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ions have revealed many species of marine life at great ocean depths, where the oxygen concentration is near zero. For example, the primitive coelacanth, a large fish recovered from depths of 4,000 m or more off the coast of South Africa, has an essentially anaerobic metabolism in virtually all its tissues. It converts... |
ds that lower the pH. The silage that results from this fermentation 8885d_c14_521-559 2/6/04 3:43 PM Page 542 mac76 mac76:385_reb: 542 Chapter 14 Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway BOX 14–2 THE WORLD OF BIOCHEMISTRY Brewing Beer Brewers prepare beer by ethanol fermentation of the carbohydra... |
sis are not identical pathways running in opposite directions, although they do share several steps (Fig. 14–16); seven of the ten enzymatic reactions of gluconeogenesis are the reverse of glycolytic reactions. However, three reactions of glycolysis are essentially irreversible in vivo and cannot be used in gluconeogen... |
o the cytosol and its reconversion there to oxaloacetate effectively moves reducing equivalents to the cytosol, where they are scarce. This path from pyruvate to PEP therefore provides an important balance between NADH produced and consumed in the cytosol during gluconeogenesis. A second pyruvate n PEP bypass predomina... |
glycolysis goes up, the flux of pyruvate toward glucose goes down, and vice versa. SUMMARY 14.4 Gluconeogenesis ■ Gluconeogenesis is a ubiquitous multistep process in which pyruvate or a related three-carbon compound (lactate, alanine) is converted to glucose. Seven of the steps in gluconeogenesis are catalyzed by the... |
s ribulose 5-phosphate to its aldose isomer, ribose 5-phosphate. In some tissues, the pentose phosphate pathway ends at this point, and its overall equation is Glucose 6-phosphate 2NADP H2O 88n ribose 5-phosphate CO2 2NADPH 2H The net result is the production of NADPH, a reductant for biosynthetic reactions, and ribose... |
y. As a result, more glucose 6-phosphate is available for glycolysis. 8885d_c14_521-559 2/6/04 3:43 PM Page 555 mac76 mac76:385_reb: SUMMARY 14.5 Pentose Phosphate Pathway of Glucose Oxidation ■ The oxidative pentose phosphate pathway (phosphogluconate pathway, or hexose monophosphate pathway) brings about oxidation an... |
yoff phase of glycolysis (with lactate as the end product), including the net standard free-energy change. 3. Pathway of Atoms in Fermentation A “pulse-chase” experiment using 14C-labeled carbon sources is carried out on a yeast extract maintained under strictly anaerobic conditions to produce ethanol. The experiment c... |
ctate Levels A congenital defect in the liver enzyme fructose 1,6-bisphosphatase results in abnormally high levels of lactate in the blood plasma. Explain. Chapter 14 Problems 559 23. Effect of Phloridzin on Carbohydrate Metabolism Phloridzin, a toxic glycoside from the bark of the pear tree, blocks the normal reabsorp... |
cussed in this chapter and in Chapter 14 are central to the metabolism of most organisms, microbial, animal, or plant. We begin with a discussion of the catabolic pathways from glycogen to glucose 6-phosphate (glycogenolysis) and from glucose 6-phosphate to pyruvate (glycolysis), then turn to the anabolic pathways from... |
itten in present-day biochemistry textbooks about the metabolism of glycogen was discovered between about 1925 and 1950 by the remarkable husband and wife team of Carl F. Cori and Gerty T. Cori. Both trained in medicine in Europe at the end of World War I (she completed premedical studies and medical school in one year... |
lycogenin. At this point, glycogen synthase takes over, further extending the glycogen chain. Glycogenin remains buried within the particle, covalently attached to the single reducing end of the glycogen molecule (Fig. 15–11b). FIGURE 15–10 Glycogenin structure. (PDB 1D 1772) Muscle glycogenin (Mr 37,000) forms dimers ... |
