Passage
Leukocyte common antigen (LCA) enzymes play several important developmental roles in both vertebrates and invertebrates. They regulate numerous cellular processes by catalyzing Reaction 1 in target proteins at specific developmental stages. Consequently, LCA enzyme expression and activity are tightly controlled in various cell types.
Reaction 1The Src protein is critical for cell differentiation and motility, and is activated by LCA enzyme activity. Two LCA enzymes, known as LAR and CD45, were purified and incubated with the phosphopeptide src527 (Figure 1) , derived from Src.
Figure 1 Structure of the src527 peptide used as a substrate in LAR and CD45 assaysThe rate of Reaction 1 was measured at various concentrations of src527 in the presence of either LAR or CD45, and kinetic parameters were determined (Table 1) .Table 1 Kinetic Parameters of LAR and CD45
Compound 1 was synthesized and tested for its effectiveness as an inhibitor of LCA enzymes.
The study in Table 1 was repeated in the presence of Compound 1, which was found to significantly reduce the activities of both LAR and CD45 against the src527 peptide. Compound 1 was subsequently used in assays to examine the in vivo function of LCA enzymes.
Adapted from Cho H, Krishnaraj R, Itoh M, et al. Substrate specificities of catalytic fragments of protein tyrosine phosphatases (HPTP beta, LAR, and CD45) toward phosphotyrosylpeptide substrates and thiophosphotyrosylated peptides as inhibitors. Protein Sci. 1993;2(6) :977-84.
-Which of the following calculations was used to determine the catalytic efficiencies in Table 1?

Question

Passage
Leukocyte common antigen (LCA) enzymes play several important developmental roles in both vertebrates and invertebrates. They regulate numerous cellular processes by catalyzing Reaction 1 in target proteins at specific developmental stages. Consequently, LCA enzyme expression and activity are tightly controlled in various cell types.

Reaction 1The Src protein is critical for cell differentiation and motility, and is activated by LCA enzyme activity. Two LCA enzymes, known as LAR and CD45, were purified and incubated with the phosphopeptide src527 (Figure 1) , derived from Src.

Figure 1 Structure of the src527 peptide used as a substrate in LAR and CD45 assaysThe rate of Reaction 1 was measured at various concentrations of src527 in the presence of either LAR or CD45, and kinetic parameters were determined (Table 1) .Table 1 Kinetic Parameters of LAR and CD45

Compound 1 was synthesized and tested for its effectiveness as an inhibitor of LCA enzymes.

The study in Table 1 was repeated in the presence of Compound 1, which was found to significantly reduce the activities of both LAR and CD45 against the src527 peptide. Compound 1 was subsequently used in assays to examine the in vivo function of LCA enzymes.
Adapted from Cho H, Krishnaraj R, Itoh M, et al. Substrate specificities of catalytic fragments of protein tyrosine phosphatases (HPTP beta, LAR, and CD45) toward phosphotyrosylpeptide substrates and thiophosphotyrosylated peptides as inhibitors. Protein Sci. 1993;2(6) :977-84.
-Which of the following calculations was used to determine the catalytic efficiencies in Table 1?

Quizplus · Accepted Answer

11ecba2d_0ea4_ab51_a3bf_63c58b0c1703_MD0008_00 Enzyme activity is characterized by several parameters, including k_cat, K_m, and catalytic efficiency. k_cat is proportional to the enzyme rate at high substrate concentrations and is equal to V_max divided by total enzyme concentration [E]. K_m is the substrate concentration at which ½V_max occurs. Catalytic efficiency is the ratio k_cat/K_m and is a measure of an enzyme's specificity for a substrate. It is proportional to the reaction rate at low substrate concentrations and is often used to compare two enzymes that catalyze the same reaction. Table 1 gives the K_m values of LAR and CD45, but does not give k_cat. However, the table does give the maximum reaction velocity V_max and the total enzyme concentration [E]. The value of k_cat is equal to V_max/[E]; therefore, k_cat can be calculated by dividing the given V_max value by the given [E]. Catalytic efficiency can then be determined by dividing the resulting k_cat value by K_m. (Choices A and B) Multiplying K_m by V_max is not a step in the calculation of catalytic efficiency. K_m is the denominator of catalytic efficiency, k_cat/K_m. (Choice D) The K_m/V_max ratio is shown in the slope of Lineweaver-Burk plots and is proportional to the inverse of catalytic efficiency. Educational objective: Catalytic efficiency measures the specificity of an enzyme for a substrate and is calculated as k_cat/K_m. k_cat can be calculated by dividing V_max by the total enzyme concentration [E].