Proteins Give Cells Structure and Perform Most Cellular Tasks

The varied, intricate structures of proteins enable them to carry out numerous functions. Cells string together 20 different amino acids in a linear chain to form a protein (see Figure 2-13). Proteins commonly range in length from 100 to 1000 amino acids, but some are much shorter and others longer. We obtain amino acids either by synthesizing them from other molecules or by breaking down proteins that we eat. The "essential" amino acids, from a dietary standpoint, are the eight that we cannot synthesize and must obtain from food. Beans and corn together have all eight, making their combination particularly nutritious. Once a chain of amino acids is formed, it folds into a complex shape, conferring a distinctive three-dimensional structure and function on each protein (Figure 1-9).

(glutamine synthetase and adenylate kinase), an antibody (immunoglobulin), a hormone (insulin), and the blood's oxygen carrier (hemoglobin). Models of a segment of the nucleic acid DNA and a small region of the lipid bilayer that forms cellular membranes (see Section 1.3) demonstrate the relative width of these structures compared with typical proteins. [Courtesy of

Gareth White.]

Glutamine synthetase

Hemoglobin

Immunoglobulin

Adenylate kinase

Lipid bilayer

Glutamine synthetase

Hemoglobin

Immunoglobulin

Adenylate kinase

Lipid bilayer

Some proteins are similar to one another and therefore can be considered members of a protein family. A few hundred such families have been identified. Most proteins are designed to work in particular places within a cell or to be released into the extracellular (extra, "outside") space. Elaborate cellular pathways ensure that proteins are transported to their proper intracellular (intra, within) locations or secreted (Chapters 16 and 17).

Proteins can serve as structural components of a cell, for example, by forming an internal skeleton (Chapters 5, 19, and 20). They can be sensors that change shape as temperature, ion concentrations, or other properties of the cell change. They can import and export substances across the plasma membrane (Chapter 7). They can be enzymes, causing chemical reactions to occur much more rapidly than they would without the aid of these protein catalysts (Chapter 3). They can bind to a specific gene, turning it on or off (Chapter 11). They can be extracellular signals, released from one cell to communicate with other cells, or intracellular signals, carrying information within the cell (Chapters 13-15). They can be motors that move other molecules around, burning chemical energy (ATP) to do so (Chapters 19 and 20).

How can 20 amino acids form all the different proteins needed to perform these varied tasks? Seems impossible at first glance. But if a "typical" protein is about 400 amino acids long, there are 20400 possible different protein sequences. Even assuming that many of these would be functionally equivalent, unstable, or otherwise discountable, the number of possible proteins is well along toward infinity.

Next we might ask how many protein molecules a cell needs to operate and maintain itself. To estimate this number, let's take a typical eukaryotic cell, such as a hepatocyte (liver cell). This cell, roughly a cube 15 ^m (0.0015 cm) on a side, has a volume of 3.4 X 10"9 cm3 (or milliliters). Assuming a cell density of 1.03 g/ml, the cell would weigh 3.5 X 10"9 g. Since protein accounts for approximately 20 percent of a cell's weight, the total weight of cellular protein is 7 X 10"10 g. The average yeast protein has a mo

▲ FIGURE 1-10 DNA consists of two complementary strands wound around each other to form a double helix.

(Left) The double helix Is stabilized by weak hydrogen bonds between the A and T bases and between the C and G bases. (Right) During replication, the two strands are unwound and used lecular weight of 52,700 (g/mol). Assuming this value is typical of eukaryotic proteins, we can calculate the total number of protein molecules per liver cell as about 7.9 X 109 from the total protein weight and Avogadro's number, the number of molecules per mole of any chemical compound (6.02 X 1023). To carry this calculation one step further, consider that a liver cell contains about 10,000 different proteins; thus, a cell contains close to a million molecules of each type of protein on average. In actuality the abundance of different proteins varies widely, from the quite rare insulin-binding receptor protein (20,000 molecules) to the abundant structural protein actin (5 X 108 molecules).

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