TABLE 2.

Kinetic parameters for the hCBS-catalyzed reactions at varying homocysteine concentrations estimated by simulations

The reaction numbers correspond to those shown in Fig. 2.

Substrates	No.	Vmaxa	Km1, Km2b	v/[E] 10 μm Hcys	v/[E] 40 μm Hcys	Fold-changed	v/[E] 200 μm Hcys	Fold-changed
		units/mg	mm	s−1c	s−1		s−1
Ser + Hcys	1	5.1 ± 0.3	2.0 ± 0.3, 2.8 ± 0.5	185 × 10−4	700 × 10−4	3.8	2717 × 10−4	14.7
Cys	2	0.46 ± 0.08	6.8 ± 1.7	6.4 × 10−4	6.4 × 10−4	0.99	6.1 × 10−4	0.96
Cys + Cys	3	0.82 ± 0.08	6.8 ± 1.7, 27.3 ± 3.7	3.9 × 10−4	3.9 × 10−4	0.99	3.8 × 10−4	0.97
Cys + Hcys	4	18.7 ± 2.6	6.8 ± 1.7, 3.2 ± 1.3	239 × 10−4	253 × 10−4	1.06	259 × 10−4	1.09

^a One unit corresponds to 1 μmol of product formed min⁻¹. The K_m and V_max values were determined as described under “Experimental Procedures” and reported in Table 1.

^b In reactions involving two substrates, the order of the K_m values reflects the substrate order in the 1st column.

^c The values for the turnover numbers at varying concentrations of homocysteine and physiological concentrations of serine (560 μm) and cysteine (100 μm) were obtained as described under “Experimental Procedures” considering a ping-pong mechanism for the bimolecular reaction and the Hill coefficients (n) reported in Table 1.

^d Fold change refers to the change in v/[E] with respect to normal conditions, i.e. 10 μm homocysteine, which is assigned a value of 1 for each reaction.