Biochemical bases for energy transformations. Biochemical bases for energy transformations. Nutrition 202 Animal Energetics R. D.

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1 Biochemical bases for energy transformations Biochemical bases for energy transformations Nutrition 202 Animal Energetics R. D. Sainz Lecture 02 Energy originally from radiant sun energy Captured in chemical form (carbohydrates) through photosynthesis Carbohydrates are ingested, digested and metabolized for energy by the animal Energy from carbohydrates captured in mediumenergy intermediates (e.g., ATP) Energy in intermediates used to drive endothermic reactions in the body ATP is the single currency of life ATP is the most important molecule for capturing and transferring free energy Hydrolysis of ATP to ADP + P i yields 7.3 kcal/mol energy that can be used to power other reactions, e.g. protein synthesis, muscle contraction or transport of molecules Energy arrives from the sun, but we are not able to use it directly to run metabolic processes. We depend on photosynthesis to convert solar energy into chemical storage energy - in the form of carbohydrate. Catabolic metabolism converts carbohydrate to energy currency i.e. ATP. ATP is the principal carrier of energy for all forms of life. It is a nucleotide consisting of adenine, ribose and a triphosphate unit. Opposite shows ATP with the blue nitrogens of adenine on the RHS, dark grey carbons of ribose in the centre, and three prominent phosphates on the LHS. In the body ATP is an electrostatic complex with Mg 2+ to give MgATP. ATP is an energy rich molecule, because it contains two phosphoanhydride bonds, and a large amount of free energy is released when one of these bonds is broken. Free energy is the intrinsic energy content of a molecule so hydrolysis of ATP to ADP + free energy represents a release of energy, and in the case of ATP, 30kJ/mole released. The released energy is taken up by other biochemicals to drive reactions e.g. that make muscles work. Release of the second phosphate releases a further 30kJ/mole. ATP is constantly being consumed/reformed all over the body, and estimated that human resting consumes about 40 kg of ATP per 24 hours. During strenuous activity rate increases to 500 g/minute. So ATP is described as energy currency, because it is continually being used and reformed. ATP cannot be stored. The adenylate cycle is the fundamental mode of energy transduction. Energy is transferred to other molecules by coupled reactions. e.g. conversion of glucose to glucose-6- P requires 16.8 kj so coupling with ATP hydrolysis to ADP releases 30 kj so reaction goes forward. Conversion of phosphoenolpyruvate (PEP) to pyruvate releases 60 kj so more than enough for the 31.5 kj required to form ATP from ATP + Pi. So ATP is not the only molecule involved in energy manipulations. ATP is about middle in energy league - ideal because it can accept energy from high energy molecules, and transfer energy to low energy molecules. The amazing design of biochemistry is that almost all of the high energy molecules that have high energy of hydrolysis e.g. PEP are formed during the oxidation of CHO, and almost all of the lower energy molecules are involved in metabolic processes that require energy to drive reactions, and ATP is in the middle as the carrier of energy from the high energy molecules to the low energy molecules, ATP spreads energy about by passing it onto other nucleotides. All reactions involve making and breaking of bonds so the thermodynamic decision as to whether a reaction goes or not depends on the energy balance between making and breaking bonds. If a reaction is not favorable then it may be possible to couple it with ATP hydrolysis so that release of 30kJ may alter the energy balance to favor the reaction. Note that there is a limit, if two P are removed from ATP then about 60 kj are free, if a reaction requires 100 kj in order to make it favorable then coupling with ATP will not be enough. 1

2 Molecular basis for ATP as Energy Carrier So what is the structural basis for ATP being an energy carrier? It is the phospho-anhydride bond. Formation of that bond between ADP + Pi to produce ATP requires larger than normal input of energy to form the bond. Why does it require extra energy to make the phospho-anhydride bond? Two reasons (a) electrostatic repulsion (b) resonance stabilization. Structure of ATP and Conversion of ATP to ADP When ATP is formed, 4 strong negative charges are placed in very close proximity so extra energy required to overcome repulsion. Also the end phosphate has less resonance stability (distribution of electrons), ADP has greater resonance stability, and AMP is very stable. Resonance allows electrons to move about to spread out the energy load. So extra energy required to maintain the resonance instability in ATP. Phospho-anhydride bonds are high energy bonds because of that amount of energy, 30 kj, is needed to form the bond against increased repulsion and reduced resonance. What keeps the structure from breaking up? ATP requires Mg2+ in order to offset and balance off the negative charges. Without Mg2+, repulsion would cause rapid breakdown to ADP + Pi. Mg2+ provides temporary stability until the molecule is required for reaction then the Mg2+ is released because the binding is simply by noncovalent electrostatic attraction. Stages of Catabolism Carbohydrate Metabolism: Initial Biochemical Pathway & Disorders 2