tion of AMPK (not to be confused with the cyclic AMP–dependent protein kinase; see Section 15.4) increases glucose transport and activates glycolysis and fatty acid oxidation, while suppressing energyrequiring processes such as the synthesis of fatty acids, cholesterol, and protein. We discuss this enzyme further, and ... |
ymes, that become active only when a proteolytic event removes an inhibitory sequence in the proenzyme. As a result of these several mechanisms of regulating enzyme level, cells can change their complement of enzymes in response to changes in metabolic circumstances. In vertebrates, liver is the most adaptable tissue; ... |
) has a high affinity for glucose—it is half-saturated at about 0.1 mM. Because glucose entering myocytes from the blood (where the glucose concentration is 4 to 5 mM) produces an intracellular glucose concentration high enough to saturate hexokinase II, the enzyme normally acts at or near its maximal rate. Muscle hexo... |
ric regulator of PFK-1 is fructose 2,6-bisphosphate, which strongly activates the enzyme. We return to this role of fructose 2,6bisphosphate later. Pyruvate Kinase Is Allosterically Inhibited by ATP At least three isozymes of pyruvate kinase are found in vertebrates, differing in their tissue distribution and their res... |
esponse to glucagon. Insulin has the opposite effect, stimulating the activity of a phosphoprotein phosphatase that catalyzes removal of the phosphoryl group from the bifunctional protein PFK-2/FBPase-2, activating its PFK-2 activity, increasing the level of fructose 2,6-bisphosphate, stimulating glycolysis, and inhibi... |
ation of Ser residues in each of the two identical subunits of glycogen phosphorylase, activating it and thus stimulating glycogen breakdown. In muscle, this provides fuel for glycolysis to sustain muscle contraction for the fight-or-flight response signaled by epinephrine. In liver, glycogen breakdown counters the low... |
the enzyme with a Ser residue at position 0, which it phosphorylates. This creates a new priming site, and the enzyme moves down the protein to phosphorylate the Ser residue at position 4, and then the Ser at 8. (b) GSK3 has a Ser residue near its amino terminus that can be phosphorylated by PKA or PKB (see Fig. 15–29... |
Glucose release to blood metabolism in muscle reflects these differences from liver. First, myocytes lack receptors for glucagon. Second, the muscle isozyme of pyruvate kinase is not phosphorylated by PKA, so glycolysis is not turned off when [cAMP] is high. In fact, cAMP increases the rate of glycolysis in muscle, pr... |
e 0.5 1.0 1.5 2.0 2.5 3.0 Enzyme added (arbitrary units) FIGURE 15–33 Dependence of glycolytic flux in a rat liver homogenate on added enzymes. Purified enzymes in the amounts shown on the x axis were added to an extract of liver carrying out glycolysis in vitro. The increase in flux through the pathway is shown on the... |
ow that enzyme’s catalytic activity changes when the concentration of a metabolite—substrate, product, or effector—changes. It is obtained from an experimental plot of the rate of the reaction catalyzed by the enzyme versus the concentration of the metabolite, at metabolite concentrations that prevail in the cell. By a... |
bons, B.J., Roach, P.J., & Hurley, T.D. (2002) Crystal structure of the autocatalytic initiator of glycogen biosynthesis, glycogenin. J. Mol. Biol. 319, 463–477. Melendez-Hevia, E., Waddell, T.G., & Shelton, E.D. (1993) Optimization of molecular design in the evolution of metabolism: the glycogen molecule. Biochem. J. ... |
ained? 4. Are All Metabolic Reactions at Equilibrium? (a) Phosphoenolpyruvate (PEP) is one of the two phosphoryl group donors in the synthesis of ATP during glycolysis. In human erythrocytes, the steady-state concentration of ATP is 2.24 mM, that of ADP is 0.25 mM, and that of pyruvate is 0.051 mM. Calculate the concen... |
ored triacylglycerols. Hans Krebs, 1900–1981 601 8885d_c16_601-630 1/27/04 8:54 AM Page 602 mac76 mac76:385_reb: 602 Chapter 16 The Citric Acid Cycle Amino acids Fatty acids Glucose Stage 1 Acetyl-CoA production Glycolysis Pyruvate e e e pyruvate dehydrogenase complex CO2 e Acetyl-CoA Stage 2 Acetyl-CoA oxidation Oxalo... |
