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