Essential Cell Biology 5th edition
110 CHAPTER 3 Energy, Catalysis, and BiosynthesisFigure 3−38 Biotin transfers a carboxylgroup to a substrate. Biotin is a vitaminthat is used by a number of enzymes totransfer a carboxyl group to a substrate.Shown here is the reaction in which biotin,held by the enzyme pyruvate carboxylase,accepts a carboxyl group from bicarbonateand transfers it to pyruvate, producingoxaloacetate, a molecule required in thecitric acid cycle (discussed in Chapter13). Other enzymes use biotin to transfercarboxyl groups to other molecules.Note that the synthesis of carboxylatedbiotin requires energy derived from ATPhydrolysis—a general feature that applies tomany activated carriers.carboxylatedbiotinO O –Chigh-energyN bondSONADPHOTRANSFER OF CH 3PENZYMECARBOXYL GROUPpyruvateC OcarboxylaseCO O –CARBOXYLATIONpyruvateOF BIOTINATPbiotinHNO O –O O – SOCCNCHH2OHOC ObicarbonateENZYMECpyruvateO O –carboxylaseoxaloacetateThe Synthesis of Biological Polymers Requiresan Energy InputThe macromolecules of the cell constitute the vast majority of its drymass—that is, the mass not due to water. These molecules are madefrom subunits (or monomers) that are linked together by bonds formedduring an enzyme-catalyzed condensation reaction. The reverse reaction—thebreakdown of ECB5 polymers—occurs e3.37-3.38 through enzyme-catalyzedhydrolysis reactions. These hydrolysis reactions are energetically favorable,whereas the corresponding biosynthetic reactions require an energyinput and are more complex (Figure 3−39).The nucleic acids (DNA and RNA), proteins, and polysaccharides are allpolymers that are produced by the repeated addition of a subunit ontoone end of a growing chain. The mode of synthesis of each of thesemacromolecules is outlined in Figure 3−40. As indicated, the condensationstep in each case depends on energy provided by the hydrolysis of anucleoside triphosphate. And yet, except for the nucleic acids, there areno phosphate groups left in the final product molecules. How, then, is theenergy of ATP hydrolysis coupled to polymer synthesis?Each type of macromolecule is generated by an enzyme-catalyzed pathwaythat resembles the one discussed previously for the synthesis ofthe amino acid glutamine (see Figure 3−32). The principle is exactly thesame, in that the –OH group that will be removed in the condensationreaction is first activated by forming a high-energy linkage to a secondmolecule. The mechanisms used to link ATP hydrolysis to the synthesisof proteins and polysaccharides, however, are more complex thanthat used for glutamine synthesis. In the biosynthetic pathways leadingFigure 3−39 In cells, macromolecules aresynthesized by condensation reactionsand broken down by hydrolysis reactions.Condensation reactions are all energeticallyunfavorable, whereas hydrolysis reactionsare all energetically favorable.H 2 OH 2 OA OH + H B A BA OH + H BCONDENSATIONHYDROLYSISenergeticallyunfavorableenergeticallyfavorable
Activated Carriers and Biosynthesis111(A) POLYSACCHARIDES(B) NUCLEIC ACIDSglucoseglycogenCH 2 OHOOHHO OHCH 2 OHOOHHOOCH 2 OHOOHOOCH 2OAOCH 2OAOHOHOHH 2Oenergy from nucleosidetriphosphate hydrolysisOOPOHO _OOPOHO _CH 2 OHOCH 2 OHOCH 2 OHORNAOCH 2OCOCH 2OCOHHOOHOOHOHOOHOHOOHOHH 2OOOHglycogen(C) PROTEINSproteinH OC CRNHRCHCOOHHHNamino acidHCRCOOHOnucleotideOHP O _OCH 2Oenergy from nucleosidetriphosphate hydrolysisGORNAP O _OCH 2OHOGOHH 2Oenergy from nucleosidetriphosphate hydrolysisOHOHH OC CRproteinNHRCHOCNHHCRCOOHFigure 3−40 The synthesis of macromolecules requires an input of energy.Synthesis of a portion of (A) a polysaccharide, (B) a nucleic acid, and (C) a protein isshown here. In each case, synthesis involves a condensation reaction in which wateris lost; the atoms involved are shaded in pink. Not shown is the consumption ofhigh-energy nucleoside triphosphates that is required to activate each subunit priorto its addition. In contrast, the reverse reaction—the breakdown of all three types ofpolymers—occurs through the simple addition of water, or hydrolysis (not shown).to these macromolecules, several high-energy intermediates are consumedin series to generate the final high-energy bond that will bebroken during the condensation