the site where a lipoyl group is attached to the lipoyl domain of E2. To make the structure clearer, about half of the complex has been cut away from the front. This model was prepared by Z. H. Zhou et al. (2001); in another model, proposed by J. L. S. Milne et al. (2002), the E3 subunits are located more toward the p... |
s required for the citric acid cycle, but also all the enzymes and proteins necessary for the last stage of respiration—electron transfer and ATP synthesis by oxidative phosphorylation. As we shall see in later chapters, mitochondria also contain the enzymes for the oxidation of fatty acids and some amino acids to acet... |
oA and CO2 The next step is another oxidative decarboxylation, in which -ketoglutarate is converted to succinyl-CoA and CO2 by the action of the -ketoglutarate dehydrogenase complex; NAD serves as electron acceptor and CoA as the carrier of the succinyl group. The energy of oxidation of -ketoglutarate is conserved in t... |
triphosphate such as ATP to an acceptor molecule—a sugar (as in hexokinase and glucokinase), a protein (as in glycogen phosphorylase kinase), another nucleotide (as in nucleoside diphosphate kinase), or a metabolic intermediate such as oxaloacetate (as in PEP carboxykinase). The reaction catalyzed by a kinase is a pho... |
OO . (c) Correct complementary fit of citrate to the binding site of aconitase. There is only one way in which the three specified groups of citrate can fit on the three points of the binding site. Thus only one of the two OCH2COO groups is bound by aconitase. In 1948, however, Alexander Ogston pointed out that althoug... |
toxic to many plants and causes decreased crop yields on 30% to 40% of the world’s arable land. Aluminum is the most abundant metal in the earth’s crust, yet it occurs mostly in chemical compounds, such as Al(OH)3, that are biologically inert. However, when soil pH is less than 5, Al3 be- comes soluble and thus can be ... |
required to keep the cell in a stable steady state while avoiding wasteful overproduction. The flow of carbon atoms from pyruvate into and through the citric acid cycle is under tight regulation at two levels: the conversion of pyruvate to acetyl-CoA, the starting material for the cycle (the pyruvate dehydrogenase comp... |
tylCoA, NADH, and fatty acids) and stimulated by metabolites that indicate a reduced energy supply (AMP, NAD, CoA). 16.4 The Glyoxylate Cycle Vertebrates cannot convert fatty acids, or the acetate derived from them, to carbohydrates. Conversion of phosphoenolpyruvate to pyruvate (p. 532) and of pyruvate to acetyl-CoA (... |
ucose ATP Amino acids, nucleotides FIGURE 16–23 Coordinated regulation of glyoxylate and citric acid cycles. Regulation of isocitrate dehydrogenase activity determines the partitioning of isocitrate between the glyoxylate and citric acid cycles. When the enzyme is inactivated by phosphorylation (by a specific protein k... |
NAD Oxidized Reduced For each of the reactions in (a) through (f), determine whether the substrate has been oxidized or reduced or is unchanged in oxidation state (see Problem 2). If a redox change has occurred, balance the reaction with the necessary amount of NAD, NADH, H, and H2O. The objective is to recognize when... |
l-CoA added 20 40 60 80 100 120 [Acetyl-CoA] ( M) On the basis of these observations, suggest how succinyl-CoA regulates the activity of citrate synthase. (Hint: See Fig. 6–29.) Why is succinyl-CoA an appropriate signal for regulation of the citric acid cycle? How does the regulation of citrate synthase control the rat... |
laries of these tissues, the extracellular enzyme lipoprotein lipase, activated by apoC-II, hydrolyzes triacylglycerols to fatty acids and glycerol (step 6 ), which are taken up by cells in the target tissues (step 7 ). In muscle, the fatty acids are oxidized for energy; in adipose tissue, they are reesterified for sto... |
