Table 1. Kinetic constants for PPIP5K1 catalytic domain.

The k_cat/K_m ratios (specificity constants) were determined from the enzyme concentration and first-order rate constants for the metabolic reactions described in Figure 1, except for Ins(1,3,4,5,6)P₅ phosphorylation. The data for intracellular concentrations were taken from various sources. Several publications have put the cellular concentration of PP-InsP₅ in the 1–5 μM range (see [4] for a summary); a mid-range value of 2 μM is given in this Table. The values for the concentrations of Ins(1,3,4,5,6)P₅ and InsP₆ [48] are at the high end of a range of estimates [49]. MDD–HPLC analysis indicates that 1/3-PP-InsP₅ must be <10 % of the cellular concentration of 5-PP-InsP₅ [5,11].

Substrate	Concentration in vivo (M)	kcat/Km (104×M−1 s−1)	[S]×kcat/Km (s −1)
5-PP-InsP5	2 × 10 −6	384 ± 10	7.7
InsP6	40 × 10 −6	4.8 ± 0.3	1.9
Ins(1,3,4,5,6)P5	90 × 10 −6	0.3 ± 0.01	0.021
1/3-PP-InsP5	0.2 × 10 −6	0.11 ± 0.04	0.0002