H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 24
Текст из файла (страница 24)
The oxidation of succinate tofumarate, which also occurs in mitochondria, is an example(Figure 2-25). Protons are soluble in aqueous solutions (asH3O), but electrons are not and must be transferred di-(b)(a)Oxidized: FADOxidized: NADHHOC+NHNH2 H 2eOC••NicotinamideHReduced: NADHReduced: FADH2HOH3CNH3CNHN 2 H 2 eNH2NNOFlavinHHH3CNH3CNHRiboseRibitolRibitol2P2P2PAdenosineAdenosineNAD H 2 eNADH▲ FIGURE 2-26 The electron-carrying coenzymes NAD andFAD.
(a) NAD (nicotinamide adenine dinucleotide) is reduced toNADH by addition of two electrons and one proton simultaneously.In many biological redox reactions (e.g., succinate n fumarate),a pair of hydrogen atoms (two protons and two electrons) areremoved from a molecule. One of the protons and both electronsNOH2PHNRiboseAdenosineOAdenosineFAD 2 H 2 eFADH2are transferred to NAD; the other proton is released into solution.(b) FAD (flavin adenine dinucleotide) is reduced to FADH2 byaddition of two electrons and two protons.
In this two-step reaction,addition of one electron together with one proton first generatesa short-lived semiquinone intermediate (not shown), which thenaccepts a second electron and proton.Key Termsrectly from one atom or molecule to another without awater-dissolved intermediate. In this type of oxidation reaction, electrons often are transferred to small electron-carrying molecules, sometimes referred to as coenzymes. The mostcommon of these electron carriers are NAD (nicotinamideadenine dinucleotide), which is reduced to NADH, and FAD(flavin adenine dinucleotide), which is reduced to FADH2(Figure 2-26). The reduced forms of these coenzymes cantransfer protons and electrons to other molecules, thereby reducing them.To describe redox reactions, such as the reaction of ferrous ion (Fe2) and oxygen (O2), it is easiest to divide theminto two half-reactions:Oxidation of Fe2:Reduction of O2:2 Fe2 On 2 Fe3 2 e2 e 1⁄2 O2 On O22In this case, the reduced oxygen (O ) readily reacts withtwo protons to form one water molecule (H2O).
The readiness with which an atom or a molecule gains an electron is itsreduction potential E. The tendency to lose electrons, the oxidation potential, has the same magnitude but opposite signas the reduction potential for the reverse reaction.Reduction potentials are measured in volts (V) from anarbitrary zero point set at the reduction potential of the following half-reaction under standard conditions (25 ºC,1 atm, and reactants at 1 M):reductionKEY CONCEPTS OF SECTION 2.4Biochemical EnergeticsThe change in free energy G is the most useful measure for predicting the direction of chemical reactions in biological systems.
Chemical reactions tend to proceed in thedirection for which G is negative.■Directly or indirectly, light energy captured by photosynthesis in plants and photosynthetic bacteria is the ultimate source of chemical energy for almost all cells.■The chemical free-energy change Gº equals 2.3 RTlog Keq. Thus the value of Gº can be calculated from theexperimentally determined concentrations of reactants andproducts at equilibrium.■A chemical reaction having a positive G can proceedif it is coupled with a reaction having a negative G oflarger magnitude.■Many otherwise energetically unfavorable cellular processes are driven by hydrolysis of phosphoanhydride bondsin ATP (see Figure 2-24).■An oxidation reaction (loss of electrons) is always coupled with a reduction reaction (gain of electrons).■Biological oxidation and reduction reactions often arecoupled by electron-carrying coenzymes such as NADand FAD (see Figure 2-26).■Oxidation-reduction reactions with a positive E havea negative G and thus tend to proceed spontaneously.H e 399999994 1⁄2 H2■oxidationThe value of E for a molecule or an atom under standardconditions is its standard reduction potential E0.
A molecule or ion with a positive E0 has a higher affinity for electrons than the H ion does under standard conditions.Conversely, a molecule or ion with a negative E0 has a loweraffinity for electrons than the H ion does under standardconditions. Like the values of Gº, standard reduction potentials may differ somewhat from those found under theconditions in a cell because the concentrations of reactants ina cell are not 1 M.In a redox reaction, electrons move spontaneously toward atoms or molecules having more positive reductionpotentials.
