Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 24
Текст из файла (страница 24)
Certain microorganisms that live in deep ocean vents, where sunlight is completely absent, derive the energy for converting ADP and Piinto ATP from the oxidation of reduced inorganic compounds. These reduced compounds originate in the centerof the earth and are released at the vents.OCO2 e 2 HOOSuccinateFumarate▲ FIGURE 2-25 Conversion of succinate to fumarate. In thisoxidation reaction, which occurs in mitochondria as part of thecitric acid cycle, succinate loses two electrons and two protons.These are transferred to FAD, reducing it to FADH2.ric) ions, a reaction that occurs as part of the process bywhich carbohydrates are degraded in mitochondria.
Eachoxygen atom receives two electrons, one from each of twoFe2 ions:2 Fe2 1⁄2 O2 On 2 Fe3 O2NAD and FAD Couple Many BiologicalOxidation and Reduction ReactionsIn many chemical reactions, electrons are transferred fromone atom or molecule to another; this transfer may or maynot accompany the formation of new chemical bonds. Theloss of electrons from an atom or a molecule is called oxidation, and the gain of electrons by an atom or a molecule iscalled reduction. Because electrons are neither created nordestroyed in a chemical reaction, if one atom or molecule isoxidized, another must be reduced. For example, oxygendraws electrons from Fe2 (ferrous) ions to form Fe3 (fer-Thus Fe2 is oxidized, and O2 is reduced. Such reactions inwhich one molecule is reduced and another oxidized oftenare referred to as redox reactions.
Oxygen is an electron acceptor in many redox reactions in aerobic cells.Many biologically important oxidation and reduction reactions involve the removal or the addition of hydrogenatoms (protons plus electrons) rather than the transfer of isolated electrons on their own. 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.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
