www.BioTopics.co.uk
Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

Adenosine triphosphate

Leaving the mouse cursor over some words in green should reveal more detail.

Adenosine triphosphate - ATP - is a molecule derived from adenosine phosphate, one of the four subunits of RNA (nucleotides).
ATPnastyle (2K)

It consists of three parts: Unlike adenosine monophosphate in RNA, ATP does not polymerise with other nucleotides; it does not condense into a polymer.
And it has 3 phosphate groups rather than the single phosphates in RNA and (DNA) nucleotides.

ATP/ADP

mouseover image to swap between triphosphate and diphosphate
atp (27K)
Elsewhere on this website a variety of biochemical molecules can be displayed in an interactive 3-D format

Bonds and energy

It takes energy to make any bonds between atoms in molecules, and this energy can be regained when bonds are broken. But the bond 'holding on' the third phosphate is of great significance because it can quite easily be broken, and re-formed. In fact the same applies to the bond holding the second phosphate.
These bonds, known as phosphoric anhydride bonds, are sometimes said to be high energy bonds, perhaps drawn with a squiggly line ~ . The energy (Δ G) released from each of these bonds is -30.5 kJ/mol

ATP - the energy currency of the cell

A cell can obtain energy from a variety of chemicals which can be taken in from external sources, or obtained by interconversion from others inside the cell.
Carbohydrates, fats and proteins can all be broken down to provide energy for the cell's functions. The main processes of respiration: glycolysis and the citric acid cycle, can take in organic compounds from a variety of sources: sugars, fatty acid and amino acid breakdown products.
So a wide variety of substances can act as fuels and become respiratory substrates, but the unique product is ATP.
Similarly, ATP is universally used to power any energy-requiring processes taking place in cells.

Hydrolysis of ATP

Do not refer to Pi as phosphorus or, even worse, as phosphorous
ATP can be broken down (hydrolysed) to give adenosine diphosphate (ADP) and an inorganic phosphate group (Pi):

ATP hydrolase
ATP (+ H2O)           →           ADP + Pi

This conversion is catalysed by the enzyme ATP hydrolase, sometimes called ATPase.

Like other hydrolysis reactions, it involves water, but this is frequently ignored.
In fact water provides OH- and H+ and the covalently bonded phosphate is released as an inorganic ion HPO42- + H+.

Uses of ATP

The hydrolysis of ATP can be coupled to energy-requiring reactions within cells.
Examples:
Movement - muscular contraction involving actin and myosin requires energy from ATP, and so do other biological structures such as flagella.

Active transport - movement of any substance against a concentration gradient - requires the input of energy, and ATP powers the Na+ /K+ pump used in neurones, and also the uptake of nitrate ions by plant roots.

A number of anabolic reactions such as synthesis of fats, polysaccharides, proteins and other categories of organic compounds require energy which is provided by ATP

Bioluminescence is another example of an energy-requiring process which is powered by ATP. This is very common in a variety of marine organisms and certain terrestrial arthropods.
In fact extracts from firefly tails containing luciferin and luciferase may be used in laboratory for the assay of ATP.

What's made in the cell stays in the cell

In a multicellular organism, there are a number of organ systems providing raw materials for cells. The digestive system breaks down a variety of food substances and deposits the soluble digestion products into the blood. Similarly, oxygen is taken into the lungs and absorbed into the bloodstream where it is carried by red blood cells using haemoglobin.

These substances are delivered by the circulatory system to all the cells of the body collectively.

However once a cell has taken in what fuel and oxygen it needs, it uses it to produce ATP.
This ATP does not leave the cell.

Although cells work together with other cells in a tissue or organ to perform its function in a co-ordinated way, in energetic terms each cell acts independently of other cells, each internally powering its own activity.

Phosphorylation

ATP can interact with other compounds. When ATP becomes hydrolysed it can contribute a phosphate group, altering the properties of that molecule.

It is said that phosphorylated compounds are more reactive.

This is used to explain the first steps in glycolysis - a cellular process leading either to aerobic or anaerobic respiration.
Initially: glucose + ATP → glucose phosphate + ADP.
A few steps later: fructose phosphate + ATP → fructose 1, 6 bisphosphate + ADP.

Proteins can also be phosphorylated, and the phosphate group is often attached to -OH groups on amino acid sidechains. This may affect their interaction with other amino-acid residues on the polypeptide chain and thus possibly change the molecular shape. In some cases phosphorylation of enzymes can cause them to become activated or deactivated, or undergo other modifications to their function.

Glycolysis and enzymes

In glycolysis, glucose is broken down in the cytoplasm in a series of steps to form pyruvate, which can either be aerobically respired in the citric acid cycle or anaerobically respired. By the addition of a phosphate group from ATP, glucose is converted into glucose 6-phosphate using the enzyme glucokinase/hexokinase, and fructose 6-phosphate is converted into fructose 1,6-bisphosphate using the enzyme phosphofructokinase.
The term kinase implies movement, i.e. activation of these sugars by conversion into sugar phosphates.