t the cytosolic side of the outer mitochondrial membrane can be transported into the mitochondrion and oxidized to produce ATP, or they can be used in the cytosol to synthesize fatty acyl–CoA synthetase 1 Adenosine ATP Fatty acid O P O O Adenosine 2 AMP R C CoA-SH fatty acyl–CoA synthetase O O O Fatty acyl–adenylate (e... |
per electron pair. In the second step of the -oxidation cycle (Fig. 17–8a), water is added to the double bond of the trans-2-enoyl-CoA to form the L stereoisomer of -hydroxyacyl-CoA (3-hydroxyacyl-CoA). This reaction, catalyzed by enoyl-CoA hydratase, is formally analogous to the fumarase reaction in the citric acid cy... |
rans double bond of the 2-enoyl-CoA generated during oxidation. Two auxiliary enzymes are needed for oxidation of the common unsaturated fatty acids: an isomerase and a reductase. We illustrate these auxiliary reactions with two examples. TABLE 17–1 Yield of ATP during Oxidation of One Molecule of Palmitoyl-CoA to CO2 ... |
ine commonly contains high levels of 6-carbon to 10-carbon dicarboxylic acids (produced by oxidation) and low levels of urinary ketone bodies (we discuss oxidation below and ketone bodies in Section 17.3). Although individuals may have no symptoms between episodes, the episodes are very serious; mortality from this dis... |
ctions also occur in glyoxysomes, as discussed below.) One difference between the peroxisomal and mitochondrial pathways is in the chemistry of the first step. In peroxisomes, the flavoprotein acyl-CoA oxidase that introduces the double bond passes electrons directly to O2, producing H2O2 (Fig. 17–13). This strong and ... |
reticulum of liver and kidney, and the preferred substrates are fatty acids of 10 or 12 carbon atoms. In mammals oxidation is normally a minor pathway for fatty acid degradation, but when oxidation is defective (because of mutation or a carnitine deficiency, for example) it becomes more important. The first step intro... |
in a reaction catalyzed by -ketoacyl-CoA transferase. The acetoacetyl-CoA is then cleaved by thiolase to yield two acetyl-CoAs, which enter the citric acid cycle. Thus the ketone bodies are used as fuels. The production and export of ketone bodies by the liver allow continued oxidation of fatty acids with only minimal... |
the form of triacylglycerols? Recall that 1.00 kcal 4.18 kJ. (b) If the basal energy requirement is approximately 8,400 kJ/day (2,000 kcal/day), how long could this person survive if the oxidation of fatty acids stored as triacylglycerols were the only source of energy? (c) What would be the weight loss in pounds per d... |
d from amino acids, whether they are derived from dietary protein or from tissue protein, varies greatly with the type of organism and with metabolic conditions. Carnivores can obtain (immediately following a meal) up to 90% of their energy requirements from amino acid oxidation, whereas herbivores may fill only a smal... |
ypsinogen, chymotrypsinogen, and procarboxypeptidases A and B, the zymogens of trypsin, chymotrypsin, and carboxypeptidases A and B, are synthesized and secreted by the exocrine cells of the pancreas (Fig. 18–3b). Trypsinogen is converted to its active form, trypsin, by enteropeptidase, a proteolytic enzyme secreted by... |
mine) H R C NH CH HO CH3 N H Carbanion R C H NH CH HO P HO P R H C C NH CH O O P CH 3 N H Quinonoid intermediate CH3 N H Schiff base intermediate (aldimine) Resonance structures for stabilization of a carbanion by PLP A B CO2 C R COO C NH H CH HO P CH3 : N H Quinonoid intermediate H R C COO NH CH HO P : CH3 N H Quinono... |
e, in concert with the Cori cycle (see Box 14–1 and Fig. 23–18), accomplishes this transaction. The energetic burden of gluconeogenesis is thus imposed on the liver rather than the muscle, and all available ATP in muscle is devoted to muscle contraction. Ammonia Is Toxic to Animals The catabolic production of ammonia p... |
l) group of citrulline, forming argininosuccinate (step 2 in Fig. 18–10). This cytosolic reaction, catalyzed by argininosuccinate synthetase, requires ATP and proceeds through a citrullyl-AMP intermediate (Fig. 18–11b). The argininosuccinate is then cleaved by argininosuccinase (step 3 in Fig. 18–10) to form free argin... |
