Re: What is the Michaelis constant for lactase?

Your question, while simple, is really quite complex. The Michaelis-Menton constant (Km) and Vmax are both important when studying the kinetics of an enzyme and can be affected by many external factors. The Km for lactase is affected by both pH and temperature. Also the substrate being used determines the Km. While lactose is the “preferred” substrate for lactase, some others are, beta-(1,3)-galactotriose and beta-(1,3)- galactobiose as well as myo-inositol and there would be a different Km using each of these substrates. Km also varies between organisms as well as within an organism.

Lactase from Bacillus acidocaldarius has a Km of 0.10 mM for lactose, while lactase from Bifidobacterium bifidium had a Km of 800 mM for lactose. Assuming you are interested in the Km for lactase from humans, some values reported range between 14 and 21 mM. Three articles characterizing lactase are listed below.

Naim HY, Lentze MJ. Impact of O-glycosylation on the function of human intestinal lactase-phlorizin hydrolase. Characterization of glycoforms varying in enzyme activity and localization of O-glycoside addition. J Biol Chem. 1992 Dec 15;267(35): 25494-504.
(http://www.ncbi.nlm.nih.gov/entrez/ query.fcgi?cmd=retrieve&db=pubmed&list_uids=1460044&dopt=Abstract)

H Skovbjerg, H Sjostrom and O Noren Purification and characterisation of amphiphilic lactase/phlorizin hydrolase from human small intestine. European Journal of Biochemistry, Vol 114, 653-661
(http://content.febsjournal.org/cgi/content/abstract/114/3/653) and (http://www.ncbi.nlm.nih.gov/entrez/ query.fcgi?cmd=Retrieve&db=PubMed&list_uids=6786877&dopt=Abstract)