ATP. 1941, Fritz Lipmann & Herman Kalckar - ATP role in metabolism. ATP: structure. adenosine triphosphate

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1 ATP Living systems need energy to do (i) mechanical work (muscles, cellular motion) (ii) transport of molecules and ions (ion channels) (iii) synthesis of macromolecules (DNA, proteins) Energy must come from environment. Two types of organisms: Chemotrophs - oxidation of foodstuffs Phototrophs - trap light energy In both organisms the energy must be converted into universal form highly accessible to any processes Most widely used free-energy donor in bioprocesses is adenosine triphosphate (ATP) 1941, Fritz Lipmann & Herman Kalckar - ATP role in metabolism ATP: structure adenosine triphosphate Energy is stored in the phosphate groups - in phosphoanhydride chemical bonds 1

2 phospoanhydride bods ATP, ADP, AMP: energy storage 3, 2 or 1 phosphoryl groups (-PO 3 2- ) hydrolysis ortophosphate (PO 4 3- ) - G o at ph7 ATP + H 2 O ADP + P i + H + ATP + H 2 O AMP + PP i + H + G o -7.3 kcal/mol kj/mol G o -7.7 kcal/mol kj/mol pyrophosphate (P 2 O 7 4- ) ATP-ADP cycle fundamental mode of energy exchange in biological systems ADP ATP is formed using energy of light (phototrophs), or when fuel molecules are oxidized (chemothrops). ATP is delivered to where energy is needed ATP ADP is used to drive endergonic reactions Photosynthesis or Oxidation of fuel molecules ADP ATP Motion Active transport Biosynthesis Signal amplification ATP is an immediate donor of free energy: consumed within a minute following its formation. Turnover of ATP is extremely high: resting human consumes ~40 kg of ATP in 24 hours could consume 0.5 kg in a minute ATP is continuously regenerated from ADP 2

3 ATP: phosphoryl potential Phosphorylation - chemical process in which a phosphate group is added to an organic molecule. Phosphoryl potential - G o for hydrolysis - depends on molecule: ATP G o -2.2 kcal/mol G o -7.3 kcal/mol G o kcal/mol Hydrolysis of one molecule can be coupled to phosphorylation of the other one with the net G o < 0, for example: ATP ADP + P i G o -7.3 kcal/mol + Glycerol + P i Glycerol 3-phosphate G o +2.2 kcal/mol ATP + Glycerol ADP + Glycerol 3-phosphate G o -5.1 kcal/mol Phosphate group can be transferred from one molecule to another + ATP: phosphoryl potential. Example Amount of ATP in muscle is sufficient for <1 second Muscle contains creatine phosphate ( G o kcal/mol) with higher phosphoryl potential than ATP. Creatine phosphate Creatine + P i G o kcal/mol ADP + P i ATP G o +7.3 kcal/mol Creatine phosphate + ADP creatine + ATP G o -3.0 kcal/mol What is the equilibrium constant for net reaction? 4 mm 13 mm [ ][ creatine] [ ADP][ creatine phosphate] G ln ATP 10RT 3/ mm 25 mm In resting muscle 162 The abundance of creatine phosphate and high phosphoryl transfer potential make it efficient ~P buffer (~P compound - compound with high phosphoryl transfer potential) 3

4 ATP: enhancing odds for unfavorable reactions Suppose we want to convert compound A into B: Equilibrium constant: A B G o +4 kcal/mol G ' / spontaneous conversion cannot occur if [B]/[A] Lets couple this reaction to ATP hydrolysis: A +ATP B + ADP + P i G o (+4 7.3) kcal/mol [ ][ ADP][ P ] [ ATP] B 36 [ ATP] [ ADP][ ] i + 3.3/ P i 500 in cell [ ] -3.3 kcal/mol B 134,000 with ATP 3 no ATP ATP enhances process ~10 8 times ATP: tilting equilibria Lets compare uncoupled and coupled to ATP reactions in general: 1. Uncoupled: A B ( G u o ) n n [ ADP] [ Pi ] n [ ATP] n [ ADP][ P ] [ ATP] [ ] K u [ ] eq A Gu ' Keq, u Coupled: A +natp B + nadp + np i ( G o c G o u +n G o ATP ) Gu ' + n GATP ' Gu ' n GATP ' Keq, c i 5.36n Keq, u 10 Keq, u 10 [ ATP] [ ADP][ P ] 5.36 ( ) n B 5.36 Keq u, 10 ~ i, 10 8n n 4

