Chapter 9A

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Announcements, Tues. Oct. 5 : 

Announcements, Tues. Oct. 5 Today: Chapter 9, Introduction to Metabolism Thurs. Oct 7: No lecture, Fall Break Tues: Oct. 12: Ch 10 Oct. 10 (Sunday): Tutoring 6:30, Room 206 Kivett, Oct. 22 Last Day to drop with w/WP or WF

Today’s Lecture: Introduction to Metabolism : 

Today’s Lecture: Introduction to Metabolism Energy and Work Laws of Thermodynamics Free Energy and Reactions ATP Oxidation-Reduction Reactions Electron Transport Chains Enzymes Ribozymes Regulation of Metabolism Posttranslational ReguLation of Enzyme Activity

An Overview of Metabolism : 

An Overview of Metabolism Metabolism: total of all chemical reactions in the cell catabolism – energy-conserving reactions, generates electrons (reducing power) and precursors for biosynthesis anabolism – the synthesis of complex organic molecules from simpler ones Requires energy and source of electrons 3

Energy and Work : 

Energy and Work ENERGY: capacity to do work or to cause particular changes 4 chemical work synthesis of complex molecules transport work take up of nutrients, elimination of wastes, and maintenance of ion balances mechanical work cell motility and movement of structures within cells

Energy Units : 

Energy Units calorie (cal) amount of heat energy needed to raise 1 gram of water from 14.5 to 15.5°C joules (J) units of work capable of being done by a unit of energy 1 cal of heat is equivalent to 4.1840 J of work 5

Laws of Thermodynamics : 

Laws of Thermodynamics First law: energy can be neither created nor destroyed total energy in universe remains constant however energy may be redistributed either within a system or between the system and its surroundings Second law: law of entropy Entropy= amount of randomness (disorder) in a system physical and chemical processes proceed in such a way that the disorder of the universe increases to the maximum possible 6

Free Energy and Reactions : 

Free Energy and Reactions G = H - TS G = the change in free energy that can occur in chemical reactions and other processes Go’= standard free energy change at pH 7 H = change in enthalpy (heat constant) TS = (Temperature Kelvin)(change in entropy) Formula used to indicate if a reaction will proceed spontaneously 7

Chemical Equilibrium : 

Chemical Equilibrium Equilibrium consider the chemical reaction A + B ↔ C + D reaction is at equilibrium when rate of forward reaction = rate of reverse reaction Equilibrium constant (Keq) expresses the equilibrium concentrations of products and reactants to one another 8

The Relationship… : 

The Relationship… 9 Figure 9.1 No free energy needed, reaction favors C+D

Adenosine 5’-triphosphate (ATP)Energy Currency of the Cell : 

Adenosine 5’-triphosphate (ATP)Energy Currency of the Cell 10 Figure 9.2

Role of ATP in Metabolism : 

Role of ATP in Metabolism Breakdown of ATP is coupled with endergonic reactions to make them more favorable 11 Figure 9.3

The Cell’s Energy Cycle : 

The Cell’s Energy Cycle 12 Figure 9.4

Oxidation-Reduction (Redox) Reactions : 

Oxidation-Reduction (Redox) Reactions Convention: Acceptor + ne- ↔ donor Example: NAD+ + 2H+ + 2e- ↔ NADH + H+ many metabolic processes involve oxidation-reduction reactions (electron transfers) electron carriers are often used to transfer electrons from an electron donor to an electron acceptor can result in energy release, which can be conserved and used to form ATP 13

Standard Reduction Potential (E0) : 

Standard Reduction Potential (E0) equilibrium constant for an oxidation-reduction reaction a measure of the tendency of the reducing agent to lose electrons more negative E0  better electron donor more positive E0  better electron acceptor 14 NAD(P)+ + H+ 2e- >>>> NAD(P)H (E’o = -0.32 volts) FAD + 2H+ 2e- >>>> FADH2 (E’o = -0.18 volts) O2 + 4H+ + 4e- >>>> 2H2O (E’o = 0.82)

