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Protein Structure Continued
©2001 Timothy Paustian, University of Wisconsin-Madison
During and after synthesis, a protein folds into alpha helices and beta sheets. These areas of secondary structure are connected by bridging sequences that will cause the protein to fold in specific ways. At the completion of this process, the protein takes on its final shape. The mature stable structure of a single peptide sequence is termed its tertiary structure. Below is pictured the tertiary structure of ribulose bisphosphate carboxylase (RubisCo) - one of the most important enzymes on this planet. Life would not exist without it. This enzyme takes energy, obtained most often from the sun, and uses it to convert carbon dioxide into carbohydrate. It is found in many photosynthetic organisms and is probably the most abundent protein on the earth. Notice in Figure 1 the alpha helices throughout the protein and the beta sheet near the bottom of the protein.
Figure 1 - RubisCo
Many enzymes and structures are actually complexes of several polypeptides. The arrangement of these polypeptides is termed the quatinary structure of a protein. Proteins complexes can contain several copies of an identical protein or they may consist of any number of polypeptides in various ratios.
Hemogobin (the oxygen carrier in blood) is an example of a protein containing identical subunits - Figure 2. Notice the heme groups associated with each polypeptide (the light blue group that is surrounded by the dark blue protein). These heme groups contain iron and the iron is the atom that actually binds the oxygen. Heme iron has an even greater affinity for carbon monoxide. If all your blood binds carbon monoxide, you literally suffocate, and this is why it is such a deadly poison.
Figure 2 - Hemoglobin. Only 3 of the four subunits are visible.
Diitrogenase is an example of a protein containing non-identical subunits. It has two copies of one protein and two copies of a second protein. This protein is responsible for the reduction of nitrogen gas to ammonia. This reaction is only carried out by microorganisms and is critical to global nitrogen cycling.
Figure 3 - Nitrogenase - 2 alpha subunits and 2 beta subunits
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