Phosphorylase enzymes also cause phosphate to combine with other compounds, but in these cases the phosphate comes not from ATP but inorganic phosphate, and it is usually associated with other energy-providing conversions. Such a process also occurs in glycolysis, where triose phosphate dehydrogenase converts phosphoglyceraldehye (PGA) into glycerate 1,3-bisphosphate (G1,3BP).

Incidentally, ATP is 'used up' at the start of glycolysis but more is 'given back' later because Pi is taken in and the final products release 2 more molecules of ATP than were used in the first place.

Phosphate groups are removed by phosphatase enzymes.

Resynthesis of ATP

ATP needs to be be replaced after use. This is achieved by rejoining ADP with Pi, with the appropriate input of energy.
       ATP synthase
ADP + Pi           →        ATP

Aerobic respiration results in the formation of ATP though oxidative phosphorylation, so called because it involves the passing of electrons and hydrogen ions (protons) to oxygen (forming water).
The light-dependent reactions of photosynthesis also produce ATP (photophosphorylation), which is used in the light-independent reactions of photosynthesis.
Both of these ATP resynthesis reactions are catalysed by enzymes called ATP synthase embedded in the inner walls of mitochondria and chloroplasts respectively. At the molecular level, these enzymes operate like a motor, driven round by an accumulation of hydrogen ions in the inter-membrane spaces.

Substrate level phosphorylation

ATP may be formed in a different biochemical context where ADP interacts directly with a (reactive) molecule which has a phosphate group. The ATP which is formed effectively picks up both a phosphate group and energy. This does not involve the ATP synthase molecule.
Substrate level phosphorylation occurs at 2 steps in glycolysis:
glycerate 1,3-bisphosphate (G1,3BP)+ ADP → glycerate 3-phosphate (G3P)+ ATP
phospho-enolpyruvate (PEP) + ADP → pyruvate + ATP
It also occurs at one stage in the citric acid cycle
and in muscles:
phosphocreatine + ADP → creatine + ATP

Small Molecule - Large Scale Recycling

Formula of ATP: C10H16N5O13P3
The energy requirement of an adult human is 8400 kJ per day (guideline daily amount).
1 mole of ATP (507g) can provide 30.5 kJ.
Therefore 275 moles of ATP are required - this is 139425 g : 139.4 kg.
World average human body weight is 62 kg.
So the human body uses more than twice its weight in ATP in a 24 hour period.
It is said that the human body contains about 50 g of ATP - about 1/10 mole.

Each ATP molecule must therefore be recycled on average 2788 times per day - twice a minute - to keep this running.

I have seen other (slightly lower) estimates than this, and I would accept criticism of my calculation if you can provide more realistic or accurate information.

Other triphosphates

There are other nucleotide triphosphates, some containing ribose, and some containing deoxyribose.

Ribonucleotides can act as substrates for the synthesis of RNA (and DNA).

Deoxyribonucleotide triphosphates are very similar compounds containing deoxyribose rather than ribose, so they have a prefix d.
These are used as substrates of DNA polymerase in the synthesis (or replication) of DNA, where they give up PPi (pyrophosphate- a double phosphate) and provide energy, ending up being incorporated in DNA up as monophosphates.
Examples:

Deoxyadenosine triphosphate (dATP) is the deoxyribonucleotide version of (ordinary) ATP - the subject of this topic.

GTP (guanosine triphosphate) This molecule is sometimes formed as a result of substrate level phosphorylation which then produces ATP from ADP. It is also used as an energy source in several important processes e.g. protein synthesis and in stabilising tubulin proteins in microtubule formation within cells. dGTP is the deoxyribonucleotide equivalent.

CTP (cytidine triphosphate) - dCTP is the deoxyribonucleotide equivalent.

dTTP (thymidine triphosphate) does not have a ribonucleotide equivalent - see UTP below.

UTP (uridine triphosphate) - only a ribonucleotide. Its main role is as substrate for the synthesis of RNA during transcription but it also functions like ATP as a source of energy or an activator of substrates in certain metabolic reactions, e.g. glycogen synthesis.

Other related topics on this site

(also accessible from the drop-down menu above)

Glycolysis pathway
Krebs, citric acid or tricarboxylic acid cycle
The mitochondrial electron transport chain
Oxidative phosphorylation
Photosynthesis: light-dependent reactions
Light-independent reactions

Interactive 3-D molecular graphic models on this site

(also accessible from the drop-down menu above)

The ATP molecule - rotatable in 3 dimensions

Web references

High-energy phosphate From Wikipedia, the free encyclopedia

ATP/ADP from the LibreTexts libraries

Bioluminescence From Wikipedia, the free encyclopedia

Luminescent Determination of ATP Concentrations Using the Clarity Luminescence Microplate Reader

Protein phosphorylation From Wikipedia, the free encyclopedia

What are 'reference intakes' on food labels? from NHS choices

Human body weight From Wikipedia, the free encyclopedia



www.BioTopics.co.uk    Home     Contents     Contact via form     Contact via email     Howlers     Books     WWWlinks     Terms of use     Privacy