d by administering carbamoyl glutamate, an analog of N-acetylglutamate that is effective in activating carbamoyl phosphate synthetase I. O C H2N COO NH C H CH2 CH2 COO Carbamoyl glutamate Supplementing the diet with arginine is useful in treating deficiencies of ornithine transcarbamoylase, argininosuccinate synthetase... |
orm spontancously. Conversion of N5-formyltetrahydrofolate to N5, N10-methenyltetrahydrofolate, requires ATP, because of an otherwise unfavorable equilibrium. Note that N5-formiminotetrahydrofolate is derived from histidine in a pathway shown in Figure 18–26. Serine Glycine H3N COO C H CH2OH COO C H H3N H H2O PLP serin... |
lay of the pyridoxal phosphate and tetrahydrofolate cofactors in serine and glycine metabolism. The first step in each of these reactions (not shown) involves the formation of a covalent imine linkage between enzyme-bound PLP and the substrate amino acid—serine in (a), glycine in (b) and (c). (a) The serine dehydratase... |
rries electrons from NADH to O2 and becomes oxidized to dihydrobiopterin in the process (Fig. 18–24). It is subsequently reduced by the enzyme dihydrobiopterin reductase in a reaction that requires NADH. In individuals with PKU, a secondary, normally little-used pathway of phenylalanine metabolism comes into play. In t... |
cid Degradation 683 Branched-Chain Amino Acids Are Not Degraded in the Liver Although much of the catabolism of amino acids takes place in the liver, the three amino acids with branched side chains (leucine, isoleucine, and valine) are oxidized as fuels primarily in muscle, adipose, kidney, and brain tissue. These extr... |
aminotransferase PLP Glutamate O C COO C CH2 O O Oxaloacetate FIGURE 18–29 Catabolic pathway for asparagine and aspartate. Both amino acids are converted to oxaloacetate. SUMMARY 18.3 Pathways of Amino Acid Degradation ■ After removal of their amino groups, the carbon skeletons of amino acids undergo oxidation to comp... |
ermediates of isoleucine degradation (I to V) shown below are not in the proper order. Use your knowledge and understanding of the citric acid cycle and -oxidation pathway to arrange the intermediates in the proper metabolic sequence for isoleucine degradation. O S-CoA C C CH3 H C CH3 I O O C OC H C CH3 CH2 CH3 II O S-... |
more than 10,000 sets of electron-transfer systems (respiratory chains) and ATP synthase molecules, distributed over the membrane surface. Heart mitochondria, which have more profuse cristae and thus a much larger area of inner membrane, contain more than three times as many sets of electron-transfer systems as liver m... |
itrogen atoms are coordinated with a central Fe ion, either Fe2 Fe3 . Iron protoporphyrin IX is found in b-type cytochromes and in hemoglobin and myoglobin (see Fig. 4–17). Heme c is covalently bound to the protein of cytochrome c through thioether bonds to two Cys residues. Heme a, found in the a-type cytochromes, has... |
first enzyme of oxidation) transfers electrons to electrontransferring flavoprotein (ETF), from which they pass to Q via ETF:ubiquinone oxidoreductase. exergonic transfer to ubiquinone of a hydride ion from NADH and a proton from the matrix, expressed by (19–1) NADH H Q On NAD QH2 and (2) the endergonic transfer of fo... |
which blocks electron flow from heme bH to Q, binds at QN, close to heme bH on the N (matrix) side of the membrane. Myxothiazol, which prevents electron flow from and protons through the complex. The net equation for the redox reactions of this Q cycle (Fig. 19–12) is QH2 2 cyt c1(oxidized) 2H N On Q 2 cyt c1(reduced) ... |
culated free-energy change for pumping protons outward is about 20 kJ/mol (of H), most of which is contributed by the electrical portion of the electrochemical potential. Because the transfer of two electrons from NADH to O2 is accompanied by the outward pumping of 10 H (Eqn 19–7), roughly 200 kJ of the 220 kJ released... |
nide (CN transfer between cytochrome oxidase and O2, inhibits both respiration and ATP synthesis. (b) Mitochondria provided with succinate respire and synthesize ATP only when ADP and Pi are added. Subsequent addition of venturicidin or oligomycin, inhibitors of ATP synthase, blocks both ATP synthesis and respiration. ... |
