Metabolism remains the focal point of the textbook, as does the goal of maintaining a one-semester book. The organization is the same, although a new chapter on coenzymes follows enzymes. This chapter can either be read in sequence to expand the chemistry of enzymes or skimmed to consult when specific coenzymes are encountered later. Some expansion of chemistry is provided, such as a more detailed treatment of acid–base and redox reactions and a fuller explanation of prochirality, as my own understanding of that concept has evolved.

Each chapter has a new addendum, providing an optional reading topic, usually of a more advanced nature than the book narrative. The organizing principles of near-equilibrium and metabolically irreversible enzymes are carried over from the first edition. In this edition, I have added two more. The first is an idea taken from the advertising industry, applied to perplexing pathway routes that are not quite linear or cyclic: repetition-variation. This description fits several pathways and provides a new way of thinking about them.

The second, introduced in the final chapter, is pathway-reversible versus pathway-irreversible as a means to discriminate between regulation that can be reverted by opposing reactions (reversible) or incorporated into proteins for their entire existence (irreversible). I have tried to make more explicit in this edition the underpinnings of chemistry in biological sciences.

In the present climate, there is a tendency for students to believe that they can look up whatever they need rather than examine the roots of the subject. This view is unfortunate; it means that every new fact or idea is searched anew and committed to memory in isolation. A few principles of biochemistry can be instead the foundation of a deeper understanding.

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