Part l Bioenergetics and Metabolism H OHH HH OHH H-C-2C--C-C-C-C-0-P-o H--C-C-C-C-C-C-0-P- O H OHOHH OH O H OHoHH O Glucose 6-pho Fructose 6-phosphate H H② This allows t a c-hbo formation of a C=C o by B B, abstracts a proton, allawi H the hydrogen ion aC=O bond. donated by b FIGURE 9 Isomerization and elimination reactions.(a) The conver- rows represent the movement of bonding electrons from nucleophile sion of glucose 6-phosphate to fructose 6-phosphate, a reaction of(pink] to electrophile(blue). B, and B2 are basic groups on the sugar metabolism catalyzed by phosphohexose isomerase(b)This re. enzyme: they are capable of donating and accepting hydrogen ions action proceeds through an enediol intermediate. The curved blue ar.(protons)as the reaction progresses. phosphoryl group (P03")serves as a leaving group charge and can therefore act as an electrophile. Ina very occur in hundreds of metabolic reactions large number of metabolic reactions, a phosphoryl group Phosphorus can form five covalent bonds. The con-(P03)is transferred from atP to an alcohol(form- ventional representation of Pi( Fig. 10a), with three ing a phosphate ester)(Fig. 10c)or to a carboxylic acid P-O bonds and one P=0 bond, is not an accurate pic- (forming a mixed anhydride). When a nucleophile at ture.In P. four equivalent phosphorus-oxygen bonds tacks the electrophilic phosphorus atom in ATP, a rela share some double-bond character, and the anion has a tively stable pentacovalent structure is formed as a re tetrahedral structure( Fig. 10b). As oxygen is more elec- action intermediate (Fig. 10d). With departure of the tronegative than phosphorus, the sharing of electrons is leaving group (ADP), the transfer of a phosphoryl group unequal: the central phosphorus bears a partial positive complete. The large family of enzymes that catalyze o-P=0 O-P-0 Adenine HRiboseF0-P-0-P-0-P-0 HO-R ATP 0→P-0 O=P-0- Adenine H Ribose 0-P-0-P-0+-0-P-0-R Glucose 6-phosphate. Z---P--w Z=R-OH FIGURE 10 Altermative ways of showing the structure of inorganic all four phosphorus-oxygen bonds with some double-bond character; orthophosphate (a) In one(inadequate)representation, three oxygens the hybrid orbitals so represented are arranged in a tetrahedron with are single- bonded to phosphorus, and the fourth is double -bonded, P at its center. (c)When a nucleophile Z (in this case, the-OH on allowing the four different resonance structures shown. (b)The four C-o of glucose)attacks ATP, it displaces ADP (W). In this SN2 reac. resonance structures can be represented more accurately by showing tion, a pentacovalent intermediate(d) forms transiently.phosphoryl group (OPO3 2) serves as a leaving group occur in hundreds of metabolic reactions. Phosphorus can form five covalent bonds. The con￾ventional representation of Pi (Fig. 10a), with three POO bonds and one PUO bond, is not an accurate pic￾ture. In Pi , four equivalent phosphorus–oxygen bonds share some double-bond character, and the anion has a tetrahedral structure (Fig. 10b). As oxygen is more elec￾tronegative than phosphorus, the sharing of electrons is unequal: the central phosphorus bears a partial positive charge and can therefore act as an electrophile. In a very large number of metabolic reactions, a phosphoryl group (OPO3 2) is transferred from ATP to an alcohol (form￾ing a phosphate ester) (Fig. 10c) or to a carboxylic acid (forming a mixed anhydride). When a nucleophile at￾tacks the electrophilic phosphorus atom in ATP, a rela￾tively stable pentacovalent structure is formed as a re￾action intermediate (Fig. 10d). With departure of the leaving group (ADP), the transfer of a phosphoryl group is complete. The large family of enzymes that catalyze Part II Bioenergetics and Metabolism 487 H 1 C 2 C B1 H O OH Glucose 6-phosphate B2 H C C H O OH C OH H C H OH C H OH C H H O P O O O H 1 C 2 C OH O Fructose 6-phosphate Enediol intermediate H C OH H C H OH C H OH C H H O P O O O (a) (b) phosphohexose isomerase 1 B1 abstracts a proton. 4 B2 abstracts a proton, allowing the formation of a C 2 This allows the formation of a C double bond. 3 Electrons from carbonyl form an 5 An electron leaves the C the hydrogen ion donated by B2. C C O bond. C bond to form a O H bond with C H bond with the proton donated by B1. B1 H H C H O O H C OH H C O B1 B2 B2 rows represent the movement of bonding electrons from nucleophile (pink) to electrophile (blue). B1 and B2 are basic groups on the enzyme; they are capable of donating and accepting hydrogen ions (protons) as the reaction progresses. FIGURE 9 Isomerization and elimination reactions. (a) The conver￾sion of glucose 6-phosphate to fructose 6-phosphate, a reaction of sugar metabolism catalyzed by phosphohexose isomerase. (b) This re￾action proceeds through an enediol intermediate. The curved blue ar￾O P O O O O O O P O O O P O O O O O O P O O O 3 O P (a) (b) O P O O O O O O Z P W (d) (c) Adenine Ribose O O P O P O HO R O P O O O O O Glucose ATP Adenine Ribose O O P O  O O P O  O P R O O O O ADP Glucose 6-phosphate, a phosphate ester Z  R OH W  ADP FIGURE 10 Alternative ways of showing the structure of inorganic orthophosphate. (a) In one (inadequate) representation, three oxygens are single-bonded to phosphorus, and the fourth is double-bonded, allowing the four different resonance structures shown. (b) The four resonance structures can be represented more accurately by showing all four phosphorus–oxygen bonds with some double-bond character; the hybrid orbitals so represented are arranged in a tetrahedron with P at its center. (c) When a nucleophile Z (in this case, the OOH on C-6 of glucose) attacks ATP, it displaces ADP (W). In this SN2 reac￾tion, a pentacovalent intermediate (d) forms transiently.