76 b-Arg182 FIGURE 19–21 Catalytic mechanism of F1. (a) 18O-exchange experiment. F1 solubilized from mitochondrial membranes is incubated with ATP in the presence of 18O-labeled water. At intervals, a sample of the solution is withdrawn and analyzed for the incorporation of 18O into the Pi produced from ATP hydrolysis.... |
changes conformation, assuming the -ATP form that tightly binds and stabilizes ATP, bringing about the ready equilibration of ADP Pi with ATP on the enzyme surface. Finally, the subunit changes to the -empty conformation, which has very low affinity for ATP, and the newly synthesized ATP leaves the enzyme surface. Ano... |
hibited, cytosolic ATP cannot be regenerated from ADP, explaining the toxicity of atractyloside. A second membrane transport system essential to oxidative phosphorylation is the phosphate translo and one case, which promotes symport of one H2PO4 H into the matrix. This transport process, too, is favored by the transmem... |
P hydrolysis (and synthesis). (Parts of IF1 that failed to resolve in crystals of F1 are shown in white outline as they occur in crystals of isolated IF1.) This complex is stable only at the low cytosolic pH characteristic of cells that are producing ATP by glycolysis; when aerobic metabolism resumes, the cytosolic pH ... |
e II) diabetes mellitus. Other mutations in mitochondrial genes are believed to be responsible for the progressive muscular weakness that characterizes mitochondrial myopathy and for enlargement and deterioration of the heart muscle in hypertrophic cardiomyopathy. According to one hypothesis on the progressive changes ... |
, cells have several forms of the enzyme superoxide dismutase, which catalyzes the reaction O2 2H 88n H2O2 O2 2 The hydrogen peroxide (H2O2) generated by this reaction is rendered harmless by the action of glutathione peroxidase (Fig. 19–35). This enzyme is remarkable for the presence of a selenocysteine residue (see F... |
apidly to its normal lower-energy orbital; the excited molecule decays to the stable ground state, giving up the absorbed quantum as light or heat or using it to do chemical work. Light emission accompanying decay of excited molecules (called fluorescence) is always at a longer wavelength (lower energy) than that of th... |
termine the action spectrum for photosynthesis. (a) Results of a classic experiment performed by T. W. Englemann in 1882 to determine the wavelength of light that is most effective in supporting photosynthesis. Englemann placed cells of a filamentous photosynthetic alga on a microscope slide and illuminated them with l... |
reen sulfur bacteria have two routes for electrons driven by excitation of P840: a cyclic route passes through a quinone to the cytochrome bc1 complex and back to the reaction center via cytochrome c, and a noncyclic route from the reaction center through the iron-sulfur protein ferredoxin (Fd), in a reaction catalyzed... |
ows) by photons absorbed in PSII and PSI. One photon is required per electron in each photosystem. After excitation, the high-energy electrons flow “downhill” through the carrier chains shown. Protons move across the thylakoid membrane during the water-splitting reaction and during electron transfer through the cytochr... |
horylates a Thr residue in the hydrophobic domain of LHCII, which reduces its affinity for the neighboring thylakoid membrane and converts appressed regions (granal lamellae) to nonappressed regions (stromal lamellae). A specific protein phosphatase reverses this regulatory phosphorylation when the [PQ]/[PQH2] ratio in... |
in the water-splitting complex. With each single-electron transfer, the Mn cluster becomes more oxidized; four single-electron transfers, each corresponding to the absorption of one photon, produce a charge of 4 on the Mn complex (Fig. 19–56): 4 Tyr [Mn complex]0 88n 4 Tyr [Mn complex]4 (19–14) In this state, the Mn c... |