Slide 15: 

15 Table 9.1

Slide 16: 

16 The greater the difference between the E0 of the donor and the E0 of the acceptor  the more negative the Go´ Figure 9.5 Convention: Acceptor + ne- ↔ donor Example: NAD+ + 2H+ + 2e- ↔ NADH + H+ (E’o = - 0.32 volts) Example: 1/2O2 +2H+ + 2e- ↔ H2O (E’o= + 0.82 volts) NADH + H+ + 1/2O2 >>>> H20 + NAD+ G = the change in free energy that can occur in chemical reactions and other processes Go’= standard free energy change at pH 7 ∆E’o = 1.14 volts

Electron Transport Chains = Electron Transport Systems : 

Electron Transport Chains = Electron Transport Systems 17 Figure 9.6 Cytochrome b (Fe3+) = e->>> cytochrome b (Fe2+) (E’o= 0.08 v) Cytochrome c (Fe3+) + e->>> cytochrome c (Fe2+) (E’o= 0.25 v) ETC: the first electron carrier has the most negative E´o (standard reduction potential) and successive carriers are each slightly less negative

Slide 18: 

18 Table 9.1 Cytochrome c (Fe3+) + e->>> cytochrome c (Fe2+) (E’o= 0.25 v) Order in the ETC: 1st electron carrier has the most negative E´o (standard reduction potential) Successive carriers are each slightly less negative Result: potential energy stored in first redox couple is released and used to form ATP

ETC Electron Carriers : 

ETC Electron Carriers NAD nicotinamide adenine dinucleotide NADP nicotinamide adenine dinucleotide phosphate 19 Figure 9.7 (a) Convention: Acceptor + ne- ↔ donor Example: NAD(P)+ + 2H+ + 2e- ↔ NAD(P)H + H+ (E’o = - 0.32 volts)

Slide 20: 

Figure 9.7 (b) NAD can accept electrons from a reduced substrate (SH2) Convention: Acceptor + ne- ↔ donor Example: NAD(P)+ + 2H+ + 2e- ↔ NAD(P)H + H+ (E’o = - 0.32 volts)

Electron Carriers : 

Electron Carriers FAD flavin adenine dinucleotide FMN flavin mononucleotide 21 Figure 9.8 riboflavin

Electron Carriers : 

Electron Carriers coenzyme Q (CoQ) also called ubiquinone 22 Figure 9.9

Electron Carriers: Heme (nonprotein component of many cytochromes : 

Electron Carriers: Heme (nonprotein component of many cytochromes Cytochromes: use iron to transfer electrons iron is part of a heme group 23 Figure 9.10

Electron Carriers : 

Electron Carriers nonheme iron proteins e.g., ferredoxin use iron to transport electrons iron is associated with sulfur atoms 24

Electron Transport Chains Summary : 

Electron Transport Chains Summary 25 Figure 9.6 Cytochrome b (Fe3+) = e->>> cytochrome b (Fe2+) (E’o= 0.08 v) Cytochrome c (Fe3+) + e->>> cytochrome c (Fe2+) (E’o= 0.25 v) ETC: the first electron carrier has the most negative E´o (standard reduction potential) and successive carriers are each slightly less negative

Enzymes = Protein Catalysts : 

Enzymes = Protein Catalysts Enzymes have great specificity for the reaction catalyzed and the molecules acted on (substrates) catalyst substance that increases the rate of a reaction without being permanently altered substrates reacting molecules products substances formed by reaction 26

Slide 27: 

27 Lock and Key Model of Enzyme Function Figure 9.13

Structure and Classification of Enzymes : 

Structure and Classification of Enzymes some enzymes are composed solely of one or more polypeptides some enzymes are composed of one or more polypeptides and nonprotein components 28

Enzyme Structure : 

