Page 863 Figure 23-25The pentose phosphate pathway.
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Transcript of Page 863 Figure 23-25The pentose phosphate pathway.
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Pag
e 86
3Figure 23-25 The pentose phosphate pathway.
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FIGURE 14-27 Role of NADPH in regulating the partitioning of glucose 6-phosphate between glycolysis and the pentose phosphate pathway. When NADPH is forming faster than it is being used for biosynthesis and glutathione reduction (see Figure 14-20), [NADPH] rises and inhibits the first enzyme in the pentose phosphate pathway. As a result, more glucose 6-phosphate is available for glycolysis.
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• LEHNINGER • PRINCIPLES OF BIOCHEMISTRY
• Fifth Edition
David L. Nelson and Michael M. Cox
© 2008 W. H. Freeman and Company
CHAPTER 15Principles of Metabolic Regulation
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Insulin’s effects on transport and hexokinase activity, not the change in glycogen synthase activity, increase the flux toward glycogen synthesis.
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Describe these differences!
When blood glucose rises above 5 mM, hexokinase IV activity increases, but hexokinase I is already operating near Vmax and cannot respond to an increase in glucose concentration.
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The protein inhibitor of hexokinase IV is a nuclear binding protein that draws hexokinase IV into the nucleus when the fructose 6-phosphate concentration in liver is high, and releases it to the cytosol when the glucose concentration is high.
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FIGURE 15-14a Phosphofructokinase-1 (PFK-1) and its regulation. Ribbon diagram of E. coli PFK-1, showing two of its four identical subunits (PDB ID 1PFK). Each subunit has its own catalytic site, where the products ADP and fructose 1,6-bisphosphate are almost in contact, and its own binding sites for the allosteric regulator ADP (blue), located at the interface between subunits.
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Fructose 2,6-bisphosphate (F26BP) has opposite effects on the enzymatic activities of phosphofructokinase-1 and fructose 1,6-bisphosphatase.
(a) PFK-1 activity in the absence of F26BP is half-maximal when the concentration of fructose 6-phosphate is 2 mM. When 0.13 μM F26BP is present, the K0.5 for fructose 6-phosphate is only 0.08 mM.
(b) FBPase-1 activity is inhibited by as little as 1 μM F26BP and is strongly inhibited by 25 μM. In the absence of this inhibitor the K0.5 for fructose 1,6-bisphosphate is 5 μM, but in the presence of 25 μM F26BP the K0.5 is >70 μM. Fructose 2,6-bisphosphate also makes FBPase-1 more sensitive to inhibition by another allosteric regulator, AMP.
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Both enzyme activities are part of the same polypeptide chain, and they are reciprocally regulated by insulin and glucagon.
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3D structure of one enzyme subunit of the testis PFK-2/FBPase-2 isoenzymeThe co-ordinates were retrieved
from the PDB database (accession code 1BIF) containing ATPγS in the PFK-2 domain. In
the upper right hand PFK-2 domain, ATP is on the left and Fru-6-P is on the right. In the
lower left hand FBPase-2 domain, two inorganic
phosphates indicate the position of the Fru-2,6-P2-binding site.
The PFK-2 domain is composed of a β-sheet surrounded by α-
helices. Two subdomains, composed of α-helices (above) form a flexible cover and are
involved in Fru-6-P binding and catalysis (Biochem J.
2004 August 1; 381(Pt 3): 561–579.
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Phosphoprotein phosphatase
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The PEP carboxykinase promoter region, showing the complexity of regulatory
input to this gene.
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Glycogen granules in a hepatocyte
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Glycogenin structure. Muscle glycogenin (Mr 37,000) forms dimers in solution. The substrate, UDP-glucose, is bound to a Rossmann fold near the amino terminus and is some distance from the Tyr194 residues—15 Å from the Tyr in the same monomer, 12 Å from the Tyr in the dimeric partner. Each UDP-glucose is bound through its phosphates to a Mn2+ ion that is essential to catalysis. Mn2+ is believed to function as an electron-pair acceptor to stabilize the leaving group, UDP. The glycosidic bond in the product has the same configuration about the C-1 of glucose as the substrate UDP-glucose, suggesting that the transfer of glucose from UDP to Tyr194 occurs in two steps. The first step is probably a nucleophilic attack by Asp162, forming a temporary intermediate with inverted configuration. A second nucleophilic attack by Tyr194 then restores the starting configuration
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“Alfonse, Biochemistry makes my head hurt!!”\