2-17 Enzymes serve as catalysts in the cytoplasm

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• The cytoplasm can be highly structured. One example is the complex assembly of enzymes that perform DNA replication.

The cytoplasm is the site of the majority of metabolism of the cell and the details of these processes are covered in the chapters on metabolism and photosynthesis. However, there are a few general issues concerning enzymes in the cytoplasm that should be mentioned here.

Though the cytoplasm appears featureless, there is actually a significant amount of local organization. A good illustration of this can be found by examining the enzymes of DNA replication. Though too small to be seen, the proteins that perform replication are in complex assemblies of many proteins. This is much more efficient that having each protein float around, and simply finding the right time and place to perform its function by random chance. For example, DNA gyrase, which unwinds and opens the DNA for copying, has to function in coordination with DNA polymerase, which inserts each new nucleotide in the growing strand. Without this coordination, the DNA would not open up for replication and the process would simply not occur. In some other processes it would certainly be possible for the enzymes to float around in the cytoplasm without any interaction, but often it is much more efficient to organize in some fashion. The glycolytic enzymes (enzymes that oxidize sugars for energy) are an example of this type of multi-enzyme complex. One enzyme directly hands over its product to the next enzyme for which it is the substrate - a sort of molecular assembly line. This is much more efficient, because substrate does not accumulate where it should not and the local concentration of substrate for each enzyme is very high.

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