Structural View of Biology
Enzymes are Nature's chemists, performing all of the chemical transformations needed for life. Enzymes catalyze chemical reactions by bringing together all of the necessary chemical tools in the proper place. They typically have an "active site" that captures the chemicals that will be modified, holding them in the perfect orientation to perform the chemical change. Researchers have separated the many types of enzymes into a few functional classes, based on the reactions that they perform. Click on any of the sub-categories below to explore a few examples of each enzyme class. You can also explore many other enzymes in the other functional categories in "Structural View of Biology."
Ligases - Making Connections
Ligases connect two molecules, creating a new chemical bond. The ligation reaction may be assisted by breakage of ATP or another nucleotide, to provide the chemical energy to drive the reaction.
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When a ribosome pairs a "CGC" tRNA with "GCG" codon, it expects to find an alanine carried by the tRNA. It has no way of checking; each tRNA is matched with its amino acid long before it reaches the ribosome. The match is made by a collection of remarkable enzymes, the aminoacyl-tRNA synthetases. These enzymes charge each tRNA with the proper amino acid, thus allowing each tRNA to make the proper translation from the genetic code of DNA into the amino acid code of proteins.
Human cells (with a few unusual exceptions) each contain their own set of 46 long strands of DNA. All of our genetic information is encoded in these strands, with thousands of genes strung along their length. The ordering of genes, and the proximity of one next to the other, can be important for the proper usage of the information, so it is important that our cells protect their DNA from breakage. If one strand in the DNA breaks, it is not a disaster, but it can lead to problems when the DNA double helix is unwound during the processes of transcription and replication. Breakage of both strands, on the other hand, is far more serious. To protect us from these dangers, our cells use DNA ligases to glue together DNA strands that have been broken.
Our cells are continually faced with a changing environment. Think about what you eat. Some days you might eat a lot of protein, other days you might eat a lot of carbohydrate. Sometimes you may eat nothing but chocolate. Your body must be able to respond to these different foods, producing the proper enzymes for capturing the nutrients in each. The same is doubly true for small organisms like bacteria, which do not have as many options in choosing their diet. They must eat whatever food happens to be close by, and then mobilize the enzymes needed to use it.
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