Enzyme Structure apoenzyme protein component of an enzyme cofactor nonprotein component of an enzyme prosthetic group – firmly attached coenzyme – loosely attached holoenzyme = apoenzyme + cofactor 29

Coenzymes : 

Coenzymes often act as carriers, transporting substances around the cell 30 Figure 9.11

Enzyme Classification : 

Enzyme Classification 31 Table 9.2

The Mechanism of Enzyme Reactions : 

The Mechanism of Enzyme Reactions a typical exergonic reaction A + B  AB‡  C + D transition-state complex – resembles both the substrates and the products 32

Slide 33: 

activation energy – energy required to form transition-state complex enzyme speeds up reaction by lowering Ea 33 Figure 9.12

How Enzymes Lower Ea : 

How Enzymes Lower Ea by increasing concentrations of substrates at active site of enzyme by orienting substrates properly with respect to each other in order to form the transition-state complex 34

The Induced Fit Model ofEnzyme Function : 

The Induced Fit Model ofEnzyme Function 35 Figure 9.14

The Effect of Environment on Enzyme Activity : 

The Effect of Environment on Enzyme Activity enzyme activity is significantly impacted by substrate concentration, pH, and temperature 36

Effect of [substrate] : 

Effect of [substrate] rate increases as [substrate] increases no further increase occurs after all enzyme molecules are saturated with substrate 37 Figure 9.15

Effect of pH and Temperature : 

Effect of pH and Temperature each enzyme has specific pH and temperature optima denaturation loss of enzyme’s structure and activity when temperature and pH rise too much above optima 38

Enzyme Inhibition : 

Enzyme Inhibition 39 competitive inhibitor directly competes with binding of substrate to active site noncompetitive inhibitor binds enzyme at site other than active site; changes enzyme’s shape so that it becomes less active

Competitive Inhibition of Enzyme Activity : 

40 Competitive Inhibition of Enzyme Activity Figure 9.16

The Nature and Significance of Metabolic Regulation : 

The Nature and Significance of Metabolic Regulation conservation of energy and materials maintenance of metabolic balance despite changes in environment three major mechanisms metabolic channeling regulation of the amount of synthesis of a particular enzyme posttranscriptional regulation 41

Metabolic Channeling : 

Metabolic Channeling differential localization of enzymes and metabolites compartmentation differential distribution of enzymes and metabolites among separate cell structures or organelles 42 Figure 9.17 can generate marked variations in metabolite concentrations

Control of Enzyme Activity : 

Control of Enzyme Activity two important reversible control measures allosteric regulation noncovalent binding of allosteric effector changes activity of enzyme covalent modification covalent binding of a phosphoryl, methyl, or adenyl group changes activity of enzyme 43

Allosteric Regulation : 

Allosteric Regulation 44 allosteric enzyme effector binding alters shape of active site enzyme inactive – can’t bind substrate enzyme catalyzes reaction Figure 9.18

Regulation of Glutamine Synthetase Activity – Covalent Modification of Enzymes : 

Regulation of Glutamine Synthetase Activity – Covalent Modification of Enzymes 45 Figure 9.19

Feedback Inhibition : 

Feedback Inhibition also called end product inhibition inhibition of one or more critical enzymes in a pathway regulates entire pathway pacemaker enzyme catalyzes the slowest or rate-limiting reaction in the pathway ensures balanced production of a pathway end product 46

Slide 47: 

47 Figure 9.20 each end product regulates its own branch of the pathway each end product regulates the initial pacemaker enzyme isoenzymes – different enzymes that catalyze same reaction

Chemotaxis : 

Chemotaxis an example of a complex behavior that is regulated by altering enzyme activity system involves a number of enzymes and other proteins that are regulated by covalent modification a major component is a phosphorelay system consisting of a sensor kinase and response regulator 48

Slide 49: 

49 modulation of the activity of the phosphorelay system determines the rotational direction of the flagella and whether the cell will run or tumble Figure 9.21 (a) Proteins and Signaling Pathways of Chemotaxis

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