Passage

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.
-Based on Table 1, the turnover number (k_cat) of CD45 exceeds that of LAR by what factor?

A) 1.5
B) 2
C) 5
D) 7

Correct Answer:

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