Structural Biochemistry/ATP

Adenosine triphosphate (ATP) is a nucleotide that consists of an adenine and a ribose linked to three sequential phosphoryl (PO32-) groups via a phosphoester bond and two phosphoanhydride bonds. ATP is the most abundant nucleotide in the cell and the primary cellular energy currency in all life forms. The primary biological importance of ATP rests in the large amount of free energy released during its hydrolysis. This provides energy for other cellular work, such as biosynthetic reactions, active transport, and cell movement. ATP is used in cellular metabolism in plants. It involved with light to create energy for plant. Besides, ATP is also one of components of DNA.

Definition
ATP also known as adenosine 5'-triphosphate. It is formed from adenosine diphosphate (ADP) and orthosphosphate (Pi). When fuel molecules are oxidized in chemotrophs or when light is trapped by phototrophs. This nucleotide is tremendously important since it is the most commonly used energy currency. The energy is released from the cleave of the triphosphate group is used to power many cellular processes.

Physical and Chemical Properties of ATP
ATP is composed of an adenine ring, ribose sugar, and three phosphate groups (triphosphate). The groups of the phosphate group are usually called the alpha (α), beta (β), and gamma (γ) phosphates. It is typically related to a monomer of RNA called adenosine nucleotide. Gamma phosphate group is the primary phosphate group on the ATP molecules that is hydrolyzed when the energy is needed to drive anabolic reactions. Basically gamma phosphate is typically located the farthest from the ribose sugar and has a higher energy of hydrolysis than either that of the alpha and beta phosphate. The bonds that are formed after hydrolysis or the phosphorylation of a residue by ATP are lower in energy than that of the phosphoanhydride bonds of ATP.

ATP is very soluble in water and is a quite stable solution that has a pH of 6.8-7.4, but is rapidly hydrolysed at extreme pH. Thus, ATP is best stored as an anhydrous salt.

Although, ATP is quite stable in solution, it is an unstable molecule in unbuffered water. This is because, once ATP gets in contact with unbuffered water, it hydrolyses to ADP and phosphate due to the strength of the bonds between the phosphate groups in ATP is commonly seen to be less than the strength of the hydrogen bonds (hydration bonds) between its products (ADP + phosphate) and water. Therefore, if ATP and ADP are in chemical equilibrium in water, almost all the ATP will form into ADP because of the reaction that will occur. Gibbs free energy is when a system is far from equilibrium and it is able to do some kind of work. It is seen that typical living cells maintain the ratio of ATP and ADP at a point ten orders of magnitude from equilibrium. However, this may only occur if ADP is thousand fold lower in concentration than that of ATP. This shows that hydrolysis of ATP in cells usually release a large amount of free energy in reaction.

However, even with releasing a large amount of free energy during reaction, any unstable system of potentially reactive molecules could potentially serve as a way of storing free energy. This is only if the cells maintain their concentration far from the equilibrium point of the reaction. However, the idea of both energy-release and entropy-increase always occur during the breakdown of RNA, DNA, and ATP into simpler monomers.

In an ATP molecule, two high-energy phosphate bonds called phsophoanhydride bonds are responsible for high energy content of this molecule. Based on biochemical reaction, these anhydride bonds are often referred to as high-energy bonds. Also the released of hydrolysis of the anhydride bonds can happen in the energy stored ATP.

Binding to Proteins
Rossmann fold is a type of protein fold that some proteins and ATP bind together as. This characteristic protein fold is a general nucleotide-binding structural domain that can also bind the coenzyme NAD. Kinase is the most common ATP-binding protein. They share a small number of common folds and there biggest kinase superfamily all share common structural features specialized for ATP binding and phosphate transfer.

ATP also requires the presence of a divalent cation that is almost as magnesium as a metal used. This metal binds to the ATP phosphate groups. This metal ion can also serve as a mechanism for kinase regulation. The presence of magnesium greatly decreases the dissociation constant of ATP from its protein binding partner without even affecting the ability of the enzyme to catalyze its reaction once the ATP has bound.

Intracellular ATP
Intracellular ATP hydrolysis is catalyzed by intracellular ATPases. For example, the (Na+-K+)-ATPase located in the plasma membranes of higher eukaryotes drives active transport of Na+ and K+ coupled to ATP hydrolysis, and generates electrochemical gradients across the cell membrane. Another important intracellular ATPase is myosin. The myosin heads form the cross-bridges to thin filaments in intact myofibrils and its ATP-powered movement is responsible for muscle contraction.

Extracellular ATP
ATP is also present in extracellular spaces in nanomolar to micromolar concentrations, which are 3-6 orders of magnitude lower than intracellular ATP concentration (1, 2). ATP is released from cells to extracellular spaces by regulated exocytosis or plasma membrane channels (Figure 1). Regulated exocytosis is an important process used to release substances such as hormones or neurotransmitters from the cell and is triggered by an increase in cytoplasmic Ca2+ concentration (2, 3). ATP efflux also occurs through plasma membrane conductance channels, transporters, or constitutive secretory pathways as residual cargo products (2). Extracellular ATP acts as a neurotransmitter and an autocrine/paracrine chemical messenger in non-neural tissues. Its effects are mediated by the P2 purinergic receptors and elicit a variety of physiological responses, such as neurotransmission, regulation of secretion, modulation of immune functions, pain transmission, apoptosis etc.