In other words, a compound having a more negative reduction potential can transfer electrons to (i.e., reduce) a compound with a more positive reductionpotential. In this type of reaction, the change in electric potential E is the sum of the reduction and oxidation potentials for the two half-reactions. The E for a redox reactionis related to the change in free energy G by the followingexpression:G (cal/mol) n (23,064) E (volts)55(2-11)where n is the number of electrons transferred. Note that aredox reaction with a positive E value will have a negativeG and thus will tend to proceed from left to right.KEY TERMSacid 48 carbon atom (C) 38amino acids 38amphipathic 29base 48buffers 48chemical potential energy 50covalent bond 30dehydration reaction 37G (free-energy change) 51disulfide bond 38energy coupling 53enthalpy (H) 51entropy (S) 51equilibrium constant 46fatty acids 43hydrogen bond 33hydrophilic 29hydrophobic 29hydrophobic effect 35ionic interactions 33molecular complementarity 36monosaccharides 41nucleosides 40nucleotides 40oxidation 55pH 47phosphoanhydride bonds 52phospholipid bilayers 45polar 32polymer 37redox reaction 54reduction 55saturated 43steady state 46stereoisomers 31unsaturated 43van der Waals interactions 3456CHAPTER 2 • Chemical FoundationsREVIEW THE CONCEPTS1.
The gecko is a reptile with an amazing ability to climbsmooth surfaces, including glass. Recent discoveries indicatethat geckos stick to smooth surfaces via van der Waals interactions between septae on their feet and the smooth surface. How is this method of stickiness advantageous overcovalent interactions? Given that van der Waals forces areamong the weakest molecular interactions, how can thegecko’s feet stick so effectively?2. The K channel is an example of a transmembrane protein (a protein that spans the phospholipid bilayer of theplasma membrane). What types of amino acids are likely tobe found (a) lining the channel through which K passes;(b) in contact with the phospholipid bilayer containing fattyacid; (c) in the cytosolic domain of the protein; and (d) inthe extracellular domain of the protein?3.
V-M-Y-Y-E-N: This is the single-letter amino acid abbreviation for a peptide. Draw the structure of this peptide.What is the net charge of this peptide at pH 7.0? An enzymecalled a protein tyrosine kinase can attach phosphates to thehydroxyl groups of tyrosine. What is the net charge of thepeptide at pH 7.0 after it has been phosphorylated by a tyrosine kinase? What is the likely source of phosphate utilizedby the kinase for this reaction?4. Disulfide bonds help to stabilize the three-dimensionalstructure of proteins.
What amino acids are involved in theformation of disulfide bonds? Does the formation of a disulfide bond increase or decrease entropy (S)?5. In the 1960s, the drug thalidomide was prescribed topregnant women to treat morning sickness. However,thalidomide caused severe limb defects in the children ofsome women who took the drug, and its use for morningsickness was discontinued. It is now known that thalidomidewas administered as a mixture of two stereoisomeric compounds, one of which relieved morning sickness and theother of which was responsible for the birth defects. Whatare stereoisomers? Why might two such closely related compounds have such different physiologic effects?6. Name the compound shown below.
Is this nucleotide acomponent of DNA, RNA, or both? Name one other function of this compound.OC6HN1C2H2NOO3N5C478 CH9CNNO5OPOOOPOOOPOOCH24OHH1HH3OH2OH7. The chemical basis of blood-group specificity resides inthe carbohydrates displayed on the surface of red blood cells.Carbohydrates have the potential for great structural diversity. Indeed, the structural complexity of the oligosaccharidesthat can be formed from four sugars is greater than that foroligopeptides from four amino acids. What properties of carbohydrates make this great structural diversity possible?8. Ammonia (NH3) is a weak base that under acidic conditions becomes protonated to the ammonium ion in the following reaction:NH3 H n NH4NH3 freely permeates biological membranes, including thoseof lysosomes.
The lysosome is a subcellular organelle with apH of about 5.0; the pH of cytoplasm is 7.0. What is the effect on the pH of the fluid content of lysosomes when cellsare exposed to ammonia? Note: Protonated ammonia doesnot diffuse freely across membranes.9. Consider the binding reaction L R n LR, where L isa ligand and R is its receptor. When 1 103 M L is addedto a solution containing 5 102 M R, 90% of the L bindsto form LR. What is the Keq of this reaction? How will theKeq be affected by the addition of a protein that catalyzes thisbinding reaction? What is the Kd?10. What is the ionization state of phosphoric acid in the cytoplasm? Why is phosphoric acid such a physiologically important compound?11.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