5 ATP: tilting equilibria The hydrolysis of n ATP molecules in a coupled reaction changes the equilibrium ratio of products to reactants by a factor in the order of 10 8n. Notes: Any thermodynamically unfavorable reaction sequence may be converted into a favorable one by coupling it to hydrolysis of a sufficient number of ATP molecules Term reaction is very general here. For example, it may represent a change in protein conformation (as in muscle contraction), or refer to an ion or molecule concentration gradient across the membrane (active transport of a nutrient) Visualizing metabolic process Sci190E Lecture 11 NMR - nuclear magnetic resonance Independently discovered (1946) Nobel Price (1952) Felix Bloch Edward Mills Purcell B.S.E.E. from Purdue electrical engineering 5

6 NMR background: spin Wolfgang Pauli Electron, neutron and protons posess an intrinsic quantummechanical property called spin (Pauli, 1924) charges spinning around in a coil produce magnetic field An elementary particle acts as a small electromagnet producing magnetic field. This field can take two discrete values called spin 1/2 and -1/2 (or up and down) NMR background: Pauli principle - No two particles with half spin (fermions) can be in exactly the same state This is the reason why there are 2 electrons on s orbital - they differ in spin, i.e. one has spin up, and one has down. This principle governs the periodicity of chemical properties of elements with increasing number of electrons (periodic table). Neutrons and protons, that constitute nucleus, follow the same exclusion principle. Whenever a new particle is added it tries to get to the level with the lowest energy. Therefore, depending on the atomic number the net spin of nucleus can be ±1/2 or zero 1 H, 13 C, 31 P - spin could be ±1/2, energy would be the same. 6

7 NMR background: nuclear spin Compass needle in magnetic field tends to turn to align along the field lines - because its energy is lowered 31 P - spin could be ±1/2, energy would be the same. However, when a nucleus with spin is brought into magnetic field it may gain or lose energy depending on orientation magnetic field E up <E down Nuclea with spin along field and against field will have energy difference: E E down - E up The photon of a radiofrequency electromagnetic wave can swap nuclear spin state if its energy, which is a function of frequency, matches E. This causes absorption of radiowave of well defined frequency. NMR spectroscopy magnetic field E up <E down The splitting energy, E E down - E up : depends on nucleus and its isotope state is proportional to the strength of magnetic field Since the magnetic field can be screened by environment: depends on chemical structure By observing NMR absorption spectrum (i.e. dependence of absorption on the frequency of incident radiation) one can quantify the contents of a single element, such as 31 P, in different compounds 7

8 Reminder: ATP example Amount of ATP in muscle is sufficient for <1 second Muscle contains creatine phosphate with higher phosphoryl potential than ATP. Creatine phosphate + ADP creatine + ATP G o -3.0 kcal/mol Equilibrium constant for net reaction: 4 mm 13 mm [ ][ creatine] [ ADP][ creatine phosphate] G ln ATP 2RT 3/ mm 25 mm In resting muscle 162 The abundance of creatine phosphate and high phosphoryl transfer potential make it efficient ~P buffer (~P compound - compound with high phosphoryl transfer potential) NMR spectroscopy of 31 P (19 minutes of exercise) Creatine ph. ADP + P i Creatine + P i ATP G.K. Radda. Science 233 (1986):641 Effect of exercise on the level of ATP, creatine phosphate, and orthophosphate in the forearm muscle of a human subject Chemical shift - shift of 31 P absorption band due to different chemical environment Note: The amount of creatine phosphate decreased and amount of P i increased, while amount of ATP stays almost constant - ~P buffer. 8