P2 receptors consist of two major subfamilies, P2X and P2Y. P2X receptors are ligand-gated ion channels and P2Y receptors are G protein-coupled receptors. The concentration of extracellular ATP is regulated by its hydrolysis that is catalyzed by extracellular ATPases. Thus the physiological responses mediated by the purinergic receptors are modulated by extracellular ATPases (Figure 2). For example, Sesti et al. reported that ATP modulates norepinephrine release from cardiac sympathetic nerve endings and this action of ATP is controlled by purinergic receptors in cardiac synaptosomes and modulated by extracellular ATPases (4). Di Virgilio et al. reported that a potent platelet aggregating factor is ADP and its amount is regulated by the activity of extracellular ATPases on endothelial cells (5). However, the precise relationship of multiple P2X and P2Y receptor subtypes to extracellular ATPases remains to be determined.

Exergonic Reaction
The role of ATP is an energy-rich molecule because its triphosphate unit contains two phosphoanhydride bonds. Large amounts of free energy is liberated when ATP is hydrolyzed to adenosine diphosphate (ADP) and orthosphosphate (Pi) or when ATP is hydrolyzed to adenosine monophosphate (AMP) and pyrophosphate (PPi). The precise $$\Delta G^\circ$$ for these reactions depend on ionic strength of the metal such as Mg 2+. The free energy is liberated in hydrolysis of ATP is harnessed to drive reactions that require an input of energy for muscle contraction. The formation of ATP from ADP and Pi is known as the ATP-ADP cycle is the fundamental mode of energy exchange in biological systems. It is intriguing to note that although, all nucleotide triphosphates are energetically equivalent, ATP is the primary cellular energy carrier. Under cellular conditions, the hydrolysis of ATP shifts the equilibrium of a coupled reaction by a factor of 108

Phosphoryl Potential
ATP has a particularly efficient phosphoryl-group donor that can best be explained by features of the ATP structure:

Resonance Structures. ADP and Pi have greater resonance stabilization than does ATP. Orthophosphate has multiple resonance forms of similar energy whereas the $$\gamma$$ phosphoryl group of ATP has a smaller number due to its unfavorability of the positively charged oxygen atom that is adjacent to a positively charged phosphorus atom.

Electrostatic Repulsion. At pH 7, triphosphate unit of ATP carries four negative charges which repel one another due to their close proximity. The repulsion between them is reduced when ATP is hydrolyzed.

Stabilization Due to Hydration. More water can bind effectively to ADP and Pi than can bind to the phosphoanhydride part of ATP, stabilizing the ADP and Pi by hydration.

Consumption of ATP
The large amounts of energy provided by the hydrolysis of ATP are necessary to overcome the large free energy changes necessary to create the large macromolecular proteins. The cleavagle of the phosphoanhydride bonds in ATP provides the source for free energy to make biological reactions spontaneous (negative free energy). Because the amount of entropy of the universe is continually increasing it is unfavorable for large macromolecules to form without the use of ATP. Because of this, the free energy generated by the ATP is always immediately consumed by nearby endergonic (energy-reguiring) biological reactions. The exergonic reaction of the ATP is only able to proceed if it is coupled to an endergonic reaction, otherwise thermodynamic equilibrium would not be obtained. The consumption of ATP proceeds with the first step of having an enzyme attache an amino acid to the a-phosphate of ATP. This results in the release of a pyrophosphate. This release is called an aminoacyl-adenylate intermediate. The reaction then proceeds to the enzyme catalyzing transfer of an amino acid to one of two -OH locations on the ribose portion of the adenosine residue. ATP is able to release energy into cells because cells maintain a concentration of ATP that is far higher above the equilibrium concentrations. The high concentration of ATP allows it to be the main provider of driving endergonic reactions in cells. This coupling of energy releasing and consuming systems through a common intermediate is vital to energy exchange in living systems. = Lehninger | firs = Albert | authorlink = |Nelson, David L. and Michael M. Cox | title = Lehninger principles of biochemistry, 4th ed | publisher = W.H. Freeman & Co | date = 2007 | location = New York, New York | pages 22–25 | isbn = 0-7167-4339-6 }}

=Importance of Oxidation of Carbon=

Formation of ATP
ATP is a principal immediate donor of free energy in biological systems meaning that it is consumed within a minute of it formation. The carbon in fuel molecules such as glucose and fats are oxidized to CO2 and the energy released is used to regenerate ATP from ADP and Pi. Oxidation in fuel takes place one carbon at a time and the carbon-oxidation energy is used in some cases to create compounds with high phosphoryl-transfer potential and other cases to create ion gradient as well with the end formation of ATP.

Coupling with Carbon Fuels
ATP is coupled with oxidation of carbon fuels directly and through the formation of ion gradients. Energy of oxidation is initially trapped as high-phosphoryl-transfer potential compound and then used to form ATP. In ion gradients the electrochemical potential, produced by oxidation of fuel molecules or by photosynthesis, which ultimately powers the synthesis of most ATP in cells. ATP hydrolysis can be used to form ion gradients of different types and functions.

Energy from Food
Described by Hans Krebs the three stages in generation of energy from oxidation of foodstuffs:

1. Large molecules in foods are broken down into smaller units in a process known as digestion. Proteins are hydrolyzed to their 20 different amino acids, polysaccharides are hydrolyzed into simple sugars and lastly fats are hydrolyzed to glycerol and fatty acids.

2. Numerous small molecules are degraded to a few simple units that play a central role in metabolism. Sugars, fatty acids, glycerol and several amino acids are converted into the acetyl unit of acetyl CoA. Some ATP is generated but not a substantial amount.

3. ATP is produced from the complete oxidation of acetyl unit of acetyl CoA. Final stage consist of citric acid cycle and oxidative phosphorylation which are the final pathways in oxidation of fuel molecules. Acetyl CoA brings acetyl units into the citric acid, where they are completely oxidized to CO2. Four pairs of electrons are transferred for each acetyl group that is oxidized. Then a proton gradient is generated as electron flows from the reduced forms of these carriers to O2 and the gradient is used to synthesize ATP.