[furaniki18]

In Kaplan lecture notes for medical students, K_m has also been defined for GLUT(glucose transporters) but my teacher tells me that K_m values are only defined for enzymes.

conceptually seeing, it makes sense to define K_m for such transporters also because it is telling us about the affinity of the transporter towards glucose. But is it correct to use it for molecules other than enzymes? if not, is there any other variable we can use for them?


Mod edit: Spelling corrected in title

[Babcock_Hall]

There are some subtle issues here, and I doubt that I can do full justice to them.  Transporters often show saturation kinetics.   Voet and Voet’s textbook uses K_M to symbolize the glucose concentration that gives half of J_max, the maximum flux.  Nelson and Cox’s textbook use K_t and indicate that it is a collection of rate constants that is analogous to K_M.  Enzymatic K_M values might or might not be the same as the thermodynamic dissociation constant for the substrate, depending on the specifics of the system.  I would imagine that the same caveat holds for transport.  I tend to use the word affinity when I am discussing dissociation constants, as opposed to kinetic constants such as K_M, but I am not sure that this is a universal practice.

[aHerraez]

I'd say that the saturability of whatever activity is being measured is more important than the affinity, if you are to "extend" the use of K_M to other processes.
 Affinity is regularly expressed in terms of association constant or dissociation constant.

An the correct spelling is Michaelis-Menten -- those are the surnames of two people.