Reactions Are Organized into Sequences or Pathways

To appreciate how biochemical reactions serve common functions we must examine the organization of biochemical reactions in some detail. Most of the enzyme-catalyzed reactions in living cells are organized into sequences or path- 

Glucose breakdown and tryptophan biosynthesis illustrate pathways consisting of a linear series of reactions. In chapter 12 we will see that the glycolytic pathway also contains branches so that common intermediates can proceed along diverging routes after the branchpoint. We will also see that the glycolytic pathway is organized so that some of the reactions within the pathway can go in either the forward or the reverse direction. 

A fundamental aspect of biochemical organizations is that the enzymes that catalyze a sequence are often clustered together in the cell. As a result, an intermediate produced in the first reaction in a pathway is passed directly to the second enzyme in the pathway, and so on. Such an arrangement might be expected to accelerate synthesis of the end product, minimize the loss of intermediates, and facilitate regulation of the pathway. 

Three types of enzyme clustering are found (fig. 11.3). In the simplest situation, all of the catalytic activities for a particular pathway are found in proteins that exist as independent soluble proteins in the same cellular compartment. In such cases the intermediates must get from one enzyme to the next in the sequence by free diffusion through the cytosol or by transfer after contact between two sequentially related enzymes—one carrying the reactive intermediate and the other ready to receive it. Such is the situation for the enzymes involved in the breakdown of glucose to pyruvate. 

The breakdown of glucose to pyruvate. The conversion of glucose to pyruvate requires ten steps and one enzyme for each step. In steps 1 and 3, ATP is consumed. In steps 7 and 10, ATP is produced. The net production of ATP is 2 moles for each mole of glucose consumed. Unlike the intermediates in the tryptophan pathway, many of the intermediates between glucose and pyruvate serve as useful substances for other purposes. A full description of all the steps in this sequence is given in chapter 12. 

---

Organisms Differ in Sources of Energy, Reducing Power, and Starting Materials for Biosynthesis Reactions Are Organized into Sequences or Pathways Sequentially Related Enzymes Are Frequently Clustered Pathways Show Functional Coupling The ATP-ADP System Mediates Conversions in Both Directions... 

---