Each sugar has its own route of entry into glycolysis. Obviously you cite glucose as the most uniform monosaccharide, but remember that glucose itself is altered slightly before entering glycolysis via hexokinase to form glucose-6-P. Then you will find that this is isomerised to fructose-6-P by glucose-6-P isomerase. Therefore, you can see that it is at this point that fructose could enter the pathway where fructose becomes fructose-6-P via the enzyme fructokinase.

As for galactose, well this is a bit of a stinker to metabolise. It is converted to glucose-6-phosphate in four steps. That much I could remember, so I went to good old Stryer and I'll cite what that has to say:

The first reaction in the galactose-glucose interconversion pathway is the phosphorylation of galactose to galactose-1-phosphate by galactokinase -

Galactose + ATP --> galactose-1-phosphate + ADP + H+

Galactose-1-phosphate then acquires a uridyl group from uridine diphosphate glucose (UDP-glucose), an intermediate in the synthesis of glycosidic linkages. The products of this reaction, which is catalysed by [wait for it] galactose-1-phosphate uridyl transferase, are UDP- galactose and glucose-1-phosphate.

The galactose moiety of UDP is then epimerised to glucose..[If you're not sure: an epimer is one of two molecules that differ only in the arrangement of atoms around a single carbon atom, which in the case of galactose/gluose is the configuration of the hydroxyl froup at C4. Epimerisation is therefore inversion of this and is catalysed by UDP- galactose-4-epimerase!]

The sum of the reactions catalysed by galactokinase, the transferase and the epimerase is:

galactose + ATP --> gluocse-1-phosphate + ADP + H+

So there you...probably more than you're looking for, but it illustrates that where there's a will, there's an enzyme just waiting to make a sugar more palatable to glycolysis!

All the best

Jim Caryl
MAD Scientist

Reference:

Stryer, L. (1995). Biochemistry. 4th Edition. W.H. Freeman & Co. New York.