step. One important example of such abiosynthetic reaction, that of protein synthesis, is discussed in detail inChapter 7.There are limits to what each activated carrier ECB5 can e3.39/3.40 do in driving biosynthesis.For example, the ΔG for the hydrolysis of ATP to ADP andinorganic phosphate (P i ) depends on the concentrations of all of the reactants,and under the usual conditions in a cell, it is between –46 and –54kJ/mole. In principle, this hydrolysis reaction can be used to drive anunfavorable reaction with a ΔG of, perhaps, +40 kJ/mole, provided that asuitable reaction path is available. For some biosynthetic reactions, however,even –54 kJ/mole may be insufficient. In these cases, the path ofATP hydrolysis can be altered so that it initially produces AMP and pyrophosphate(PP i ), which is itself then hydrolyzed in solution in a subsequentstep (Figure 3−41). The whole process makes available a total ΔG of about–109 kJ/mole. The biosynthetic reaction involved in the synthesis ofnucleic acids (polynucleotides) is driven in this way (Figure 3−42).QUESTION 3–9Which of the following reactions willoccur only if coupled to a second,energetically favorable reaction?A. glucose + O 2 → CO 2 + H 2 OB. CO 2 + H 2 O → glucose + O 2C. nucleoside triphosphates →DNAD. nucleotide bases → nucleosidetriphosphatesE. ADP + P i → ATP
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Activated Carriers and Biosynthesis
111
(A) POLYSACCHARIDES
(B) NUCLEIC ACIDS
glucose
glycogen
CH 2 OH
O
OH
HO OH
CH 2 OH
O
OH
HO
O
CH 2 OH
O
OH
O
O
CH 2
O
A
O
CH 2
O
A
OH
OH
OH
H 2O
energy from nucleoside
triphosphate hydrolysis
O
O
P
OH
O _
O
O
P
OH
O _
CH 2 OH
O
CH 2 OH
O
CH 2 OH
O
RNA
O
CH 2
O
C
O
CH 2
O
C
OH
HO
OH
O
OH
OH
O
OH
OH
O
OH
OH
H 2O
O
OH
glycogen
(C) PROTEINS
protein
H O
C C
R
N
H
R
C
H
C
O
OH
H
H
N
amino acid
H
C
R
C
O
OH
O
nucleotide
OH
P O _
O
CH 2
O
energy from nucleoside
triphosphate hydrolysis
G
O
RNA
P O _
O
CH 2
OH
O
G
OH
H 2O
energy from nucleoside
triphosphate hydrolysis
OH
OH
H O
C C
R
protein
N
H
R
C
H
O
C
N
H
H
C
R
C
O
OH
Figure 3−40 The synthesis of macromolecules requires an input of energy.
Synthesis of a portion of (A) a polysaccharide, (B) a nucleic acid, and (C) a protein is
shown here. In each case, synthesis involves a condensation reaction in which water
is lost; the atoms involved are shaded in pink. Not shown is the consumption of
high-energy nucleoside triphosphates that is required to activate each subunit prior
to its addition. In contrast, the reverse reaction—the breakdown of all three types of
polymers—occurs through the simple addition of water, or hydrolysis (not shown).
to these macromolecules, several high-energy intermediates are consumed
in series to generate the final high-energy bond that will be
broken during the condensation step. One important example of such a
biosynthetic reaction, that of protein synthesis, is discussed in detail in
Chapter 7.
There are limits to what each activated carrier ECB5 can e3.39/3.40 do in driving biosynthesis.
For example, the ΔG for the hydrolysis of ATP to ADP and
inorganic phosphate (P i ) depends on the concentrations of all of the reactants,
and under the usual conditions in a cell, it is between –46 and –54
kJ/mole. In principle, this hydrolysis reaction can be used to drive an
unfavorable reaction with a ΔG of, perhaps, +40 kJ/mole, provided that a
suitable reaction path is available. For some biosynthetic reactions, however,
even –54 kJ/mole may be insufficient. In these cases, the path of
ATP hydrolysis can be altered so that it initially produces AMP and pyrophosphate
(PP i ), which is itself then hydrolyzed in solution in a subsequent
step (Figure 3−41). The whole process makes available a total ΔG of about
–109 kJ/mole. The biosynthetic reaction involved in the synthesis of
nucleic acids (polynucleotides) is driven in this way (Figure 3−42).
QUESTION 3–9
Which of the following reactions will
occur only if coupled to a second,
energetically favorable reaction?
A. glucose + O 2 → CO 2 + H 2 O
B. CO 2 + H 2 O → glucose + O 2
C. nucleoside triphosphates →
DNA
D. nucleotide bases → nucleoside
triphosphates
E. ADP + P i → ATP
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