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4 Oxidative phosphorylation is the last stage of catabolism Glycolysis, TCA cycle and fatty acid oxidation generate NADH and FADH 2 NADH and FADH 2 are energy rich molecules because each contains a pair of electrons that have a high transfer potential In oxidative phosphorylation the electron transferring potential of NADH and FADH 2 is converted to phosphate-transfer potential of ATP ATP is formed as electrons are transferred from NADH or FADH 2 to 0 2 by a series of electron carriers. Proton motive force and chemiosmotic coupling The immediate energy sources that power ATP synthesis are proton gradient and electric potential (voltage gradient) across the membrane. Proton gradient and electric potential are collectively called proton-motive force. The proton motive force is generated by stepwise movement of electrons by electron carriers that leads to pumping of protons out of the mitochondrial matrix. Oxidation of NADH and phosphorylation of ADP are coupled by a generation of proton gradient. Energy is released gradually in the electron transfer chain Oxidative phosphorylation Most free energy released when glucose is oxidised to carbon dioxide is retained in the reduced coenzymes NADH and FADH 2 Respiration: electrons are released from from NADH and FADH 2 to oxygen NADH + H + + 1/2 0 2 = NAD + +H kcal/mol ADP + P i = ATP +7.6 kcal/mol ATP production is maximised by releasing the free energy in small increments in the electron transfer chain (a.k.a respiratory chain). The electron transfer chain contains four multiprotein complexes. Three of these are electron driven proton pumps that create the proton gradient ATP Synthase ATP synthase or F 0 F 1 complex has two components that are both itself multiprotein complexes F 0 is transmembrane complex that forms a regulated H + channel F 1 is protrudes in the matrix and contains the sites for ATP formation Proton translocation through F 0 powers rotation of one subunits of F 1 Three conformations: one binds ADP and P i so tightly that they spontaneously form ATP. Redox (oxidation-reduction) potential Oxidant + electron = reductant Substance that can exist as a reduced and oxidized form is referred to a redox couple Redox potential of such couple is measured against the H + -> H 2 couple. Redox potential of H + -> H 2 couple is defined as 0 V (volts). A negative redox potential means that a substance has lower affinity for electrons than hydrogen. Positive redox potential means higher affinity. Strong oxidizing agents have positive redox potential In the respiratory chain the electrons are transferred to higher redox potential values, that is, to higher affinity electron carriers. Electron transfer is driven by redox potential 4

5 Cytochromes are heme containing proteins Respiratory control Cytochromes are covalently linked to heme, an ironcontaining prosthetic group similar to that in hemoglobin or myoglobin. Electron transport occurs by by oxidation and reduction of the Fe atom in the centre of the heme Different cytochromes have slightly different heme groups that generate different environment for Fe-ion and thus different tendency to accept an electron Mitochondria can only oxidise FADH 2 and NADH only as long as there is ADP and Pi available. Electron flow ceases if ATP is not produced. ADP increases when ATP is consumed e.g. in muscle work. Oxidative phosphorylation is regulated by ATP consumption. Exception: brown fat Brown-fat mitochondria contain an uncoupler of oxidative phosphorylation Brown fat is specialised to produce heat Newborns: brown-fat thermogenesis Thermogenin protein, a proton transporter that is not connected to ATP synthesis. Energy released by NADH oxidation converted to heat. The schematic diagram above illustrates a mitochondrion. In the animation, watch as NADH transfers H+ ions and electrons into the electron transport system. Key points: Protons are translocated across the membrane, from the matrix to the intermembrane space Electrons are transported along the membrane, through a series of protein carriers Oxygen is the terminal electron acceptor, combining with electrons and H+ ions to produce water As NADH delivers more H+ and electrons into the ETS, the proton gradient increases, with H+ building up outside the inner mitochondrial membrane, and OH- inside the membrane. Animation of ATP synthesis in Mitochondria Step 1: Proton gradient is built up as a result of NADH (produced from oxidation reactions) feeding electrons into electron transport system. Step 2: Protons (indicated by + charge) enter back into the mitochondrial matrix through channels in ATP synthase enzyme complex. This entry is coupled to ATP synthesis from ADP and phosphate (P i ) The schematic diagram above illustrates a mitochondrion. In the animation, watch as H+ ions accumulate in the outer mitochondrial compartment whenever NADH is made from oxidation reactions, generating a proton gradient (upper image). Protons re-enter the cell through the ATP synthase complex, generating ATP (lower image). Key points: Protons are translocated across the membrane, from the matrix to the intermembrane space, as a result of electron transport resulting from the formation of NADH by oxidation reactions. (See the animation of electron transport.) The continued buildup of these protons creates a proton gradient. ATP synthase is a large protein complex with a proton channel that allows re-entry of protons. ATP synthesis is driven by the resulting current of protons flowing through the membrane: ADP + Pi ---> ATP 5

6 Sources Regulation of Adenyl Cyclase Adenyl cyclase catalyzes the conversion of ATP to cyclic AMP (camp) and inorganic phosphate (see below). camp functions as a cell-specific and hormone specific "second messenger" in response to several different nonsteroid hormones. Within the cell, camp activates protein kinases which utilize ATP to phosphorylate specific substrate proteins. In the phosphorylated form, these protein substrates become active and carry out the specific function corresponding to hormone stimulation. Regulation of Adenyl Cyclase The animation to the right depicts the regulation of adenyl cyclase by hormone stimulation. (Note: the animation cycles 5 times and stops; "reload" the page to run it again.) 1.Hormone (in red) interacts with receptor (R) 2.Receptor binds to G complex (Gb -Ga) 3.Receptor - G complex interaction causes Ga subunit to bind GTP in place of GDP 4.Binding of GTP causes Ga subunit to activate adenyl cyclase 5.Activated adenyl cyclase converts ATP to camp as long as the Ga subunit is activated 6.Hormone binding is transient and loss of binding to receptor results in conversion of GTP bound to subunit Ga to GDP leading to inactive state 7.Inactivation of the Ga subunit leads to inactivation of adenyl cyclase 8.Cholera toxin inhibits the conversion of GTP bound to Ga to GDP thus maintaining the activation of adenyl cyclase. 6