Chapter 8 Energy and Enzymes 21 September 2014 Energy and enzymes drive actions in cells o Enzymes direct reactions life creates order from naturally disordered environment Free energy amount of energy available to do work o Overall change in free energy G product energy reactant energy o all events of life decrease free energy o organisms replace losses by taking energy from outside ie the sun Kinetic energy energy of motion ex thermal energy molecular level Potential energy energy associated with position or configuration ex chemical energy position of electrons in molecules o PE in e based on position with other e and p in nucleus and other atoms High PE close to other charges and far from charges If falls to lower shell PE decreases because converted to KE First law of thermodynamics energy can t be created or destroyed Enthalpy H total energy in a molecule o Includes PE of molecule and effect of pressure and volume on surroundings o Changes represented by H based on difference in heat content because most reactions don t change pressure and volume o Exothermic releases heat H products have less PE than reactants o Endothermic absorbs heat H products have more PE than reactants Entropy S amount of disorder in a system o S positive when products less ordered than reactants o Increases in spontaneous reaction Second law of thermodynamics total entropy increases in isolated system universe surroundings and products of reaction Asses changes in heat and disorder to determine if reaction is spontaneous o Gibbs free energy change G G H T S T temperature in Kelvin Thermal energy increases disorder molecules move faster o Exergonic spontaneous reaction release energy G 0 products have lower free energy than reactants ex cellular respiration o Endergonic nonspontaneous reaction absorb energy G 0 ex photosynthesis o G 0 equilibrium o Spontaneous reactions occur in direction that lowers free energy increase entropy or with temperature increase o Spontaneous reactions don t always happen quickly Reactions require collisions of substances in specific orientation to bring e close to break or form bonds o Amount of collisions in mixture depends on temperature and concentration Higher concentration causes more collisions faster reaction Higher temperature causes faster movement cause frequent collisions faster reaction Energetic coupling between exergonic and endergonic reactions allows chemical energy released from one reaction to be used for another o Occurs in cells with transfer of high energy e or transfer of P group o Redox and ATP hydrolysis Reduction oxidation redox reactions involve loss or gain of one or more e to transfer energy o Oxidation atom loses one or more e removes H exergonic O pulls e from atoms because highly electronegative o Reduction atom molecule gains one or more e adds H endergonic o OIL RIG Oxidation Is Loss of e Reduction Is Gain of e o Oxidation and reduction always paired atom loses e another has to gain Two half reactions exergonic and endergonic Energy lost by oxidized molecule increases PE of reduced Overall reaction exergonic when more free energy released by oxidation that necessary o e can completely transfer from one atom to another or change position in covalent bond o ex reduced carbons in glucose C6H12O6 oxidized as sugar is burned in presence of oxygen C6H12O6 6O 6CO2 6H2O energy e move from carbon in CO2 carbon oxidized lost e because O has greater electronegativity e move closer to O in H2O than in O molecules O atoms reduced gained e spontaneous and exergonic reaction because e held tighter in products molecules than reactant molecules e PE decreases and S of products higher more molecules o electron donor atom that gives up an e o electron acceptor atom that accepts e e may be accompanied by a proton H neutral H atom added to electron acceptor molecules that gain H have increased PE because C H bonds share electrons equally and are far from charged nucleus molecules with many C H have greater PE oxidized molecules often lose H and e and lose PE usually more C O higher electronegativity and lower PE o electron carrier readily donates high energy e reducing power ex Flavin adenine dinucleotide FAD reduced by 2 e with 2 p to form FADH2 which donates high energy e ex nicotinamide adenine dinucleotide NAD reduced by 2 e to form NADH gets one H and releases second H to environment o all redox reactions involve e transfer not all involve H transfer Adenosine triphosphate ATP made of 3 phosphate groups ribose sugar and adenine nucleotide ATP has great PE because 4 charges of P group repel eachother ATP and H2O react during hydrolysis o bond between outer P group and neighbor broken to form ADP and inorganic phosphate Pi H2PO4 o exergonic because entropy of products higher than entropy of reactants and PE decreases o PE change because e from ATP phosphate groups now spread across two molecules instead of one less electrical repulsion Phosphorylation phosphate group transferred to substrate with energy from ATP hydrolysis o Exergonic when ATP is phosphate donor because e in ADP and phosphate added to substrate have less PE than when in ATP Substrate reactant that interacts with a catalyst during chemical reaction o Activated substrate endergonic reactant molecules phosphorylated to cause free energy release which makes reaction exergonic Catalyst enzymes that bring substrates together in specific orientation to make o Enzyme specificity results from geometry and chemical properties of reaction more likely binding sites o Catalyze single reaction by lowering activation energy o Composition exactly same before and after reaction For reaction to occur 1 reactants need to collide in precise orientation 2 reactants need enough KE to overcome repulsion from bonding e Active site site on enzyme for substrate binding cleft or cavity within large globular enzymes o Substrates held together in active site with hydrogen bonds or other interactions with amino acids in active site R groups R groups may form temporary covalent bonds to transfer atoms between reactants Acidic or basic R groups allow reactants to lose gain e Enzymes are flexible and dynamic o only change free energy of transition state Induced fit enzyme shape change when reactants bind to active site Transition state temporary unstable intermediate condition when enzyme and substrate bind o Substrate and enzyme interaction at maximum degree o High free energy because bonds in substrates are
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