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    D-mannose isomerase

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    Is D-mannose or D-lyxose a better substrate for D-mannose isomerase and why? Would a L form also be an substrate?

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    D-Mannose Isomerase and Substrate Specificity

    D-mannose isomerase catalyzes the conversion of D-mannose and D-lyxose to ketoses. The ketose formed from D-mannose is D- fructose. The one formed from D-lyxose is D-xylulose.

    Is it odd for a single enzyme to have more than one specificity? No, it is not. In fact, numerous enzymes act of variant substrates. However, when this is the case, the substrates must share a significant degree of similarity. After all, enzymes are chemical catalysts and show a high degree of specificity.

    When we look at the data, we find out that D-mannose is a much better substrate than D-lyxose. For example, if you look at the article entitled "Some properties of D-mannose isomerase from Escherichia coli K12." by Stevens, F.J. et al. (J. Gen. Microbiol., 1981, May 124(1):219-23) you will find that the Km for D-mannose is 80 mM while the corresponding Km for D-lyxose is significantly higher, at 300 mM.

    What do these Km values mean? Remember, the Km of an enzyme is the ...

    Solution Summary

    The solution discusses the better substrate for D-mannose isomerase.