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Chemical Reactions in Cells Energetics, Enzymes and Metabolic Reactions Energy • Energy is the capacity for work or change. • Kinetic Energy = energy of movement • Potential Energy = stored energy • 1st Law of Thermodynamics – Energy can be transferred and transformed from one form to another but it cannot be created or destroyed. • 2nd Law of Thermodynamics Energy – Energy transfer or transformation increases the entropy of the universe – Increase in entropy = randomness – Energy conversions result in a loss of useful energy usable usable usable usable Free Energy = Energy Useful for Change •Spontaneity of a reaction depends on free energy change G Greaction = Gproducts – Greactants •If G is negative, free energy is released and the reaction is spontaneous •If G is positive, free energy is consumed Free Energy = Energy Useful for Change •Free Energy change depends on changes in –total energy –entropy S H (enthalpy) (unusable energy, disorder) •In living systems, entropy changes have substantial influence –when S is positive, and the term T S is large, a negative G value predicts a spontaneous reaction G = H –T S Chemical Reactions • Involve the breaking and formation of chemical bonds – Reactants are converted to products. – Two types of reactions based on energy use: • Exergonic– free energy released • Endergonic – free energy consumed Exergonic Reactions Burning glucose (sugar): an exergonic reaction high Reactants changed to transition-state species Activation energy needed to ignite glucose Energy Glucose + O2 content of Energy released by molecules burning glucose G C O2 + H2O low Progress of reaction Endergonic Reactions Photosynthesis: an endergonic reaction high Glucose Energy content of molecules Net energy Activation captured by energy from light captured synthesizing glucose by photosynthesis CO2 + H2O low Progress of reaction G Applying Your Knowledge 1. Endergonic Reaction 2. Exergonic Reaction A. Which type of reaction would be spontaneous? B. For which type of reaction will the products have a higher energy than the reactants? C. Which type of reaction releases energy? D. Which type of reaction would have a positive value for G? ATP Provides Energy for Cellular Reactions G 7.3 kcal/mol + H2O + Pi + free energy Short-Term Energy Storage • Chemical Energy is stored in the bonds of ATP – ATP = adenosine triphosphate – ADP = adenosine diphosphate – to store energy • ADP + Phosphate + Energy ATP – to release energy • ATP ADP + Phosphate + Energy Coupled Reactions Pairing of an Exergonic reaction, often involving ATP, with an Endergonic reaction Note that overall free energy change is negative Metabolic Reactions • Anabolic – link simple molecules to produce complex molecules (eg. dehydration synthesis of starch) – require energy • Catabolic – break down complex molecules to release simple ones (eg. hydrolysis of starch sugars) – release energy stored in chemical bonds Metabolic Pathways Initial Reactants A Intermediates B Enzyme 1 Pathway 1 C Enzyme 2 Pathway 2 Final Products D Enzyme 3 Enzyme 4 F Enzyme 5 E Enzyme 6 G Enzymes Assist in Biological Reactions Enzymes are biological catalysts. biological: composed of protein or, rarely, RNA catalyst: speeds up a reaction without being changed by the reaction Properties of Enzymes • Enzymes speed up biological reactions by lowering the activation energy for the reaction. – Enzymes provide a surface where the catalysis takes place – The reaction reaches equilibrium more rapidly – The value of G and the ratio of reactants and products at equilibrium is the same as for an uncatalyzed reaction Activation Energy: Controls Rate of Reaction high Energy content of molecules transition state Amount of energy required for reaction to occur Activation energy without catalyst Activation energy with catalyst low Progress of reaction G Properties of Enzymes • Enzymes are SPECIFIC for the reactants (substrates) in the reactions that they catalyze. • Only substrates that fit the active site of the enzyme can bind and complete the reaction – active site: region on enzyme where substrates bind Enzyme-Substrate Interactions Substrate 1 Substrates enter active site Substrate Active Site Enzyme 2 Shape change promotes reaction 3 Product released; enzyme ready again induced fit Chemical Events at Active Sites • Enzymes hold substrates in the proper orientation for the reaction to take place Chemical Events at Active Sites • Enzymes induce strain in the substrate to produce a transition state favorable to reaction • Active site provides a microenvironment that favors the chemical reaction Chemical Events at Active Sites • Active site directly participates in the reaction – covalent bonding can occur between enzyme and substrate – R groups of the enzyme’s amino acids can temporarily add chemical groups to the substrates Molecules that Assist Enzymes • Cofactors: inorganic ions that bind to enzymes, eg. zinc • Coenzymes: small organic factors that temporarily bind to enzymes, eg. biotin, NAD, ATP • Prosthetic groups: non-protein factors that are permanently bound an enzyme, eg. heme Factors Influencing Reaction Rate • Substrate Concentration Rate no longer increases since the active sites of all enzymes are saturated with substrate Rate is more rapid Rate is proportional to substrate concentration Factors Influencing Reaction Rate • Competitive Inhibitors: Bind at the active site, compete for binding with substrate – Irreversible: form covalent bond with amino acids in the active site DIPF Factors Influencing Reaction Rate • Competitive Inhibitors: Bind at the active site, compete for binding with substrate – Reversible: molecule similar to substrate occupies active site but does not undergo reaction Factors Influencing Reaction Rate • Non-Competitive Inhibitors: Bind to a different site, cause a conformational change in the enzyme that alters the active site – Reversible Factors Influencing Reaction Rate • Allosteric Regulation – Conversion between active and inactive forms of an enzyme due to binding of regulatory molecules at an allosteric site • Activators stabilize the active form • Allosteric inhibitors stabilize the inactive form Factors Influencing Reaction Rate • Allosteric Regulation – Cooperativity: a substrate causing induced fit in one enzyme subunit can cause a change to the active form in all the other subunits Enzyme Regulation: Feedback Inhibition Commitment step CH3 CH3 CH2 A B C D H C OH H C CH3 Enz. 1 Enz. 2 Enz. 3 Enz. 4 Enz. 5 H C NH3 H C NH3 COOH COOH Threonine Feedback Inhibition (substrate) Isoleucine allosterically Isoleucine inhibits enzyme 1 (end product) Feedback Inhibition: The product of a pathway inhibits an initial step in the pathway to decrease its own production Properties of Enzymes • Three dimensional structure of an enzyme preserves its ACTIVE SITE • Conditions that can affect three dimensional structure include: heat, pH (acid/base balance) and other chemicals (salt, charged ions) Effects of Temperature and pH on Enzymatic Activity fewer collisions between enzyme and substrate enzyme unfolds (denatures) enzyme unfolds (denatures) Applying Your Knowledge 1. 2. 3. 4. 5. Active Site Activation Energy Allosteric Site Commitment step Induced fit A. Where can an inhibitor bind to stabilize the inactive form of an enzyme? B. Where do the substrates bind? C. Enzymes (raise or lower) the (1, 2, 3, 4 or 5) of a reaction. D. What is the model for a shape change caused by substrate binding to the enzyme?