"Alchemy" in a Nutshell |
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| To say that enzymes are essential to life is quite an understatement. Over 1500 enzymes were known as of 1996 (Birkbeck), and more are being catalogued daily. All these enzymes are needed to catalyze the chemical reactions necessary to living organisms. But how did these enzymes come into being? The consensus seems to be that they evolved gradually, being selected at each step for better substrate fit and higher catalytic activity. (Figure 1). | |||||||||||||||
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| Figure 1 The multistep evolutionary process of enzymes | |||||||||||||||
| Recently,
Altamirano
et al. (2000) in Cambridge, England, side-stepped the lengthy evolutionary
process through a sort of
"enzyme alchemy." They created a new enzyme,
ivePRAI, by modifying indole-3-glycerol phosphate synthase (IGPS) to give
it the catalytic activity of phosphoribosylanthranilate isomerase (PRAI).
(Figure 2).
The approach used to accomplish this feat of alchemy is novel in its reliance upon a framework common to both enzymes. In this case, scientists approached the problem -- creation of a new enzyme -- in a logical series of steps: |
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| Figure 2 Shortcutting the evolutionary process via Altamirano's "alchemy." | |||||||||||||||
| 1. Choosing precursors for binding similarity Two enzymes were identified that contained some degree of similarity in binding of a specific substrate. These enzymes occur in the tryptophan synthesis pathway, and both enzymes are able to bind the same molecule, CdRP. CdRP is the product of one enzyme, IGPS, and the substrate for the second enzyme, PRAI. | |||||||||||||||
| 2.
Modification
of active sites Protein segments directly involved in the catalytic
activity were identified. The amino acid sequences of these segments were
modified while the binding "domain" (TIM
a/b
barrel) was preserved,
resulting in a new "family" of related enzymes with differing levels of
catalytic activity.
3. Selection The newly synthesized enzymes were screened for the desired activity, and the enzyme with the greatest activity was selected. The enzyme that was ultimately selected, ivePRAI, had an activity far greater than that of wild-type PRAI (figure 3). |
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| Figure 3 Relative catalytic activity of ivePRAI compared to that of PRAI. | |||||||||||||||
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On to Background
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Nikki
Jarrett, 11/20/00
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