cause electrons to cycle continuously out of and back into the reaction center of PSI, each electron propelled around the cycle by the energy yielded by the absorption of one photon. Cyclic electron flow is not accompanied by net formation of NADPH or evolution of O2. However, it is accompanied by proton pumping by th... |
mplex lowered O C O O C Glu194 Glu204 Proton release mational changes in the protein that alter the distance between the Schiff base and its neighboring amino acid residues. Interaction with these neighbors lowers the pKa of the protonated Schiff base, and the base gives up its proton to a nearby carboxyl group on Asp8... |
vanced review of kinetic, structural, and biochemical evidence for the ATP synthase mechanism. Yasuda, R., Noji, H., Kinosita, K., Jr., & Yoshida, M. (1998) F1-ATPase is a highly efficient molecular motor that rotates with discrete 120 steps. Cell 93, 1117–1124. Graphical demonstration of the rotation of ATP synthase. ... |
ects of Valinomycin on Oxidative Phosphorylation When the antibiotic valinomycin is added to actively respiring mitochondria, several things happen: the yield of ATP decreases, the rate of O2 consumption increases, heat is released, and the pH gradient across the inner mitochondrial membrane increases. Does valinomycin... |
V for H2S and 320 mV for NAD. See Figure 19–39 for energy equivalents of wavelengths of light. 27. Equilibrium Constant for Water-Splitting Reactions The coenzyme NADP is the terminal electron acceptor in chloroplasts, according to the reaction 2H2O 2NADP On 2NADPH 2H O2 Use the information in Table 19–2 to calculate t... |
synthesis, can be used to make hexoses for fuel and building materials, sucrose for transport to nonphotosynthetic tissues, or starch for storage. Thus the overall process is cyclical, with the continuous conversion of CO2 to triose and hexose phosphates. Fructose 6-phosphate is a key intermediate in stage 3 of CO2 ass... |
this sugar, followed by aldol cleavage 4 , forms one molecule of 3-phosphoglycerate, which leaves the enzyme active site. 5 The carbanion of the remaining three-carbon fragment is protonated by the nearby side chain of Lys175, generating a second molecule of 3-phosphoglycerate. The overall reaction therefore accomplish... |
hree-carbon sugars to two pentoses (step 6 of Fig. 20–10). Transketolase catalyzes the reversible transfer of a 2-carbon ketol group (CH2OHOCOO) from a ketose phosphate donor, fructose 6-phosphate, to an aldose phosphate acceptor, glyceraldehyde 3-phosphate (Fig. 20–11a, b), forming the pentose xylulose 5-phosphate and... |
et to an as yet undetermined degree by mitochondria, but a second potential source of energy is the ATP and NADPH generated in the chloroplast stroma during the light reactions. However, neither ATP nor NADPH can cross the chloroplast membrane. The Pi–triose phosphate antiport system has the indirect effect of moving A... |
ses some previously fixed CO2. Given that the reaction with oxygen is deleterious to the organism, why did the evolution of rubisco produce an active site unable to discriminate well between CO2 and O2? Perhaps much of this evolution occurred before the time, about 2.5 billion years ago, when production of O2 by photos... |
rboxylated to yield pyruvate and CO2 by the action of malic enzyme, reducing NADP. In plants that use aspartate as the CO2 carrier, aspartate arriving in bundle-sheath cells is transaminated to form oxaloacetate and reduced to malate, then the CO2 is released by malic enzyme or PEP carboxykinase. As labeling experiment... |
orage it is synthesized in the amyloplasts of the nonphotosynthetic parts of plants—seeds, roots, and tubers (underground stems). Most of the triose phosphate generated by CO2 fixation in plants is converted to sucrose (Fig. 20–25) or starch. In the course of evolution, sucrose may have been selected as the transport f... |
esis is ADP-glucose pyrophosphorylase (Fig. 20–28); it is activated by 3-phosphoglycerate (which accumulates during active photosynthesis) and inhibited by Pi (which accumulates when light-driven condensation of ADP and Pi slows). When sucrose synthesis slows, 3-phosphoglycerate formed by CO2 fixation accumulates, acti... |
