n-c-cooh + h2o
In this chemical reaction, a hydroxyl radical (OH-) is removed from the COOH portion of the first amino acid, and a hydrogen (H+) of the NH2 portion of the other amino acid is removed. These combine to form water, and the two reactive sites left on the two successive amino acids bond with each other, resulting in a single molecule. This process is called peptide linkage. As each additional amino acid is added, an additional peptide linkage is formed.
Many thousand protein enzymes formed in the manner just described control essentially all the other chemical reactions that take place in cells. These enzymes promote synthesis of lipids, glycogen, purines, pyrimidines, and hundreds of other substances. We discuss many of these synthetic processes in relation to carbohydrate, lipid, and protein metabolism in Chapters 67 through 69. It is by means of all these substances that the many functions of the cells are performed.
Control of Gene Function and Biochemical Activity in Cells
From our discussion thus far, it is clear that the genes control both the physical and the chemical functions of the cells. However, the degree of activation of respective genes must be controlled as well; otherwise, some parts of the cell might overgrow or some chemical reactions might overact until they kill the cell. Each cell has powerful internal feedback control mechanisms that keep the various functional operations of the cell in step with one another. For each gene (more than 30,000 genes in all), there is at least one such feedback mechanism.
There are basically two methods by which the biochemical activities in the cell are controlled. One of these is genetic regulation, in which the degree of activation of the genes themselves is controlled, and the other is enzyme regulation, in which the activity levels of already formed enzymes in the cell are controlled.
The "Operon" of the Cell and Its Control of Biochemical Synthesis—Function of the Promoter. Synthesis of a cellular biochemical product usually requires a series of reactions, and each of these reactions is catalyzed by a special protein enzyme. Formation of all the enzymes needed for the synthetic process often is controlled by a sequence of genes located one after the other on the same chromosomal DNA strand. This area of the DNA strand is called an operon, and the genes responsible for forming the respective enzymes are called structural genes. In Figure 3-12, three respective structural genes are shown in an operon, and it is demonstrated that they control the formation of three respective enzymes that in turn cause synthesis of a specific intracellular product.
Note in the figure the segment on the DNA strand called the promoter. This is a group of nucleotides that has specific affinity for RNA polymerase, as already discussed. The polymerase must bind with this promoter before it can begin traveling along the DNA strand to synthesize RNA. Therefore, the promoter is an essential element for activating the operon.
Control of the Operon by a "Repressor Protein"—The "Repressor Operator." Also note in Figure 3-12 an additional band of nucleotides lying in the middle of the promoter. This area is called a repressor operator because a "regulatory" protein can bind here and prevent attachment of RNA polymerase to the promoter, thereby blocking transcription of the genes of
Activator Repressor operator operator
Activator Repressor operator operator
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