BCH 5045 Graduate Survey of Biochemistryhort.ifas.ufl.edu/faculty/guy/bch5045/Lecture Files/Lecture 3.pdf · Bioenergetics and Biochemical Reaction Types. ... The reaction would not
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Images from the Text are protected by Copyright (c) 2008 by W. H. Freeman and Company, and by the licensors of W. H. Freeman and Company. Living Graphs software (c) 2008 Sumanas, Inc. ALL RIGHTS RESERVED. Commentary by the instructor is protected by Copyright (c) 2010. ALL RIGHTS RESERVED. BCH 5045 Graduate Survey of Biochemistry Instructor: Charles Guy Producer: Ron Thomas Director: Glen Graham Lecture 3 Slide sets available at: http://hort.ifas.ufl.edu/teach/guyweb/bch5045/index.html
Images from the Text are protected by Copyright (c) 2008 by W. H. Freeman and Company, and by the licensors of W. H. Freeman and Company. Living Graphs software (c) 2008 Sumanas, Inc. ALL RIGHTS RESERVED.
Commentary by the instructor is protected by Copyright (c) 2010. ALL RIGHTS RESERVED.
BCH 5045
Graduate Survey of Biochemistry
Instructor: Charles GuyProducer: Ron ThomasDirector: Glen Graham
Images from the Text are protected by Copyright (c) 2008 by W. H. Freeman and Company, and by the licensors of W. H. Freeman and Company. Living Graphs software (c) 2008 Sumanas, Inc. ALL RIGHTS RESERVED.
Commentary by the instructor is protected by Copyright (c) 2010. ALL RIGHTS RESERVED.
Images from the Text are protected by Copyright (c) 2008 by W. H. Freeman and Company, and by the licensors of W. H. Freeman and Company. Living Graphs software (c) 2008 Sumanas, Inc. ALL RIGHTS RESERVED.
Commentary by the instructor is protected by Copyright (c) 2010. ALL RIGHTS RESERVED.
Formation and Breaking Bonds
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FIGURE 13-1 Two mechanisms for cleavage of a C—C or C—H bond. In a homolytic cleavage, each atom keeps one of the bonding electrons, resulting in the formation of carbon radicals (carbons having unpaired electrons) or uncharged hydrogen atoms. In a heterolytic cleavage, one of the atoms retains both bonding electrons. This can result in the formation of carbanions, carbocations, protons, or hydride ions.
Images from the Text are protected by Copyright (c) 2008 by W. H. Freeman and Company, and by the licensors of W. H. Freeman and Company. Living Graphs software (c) 2008 Sumanas, Inc. ALL RIGHTS RESERVED.
Commentary by the instructor is protected by Copyright (c) 2010. ALL RIGHTS RESERVED.
Properties of carbonyl groups and imines
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FIGURE 13-3 Chemical properties of carbonyl groups. (a) The carbon atom of a carbonyl group is an electrophile by virtue of the electron-withdrawing capacity of the electronegative oxygen atom, which results in a resonance hybrid structure in which the carbon has a partial positive charge. (b) Within a molecule, delocalization of electrons into a carbonyl group stabilizes a carbanion on an adjacent carbon, facilitating its formation. (c) Imines function much like carbonyl groups in facilitating electron withdrawal. (d) Carbonyl groups do not always function alone; their capacity as electron sinks often is augmented by interaction with either a metal ion (Me2+, such as Mg2+) or a general acid (HA).
Images from the Text are protected by Copyright (c) 2008 by W. H. Freeman and Company, and by the licensors of W. H. Freeman and Company. Living Graphs software (c) 2008 Sumanas, Inc. ALL RIGHTS RESERVED.
Commentary by the instructor is protected by Copyright (c) 2010. ALL RIGHTS RESERVED.
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FIGURE 13-4 Some common reactions that form and break C—C bonds in biological systems. For both the aldol condensation and the Claisen condensation, a carbanion serves as nucleophile and the carbon of a carbonyl group serves as electrophile. The carbanion is stabilized in each case by another carbonyl at the adjoining carbon. In the decarboxylation reaction, a carbanion is formed on the carbon shaded blue as the CO2 leaves. The reaction would not occur at an appreciable rate without the stabilizing effect of the carbonyl adjacent to the carbanion carbon. Wherever a carbanion is shown, a stabilizing resonance with the adjacent carbonyl, as shown in Figure 13-3b, is assumed. An imine (Figure 13-3c) or other electron-withdrawing group (including certain enzymatic cofactors such as pyridoxal) can replace the carbonyl group in the stabilization of carbanions.
Images from the Text are protected by Copyright (c) 2008 by W. H. Freeman and Company, and by the licensors of W. H. Freeman and Company. Living Graphs software (c) 2008 Sumanas, Inc. ALL RIGHTS RESERVED.
Commentary by the instructor is protected by Copyright (c) 2010. ALL RIGHTS RESERVED.
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FIGURE 13-5 Carbocations in carbonﾐcarbon bond formation. In one of the early steps in cholesterol biosynthesis, the enzyme prenyl transferase catalyzes condensation of isopentenyl pyrophosphate and dimethylallyl pyrophosphate to form geranyl pyrophosphate (see Figure 21-36). The reaction is initiated by elimination of pyrophosphate from the dimethylallyl pyrophosphate to generate a carbocation, stabilized by resonance with the adjacent C=C bond.
Images from the Text are protected by Copyright (c) 2008 by W. H. Freeman and Company, and by the licensors of W. H. Freeman and Company. Living Graphs software (c) 2008 Sumanas, Inc. ALL RIGHTS RESERVED.
Commentary by the instructor is protected by Copyright (c) 2010. ALL RIGHTS RESERVED.
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FIGURE 13-6 Isomerization and elimination reactions. (a) The conversion of glucose 6-phosphate to fructose 6-phosphate, a reaction of sugar metabolism catalyzed by phosphohexose isomerase. (b) This reaction proceeds through an enediol intermediate. The curved blue arrows represent movement of bonding electron pairs. Pink screens follow the path of oxidation from left to right. B1 and B2 are ionizable groups on the enzyme; they are capable of donating and accepting protons (acting as general acids or general bases) as the reaction proceeds. Pink screens indicate nucleophilic groups; blue, electrophilic.
Images from the Text are protected by Copyright (c) 2008 by W. H. Freeman and Company, and by the licensors of W. H. Freeman and Company. Living Graphs software (c) 2008 Sumanas, Inc. ALL RIGHTS RESERVED.
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