se synthase spans the plasma membrane and uses cytosolic UDP-glucose as the precursor for extracellular cellulose synthesis. In another, a membrane-bound form of sucrose synthase forms a complex with cellulose synthase, feeding UDP-glucose from sucrose directly into cell wall synthesis (Fig. 20–32). In the activated pr... |
antibiotics. By the early 1990s, 20% to 40% of Staphylococcus aureus (the causative agent of “staph” infections) was resistant to methicillin, and 32% of Neisseria gonorrhoeae (the causative agent of gonorrhea) was resistant to penicillin. By 1986, 32% of Shigella (a pathogen responsible for severe forms of dysentery,... |
Fructose 1,6-bisphosphate Ribose 5-phosphate Glyceraldehyde 3-phosphate Dihydroxyacetone phosphate Ribulose 5-phosphate Xylulose 5-phosphate 8885d_c20_783 2/20/04 12:03 PM Page 783 mac76 mac76:385_reb: Key Terms 752 752 752 752 Terms in bold are defined in the glossary. Calvin cycle plastids chloroplast amyloplast car... |
Illuminated Chlorella were grown with unlabeled CO2, then the light was turned off and 14CO2 was added (vertical dashed line in the graph below). Under these conditions, X was the first compound to become labeled with 14C; Y was unlabeled. CO2, light 14CO2, dark Time (b) Illuminated Chlorella cells were grown with 14C... |
y of fatty acids into triacylglycerols and the simpler membrane phospholipids. Finally, we consider the synthesis of cholesterol, a component of some membranes and the precursor of steroids such as the bile acids, sex hormones, and adrenocortical hormones. 21.1 Biosynthesis of Fatty Acids and Eicosanoids After the disc... |
RE 21–4 Acyl carrier protein (ACP). The prosthetic group is 4phosphopantetheine, which is covalently attached to the hydroxyl group of a Ser residue in ACP. Phosphopantetheine contains the B vitamin pantothenic acid, also found in the coenzyme A molecule. Its OSH group is the site of entry of malonyl groups during fatt... |
rates are also multienzyme complexes, and their integration is even more complete than in E. coli and plants. In yeast, the seven distinct active sites reside in two large, multifunctional polypeptides, with three activities on the subunit and four on the subunit. In vertebrates, a single large polypeptide (Mr 240,000)... |
a phosphorylationdephosphorylation cycle. The plant enzyme is activated by an increase in stromal pH and [Mg2], which occurs on illumination of the plant (see Fig. 20–18). Bacteria do not use triacylglycerols as energy stores. In E. coli, the primary role of fatty acid synthesis is to provide precursors for membrane l... |
rome P-450 is also important in the hydroxylation of many different drugs, such as barbiturates and other xenobiotics (substances foreign to the organism), particularly if they are hydrophobic and relatively insoluble. The environmental carcinogen benzo[a]pyrene (found in cigarette smoke) undergoes cytochrome P-450–dep... |
earch is aimed at developing new NSAIDs that inhibit COX-2 specifically, and several such drugs have become available. The development of COX-2–specific inhibitors has been helped immensely by knowledge of the detailed three-dimensional structures of COX-1 and COX-2 (Fig. 1). Both proteins are homodimers. Each monomer ... |
glucose properly but also fail to synthesize fatty acids from 8885d_c21_787-832 2/26/04 9:35 AM Page 805 mac76 mac76:385_reb: Glucose glycolysis CH2OH C O O CH2 O P O Dihydroxyacetone phosphate O CH2OH CHOH CH2OH Glycerol NADH H glycerol 3-phosphate dehydrogenase NAD CH2OH HO C H O O CH2 P O L-Glycerol 3-phosphate ATP ... |
the gene encoding PEP carboxykinase in adipose tissue. This results in a decrease in glyceroneogenesis in adipose tissue; recycling of fatty acids declines as a result, and more free fatty acids are released into the blood. Thus glyceroneogenesis is regulated reciprocally in the liver and adipose tissue, affecting lipi... |
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