In the above mechanism, k 1 , k -1 , k 3 , k -3 , k 2 and k 4 are rate constants. The coupled auxiliary enzyme reaction is by far the most common type of coupled assay, and there are many examples reported in the literature (see, Tables II and III in and Table .5 in ). One specific example is the phosphorylation of glucose to glucose-6 phosphate. The primary reaction is catalyzed by hexokinase, and is non-observable in typical steady-state kinetic experiments.