Part l Bioenergetics and Metabolism phosphoryl group transfers with ATP as donor are called chemiosmotic energy coupling, a universal mechanism kinases(Greek kinein. " to move"). Hexokinase, for ex- in which a transmembrane electrochemical potential ample, "moves"a phosphoryl group from ATP to glucose. produced either by substrate oxidation or by light ab- Phosphoryl groups are not the only activators of this sorption, drives the synthesis of ATP type. Thioalcohols(thiols), in which the oxygen atom Chapters 20 through 22 describe the major anabolic of an alcohol is replaced with a sulfur atom, are also pathways by which cells use the energy in atP to pro- good leaving groups. Thiols activate carboxylic acids by duce carbohydrates, lipids, amino acids, and nucleotides forming thioesters(thiol esters)with them. We will dis- from simpler precursors. In Chapter 23 we step back cuss a number of cases, including the reactions cat- from our detailed look at the metabolic pathways-as alyzed by the fatty acyl transferases in lipid synthesis they occur in all organisms, from Escherichia coli to (see Fig 21-2), in which nucleophilic substitution at the humans-and consider how they are regulated and in carbonyl carbon of a thioester results in transfer of the tegrated in mammals by hormonal mechanisms. acyl group to another moiety As we undertake our study of intermediary metab olism, a final word. Keep in mind that the myriad re- 5. Free radical reactions Once thought to be rare, the actions described in these pages take place in, and play homolytic cleavage of covalent bonds to generate free crucial roles in, living organisms. As you encounter each adicals has now been found in a range of biochemical reaction and each pathway ask, What does this chemi processes. Some examples are the reactions of methyl- cal transformation do for the organism? How does this malonyl-CoA mutase(see Box 17-2), ribonucleotide pathway interconnect with the other pathways operat- reductase(see Fig. 22-41), and DNa photolyase(see ing simultaneously in the same cell to produce the en Fig25-25) ergy and products required for cell maintenance and growth? How do the multilayered regulatory mecha- We begin Part II with a discussion of the basic en- nisms cooperate to balance metabolic and energy in- ergetic principles that govern all metabolism( Chapter puts and outputs, achieving the dynamic steady state 13). We then consider the major catabolic pathways by of life? Studied with this perspective, metabolism pro which cells obtain energy from the oxidation of various vides fascinating and revealing insights into life, with fuels( Chapters 14 through 19). Chapter 19 is the piv- countless applications in medicine, agriculture, and otal point of our discussion of metabolism; it concerns biotechnology.phosphoryl group transfers with ATP as donor are called kinases (Greek kinein, “to move”). Hexokinase, for ex￾ample, “moves” a phosphoryl group from ATP to glucose. Phosphoryl groups are not the only activators of this type. Thioalcohols (thiols), in which the oxygen atom of an alcohol is replaced with a sulfur atom, are also good leaving groups. Thiols activate carboxylic acids by forming thioesters (thiol esters) with them. We will dis￾cuss a number of cases, including the reactions cat￾alyzed by the fatty acyl transferases in lipid synthesis (see Fig. 21–2), in which nucleophilic substitution at the carbonyl carbon of a thioester results in transfer of the acyl group to another moiety. 5. Free radical reactions Once thought to be rare, the homolytic cleavage of covalent bonds to generate free radicals has now been found in a range of biochemical processes. Some examples are the reactions of methyl￾malonyl-CoA mutase (see Box 17–2), ribonucleotide reductase (see Fig. 22–41), and DNA photolyase (see Fig. 25–25). We begin Part II with a discussion of the basic en￾ergetic principles that govern all metabolism (Chapter 13). We then consider the major catabolic pathways by which cells obtain energy from the oxidation of various fuels (Chapters 14 through 19). Chapter 19 is the piv￾otal point of our discussion of metabolism; it concerns chemiosmotic energy coupling, a universal mechanism in which a transmembrane electrochemical potential, produced either by substrate oxidation or by light ab￾sorption, drives the synthesis of ATP. Chapters 20 through 22 describe the major anabolic pathways by which cells use the energy in ATP to pro￾duce carbohydrates, lipids, amino acids, and nucleotides from simpler precursors. In Chapter 23 we step back from our detailed look at the metabolic pathways—as they occur in all organisms, from Escherichia coli to humans—and consider how they are regulated and in￾tegrated in mammals by hormonal mechanisms. As we undertake our study of intermediary metab￾olism, a final word. Keep in mind that the myriad re￾actions described in these pages take place in, and play crucial roles in, living organisms. As you encounter each reaction and each pathway ask, What does this chemi￾cal transformation do for the organism? How does this pathway interconnect with the other pathways operat￾ing simultaneously in the same cell to produce the en￾ergy and products required for cell maintenance and growth? How do the multilayered regulatory mecha￾nisms cooperate to balance metabolic and energy in￾puts and outputs, achieving the dynamic steady state of life? Studied with this perspective, metabolism pro￾vides fascinating and revealing insights into life, with countless applications in medicine, agriculture, and biotechnology. 488 Part II Bioenergetics and Metabolism