. Energy for protein synthesis is also provided by ATP. The transfer RNA carries amino acids to ribosomes for their assembly. The activation of amino acid and its attachment to tRNA requires ATP. Aminoacyl tRNA synthetase complex uses ATP and tRNA to form AMP-tRNA complex, and pyrophosphate is released. Amino acid then binds to this AMP-tRNA complex and is carried to the ribosomes.

6. Movement and Muscle contraction

Movement of body parts or movement of an individual from one place to another takes place as a result of muscle contraction that is also dependent on ATP. Muscle contraction occurs as a result of the interaction between actin and myosin proteins in muscle fibers. A molecule of ATP is used each time to break the actin-myosin cross-bridges and continue the cycle again. In the absence of ATP, actin and myosin bridges will not be broken and muscles will remain in a state of permanent contracture, as happen after death. The body stiffness after death due to lack of ATP is called rigor mortis.

7. Phagocytosis

Phagocytosis is a process by which pathogenic foreign particles are engulfed by macrophages. It is extremely important in defense of the body against harmful pathogens. ATP is also required for this purpose. Phagocytes engulf foreign particles by extending membrane processes around it, forming a phagocytic cup. ATP is required for actin polymerization to extend these membranous processes outwards.

8. Neurotransmitter

ATP also acts as a neurotransmitter for several cells. Smooth muscles are an example of such cells that are activated by ATP released from neurons. Upon activation by A^TP, these muscles do undergo contraction. 

Summary

ATP is an organic molecule having prime importance in living structures. 

It is a nucleotide that acts as energy-carrier, capturing energy in some reactions and providing it for others. 

It is made up of; 

• Ribose Sugar
• Adenine base
• Three phosphate groups

Its precursor is adenosine monophosphate (AMP) that is made from IMP in nucleotide synthesis. AMP can be converted to ADP by adenosine kinases. 

The phosphorylation of ADP to ATP requires energy that is provided by the electron transport chain during, 

1. Oxidative Phosphorylation
2. Z-reaction (only in plants)

Other reactions that can directly cause phosphorylation include;

• Glycolysis
• Krebs cycle

ATP has two high energy phosphate bonds that liberate energy upon hydrolysis. 

This energy is used in various processes. Some of these processes include;

• Gluconeogenesis
• Active transport of substances across the membranes
• Synthesis of DNA and other nucleic acids
• Protein synthesis
• Phagocytosis of foreign particles
• Muscle contraction and movements

Besides, ATP also acts as a precursor of second messenger cAMP, activator of signaling kinases like MAP kinase, as well as a neurotransmitter for some cells like some smooth muscles. 

Frequently Asked Questions

What is ATP?

ATP stands for adenosine triphosphate, an energy-rich phosphate-containing molecule found in living cells. It plays a crucial role in energy transformation in the cells.

How is ATP synthesized?

ATP is synthesized from ADP during different energy-yielding reactions in the body. ADP comes from AMP, synthesized from inosine monophosphate (IMP) during nucleotide metabolism.

How much energy is yielded upon hydrolysis of ATP?

One ATP molecule contains two high-energy phosphate bonds. Hydrolysis of one high-energy phosphate bond of ATP yields 7.3 Kcal/mol energy.

What are the functions of ATP?

ATP is the primary energy-rich molecule involved in biochemical processes of the body like glucose metabolism, amino acid metabolism, nucleotide metabolism etc. It also acts as a neurotransmitter.

References

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3. Berg, Jeremy M.; Tymoczko, John L.; Stryer, Lubert (2007). Biochemistry (6th ed.). New York, NY: W. H. Freeman. p. 413. ISBN 978-0-7167-8724-2.
4. Beis, I.; Newsholme, E. A. (October 1, 1975). “The contents of adenine nucleotides, phosphagens and some glycolytic intermediates in resting muscles from vertebrates and invertebrates”. Biochem. J. 152 (1): 23–32. doi:10.1042/bj1520023. PMC 1172435. PMID 1212224.
5. https://commons.wikimedia.org/wiki/File:Adenine.svg