4. The table below summarizes the results of an experiment using a group of norm

Question

4. The table below summarizes the results of an experiment using a group of normal cells and three other cell lines of the same type of cells that each carry a different mutation in a membrane receptor protein. The receptor is part of a signal transduction pathway that is stimulated by glucose-6-phosphate. Cells were tested for their ability to respond, via the signal transduction pathway, to a variety of ligand molecule, as shown in the table Based on these data, which statement about these three mutations is plausible? Percent cellular response via signal transduction pathway Ligand Normal (nonmutant) Mutant 1 Mutant 2 Mutant 3 Glucose-6- phosphate Glucose Galactose Mannose 100 100 95 97 94 89 95 Mutation 3 has no effect on ligand binding, while mutations 1 and 2 have dramatic effects on ligand binding Mutations 1 and 3 have no effect on ligand binding, while mutation 2 has a dramatic effect on ligand binding Mutations 1 and 3 broaden the specificity of ligand binding, while mutation 2 eliminates ligand binding Mutation 1 broadens the specificity of ligand binding, while mutations 2 and 3 eliminate ligand binding Mutations 1, 2, and 3 each broaden the specificity of ligand binding in different ways o

Explanation / Answer

Membrane receptor proteins facilitates the binding of ligand molecules, in form of hormones, neurotransmitters, etc., to bind to them and trigger specific effector activities in the cells. These ligands, on binding to receptors, (sometimes through second messengers) initiate a signal transduction pathway. Mutations in the membrane receptor proteins, may alter the protein structure and conformation, which in turn may increase or decrease the specificity of the ligand to bind with the receptor. In this case the signal transduction pathway is induced by glucose-6-phosphate. The normal cell (non-mutant) and mutant-1 has same by glucose-6-phosphate, and higher values for glucose, galactose, and mannose. This reflects, that mutation type 1, though have similar stimulatory effect, but can now bind to more types of ligands. Thus, mutation I has broaden the specificity of ligand binding. In case of mutation 2 and 3 values reflect the negative effect on ligand binding, due to change in three dimensional structure, and access to binding sites. Hence correct option is 4.

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