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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 1 st Law of Thermodynamics
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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.
usable usable usable usable Energy • 2nd Law of Thermodynamics • Energy transfer or transformation increases the entropy of the universe • Increase in entropy = randomness • Energy conversions result in a loss of useful energy
Spontaneity of a reaction depends on free • energy change G reaction = Gproducts – Greactants G G • If is negative, free energy is released and the reaction is spontaneous G • If is positive, free energy is consumed Free Energy = Energy Useful for Change
Free Energy change depends on changes in • total energy (enthalpy) • entropy (unusable energy, disorder) H S • In living systems, entropy changes have substantial influence • when is positive, and the term is large, a negative value predicts a spontaneous reaction S T S G H – G = T S Free Energy = Energy Useful for Change
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
Reactants changed to transition-state species high Energycontentofmolecules G low Progress of reaction Exergonic Reactions Burning glucose (sugar):an exergonic reaction Activation energy neededto ignite glucose Glucose + O2 Energy released byburning glucose C O2 + H2O
high Energycontentofmolecules G low Progress of reaction Endergonic Reactions Photosynthesis:an endergonic reaction Glucose Net energycaptured bysynthesizingglucose Activationenergy fromlight capturedby photosynthesis CO2 + H2O
+ H2O + Pi+ free energy ATP Provides Energy for Cellular Reactions
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
D E Metabolic Pathways InitialReactants Intermediates FinalProducts B C A Enzyme 1 Enzyme 2 Enzyme 3 Enzyme 4 Pathway 1 F G Pathway 2 Enzyme 5 Enzyme 6
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
G –The value of and the ratio of reactants and products at equilibrium is the same as for an uncatalyzed 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
high Energycontentofmolecules G low Progress of reaction Activation Energy: Controls Rate of Reaction Amount of energy required for reaction to occur transition state Activationenergy withoutcatalyst Activationenergy withcatalyst
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
induced fit Enzyme-Substrate Interactions Substrate Substrate 1 Substrates enter active site ActiveSite 2 Shape change promotes reaction Enzyme Product released;enzyme ready again
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 Rate no longer increases since the active sites of all enzymes are saturated with substrate • Substrate Concentration Rate is more rapid Rate is proportional to substrate concentration
DIPF 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
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
CH3 CH3 CH2 OH H C CH3 H C NH3 H C NH3 H C COOH COOH Feedback InhibitionIsoleucine allosterically inhibits enzyme 1 Enzyme Regulation: Feedback Inhibition Commitment step A B C D Enz. 1 Enz. 2 Enz. 3 Enz. 4 Enz. 5 Threonine(substrate) Isoleucine(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)
fewer collisions between enzyme and substrate enzyme unfolds(denatures) enzyme unfolds(denatures) Effects of Temperature and pH on Enzymatic Activity