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.
-The information in the passage suggests that the effects of LCA activity could be counteracted by which type of enzymatic activity?

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.
-The information in the passage suggests that the effects of LCA activity could be counteracted by which type of enzymatic activity?

Quizplus · Accepted Answer

11ecba2d_0ea5_bcc2_a3bf_35a1d3264c47_MD0008_00 Enzymes are divided into classes and subclasses based on the kinds of reactions they catalyze. Biological processes are frequently regulated by opposing reactions such as phosphorylation and dephosphorylation, with each reaction counteracting the other. Phosphorylation reactions are most commonly carried out by kinases, a type of transferase that moves a phosphate group (HPO₄²⁻) from a donor molecule (usually ATP) to an acceptor molecule. Dephosphorylation reactions are catalyzed by phosphatases that remove an inorganic phosphate group by adding water across a bond (hydrolysis). Reaction 1 shows that LCA enzymes catalyze the hydrolysis of the phosphoester bond in phosphotyrosine residues. Enzymes that hydrolyze bonds are known as hydrolases, and the subset of hydrolases that remove phosphate groups, including LCA enzymes, are classified as phosphatases. The question asks which type of enzymatic activity would most likely counteract the effects of LCA enzymes. Because LCA enzymes are phosphatases that facilitate the removal of phosphate groups, their activity is most likely counteracted by kinase activity, which adds phosphate. (Choice A) Dehydrogenases are a type of oxidoreductase that transfer electrons through hydrogen atoms or hydride ions. They do not counteract phosphatase activity. (Choice C) Synthetases are a type of ligase that link two molecules together and typically use ATP hydrolysis as a source of energy. They should not be confused with synthases, which are lyases and do not require energy input. Neither synthetases nor synthases phosphorylate acceptor molecules. (Choice D) Mutases are a type of isomerase that rearrange functional groups within a molecule. They do not counteract phosphatase activity. Educational objective: Phosphatases catalyze the dephosphorylation of their substrates by hydrolysis, producing inorganic phosphate. Kinases catalyze the phosphorylation of their substrates by transferring organic phosphate from a donor molecule, usually ATP. The two enzyme types often regulate biological processes by counteracting each others' effects.