84 Part l Bioenergetics and Metabolism Steroid pigments hormones Phospholipids pyrophosphate → Cholesterol bl. Mevalonate Vitamin K Alanine Phenyl- anine Acetoacetyl-CoAEicosanoids acrose Isoleucine Fatty acids Triacylglycerols a)Converging catabolism Citrate CDP-diacylglycerol>Phospholipid Oxaloacetate (b)Diverging anabolism CO2 (c)Cyclic pathway FIGURE 4 Three types of nonlinear metabolic pathways. (a)Con- the breakdown product of a variety of fuels(a), serves as the precur verging, catabolic; (b) diverging, anabolic; and (c)cyclic, in which sor for an array of products(b), and is consumed in the catabolic path one of the starting materials (oxaloacetate in this case) is regenerated way known as the citric acid cycle(c) and reenters the pathway. Acetate, a key metabolic intermediate, is messengers that modify the activity of existing enzyme Before reviewing the five main reaction classes of molecules by allosteric mechanisms or by covalent mod- biochemistry, lets consider two basic chemical princi- ification such as phosphorylation. In other cases, the ex- ples. First, a covalent bond consists of a shared pair of tracellular signal changes the cellular concentration of electrons, and the bond can be broken in two general an enzyme by altering the rate of its synthesis or degra- ways(Fig. 5). In homolytic cleavage, each atom leaves dation, so the effect is seen only after minutes or hours. the bond as a radical, carrying one of the two electrons The number of metabolic transformations taking (now unpaired)that held the bonded atoms together place in a typical cell I overwhelming to a be- In the more common, heterolytic cleavage, one atom re- ginning student. Most cells have the capacity to carry tains both bonding electrons. The species generated out thousands of specific, enzyme-catalyzed reactions: when C-C and C-H bonds are cleaved are illustrated for example, transformation of a simple nutrient such in Figure 5. Carbanions, carbocations, and hydride ions as glucose into amino acids, nucleotides, or lipids; ex- are highly unstable; this instability shapes the chemistry traction of energy from fuels by oxidation; or polymer- of these ions, as described further below. ization of monomeric subunits into macromolecules The second chemical principle of interest here is that Fortunately for the student of biochemistry, there are many biochemical reactions involve interactions between patterns within this multitude of reactions you do not nucleophiles(functional groups rich in electrons and need to learn all these reactions to comprehend the capable of donating them) and electrophiles(electron molecular logic of biochemistry. Most of the reactions deficient functional groups that seek electrons). Nucle in living cells fall into one of five general categories: ophiles combine with, and give up electrons to, elec (1)oxidation-reductions;(2) reactions that make or trophies. Common nucleophiles and electrophiles are break carbon-carbon bonds; (3)internal rearrangements, listed in Figure 6-21. Note that a carbon atom can act isomerizations, and eliminations;(4) group transfers; as either a nucleophile or an electrophile, depending on and(5) free radical reactions. Reactions within each which bonds and functional groups surround it. general category usually proceed by a limited set of We now consider the five main reaction classes you mechanisms and often employ characteristic cofactors. will encounter in upcoming chaptersmessengers that modify the activity of existing enzyme molecules by allosteric mechanisms or by covalent mod￾ification such as phosphorylation. In other cases, the ex￾tracellular signal changes the cellular concentration of an enzyme by altering the rate of its synthesis or degra￾dation, so the effect is seen only after minutes or hours. The number of metabolic transformations taking place in a typical cell can seem overwhelming to a be￾ginning student. Most cells have the capacity to carry out thousands of specific, enzyme-catalyzed reactions: for example, transformation of a simple nutrient such as glucose into amino acids, nucleotides, or lipids; ex￾traction of energy from fuels by oxidation; or polymer￾ization of monomeric subunits into macromolecules. Fortunately for the student of biochemistry, there are patterns within this multitude of reactions; you do not need to learn all these reactions to comprehend the molecular logic of biochemistry. Most of the reactions in living cells fall into one of five general categories: (1) oxidation-reductions; (2) reactions that make or break carbon–carbon bonds; (3) internal rearrangements, isomerizations, and eliminations; (4) group transfers; and (5) free radical reactions. Reactions within each general category usually proceed by a limited set of mechanisms and often employ characteristic cofactors. Before reviewing the five main reaction classes of biochemistry, let’s consider two basic chemical princi￾ples. First, a covalent bond consists of a shared pair of electrons, and the bond can be broken in two general ways (Fig. 5). In homolytic cleavage, each atom leaves the bond as a radical, carrying one of the two electrons (now unpaired) that held the bonded atoms together. In the more common, heterolytic cleavage, one atom re￾tains both bonding electrons. The species generated when COC and COH bonds are cleaved are illustrated in Figure 5. Carbanions, carbocations, and hydride ions are highly unstable; this instability shapes the chemistry of these ions, as described further below. The second chemical principle of interest here is that many biochemical reactions involve interactions between nucleophiles (functional groups rich in electrons and capable of donating them) and electrophiles (electron￾deficient functional groups that seek electrons). Nucle￾ophiles combine with, and give up electrons to, elec￾trophiles. Common nucleophiles and electrophiles are listed in Figure 6–21. Note that a carbon atom can act as either a nucleophile or an electrophile, depending on which bonds and functional groups surround it. We now consider the five main reaction classes you will encounter in upcoming chapters. 484 Part II Bioenergetics and Metabolism Rubber Bile acids Steroid hormones (a) Converging catabolism Oxaloacetate (b) Diverging anabolism CO2 CO2 (c) Cyclic pathway Acetate (acetyl-CoA) Citrate Glycogen Glucose Pyruvate Phospholipids Alanine Fatty acids Leucine Phenyl￾alanine Isoleucine Starch Sucrose Serine Eicosanoids Phospholipids Carotenoid pigments Vitamin K Triacylglycerols Cholesteryl esters Triacylglycerols Mevalonate Isopentenyl￾pyrophosphate Fatty acids Acetoacetyl-CoA CDP-diacylglycerol Cholesterol FIGURE 4 Three types of nonlinear metabolic pathways. (a) Con￾verging, catabolic; (b) diverging, anabolic; and (c) cyclic, in which one of the starting materials (oxaloacetate in this case) is regenerated and reenters the pathway. Acetate, a key metabolic intermediate, is the breakdown product of a variety of fuels (a), serves as the precur￾sor for an array of products (b), and is consumed in the catabolic path￾way known as the citric acid cycle (c)