a) Some of the membrane proteins are associated with the outer phospholipid laye
ID: 192889 • Letter: A
Question
a) Some of the membrane proteins are associated with the outer phospholipid layer of the plasma membrane and protrude out from the membrane sufficiently to allow exposed tyrosine groups to be labeled by the LP reaction. Lactoperoxidase O 1251 600 bilayer Hypotonic medium Lactoperoxidase A Method for Labeling Proteins Exposed on One or Both Surfaces of a Membrane Vesicle. (a) Whern lactoperoxidase (LP) and 125I are present in the solution outside a membrane vesicle, LP catalyzes the labeling of membrane proteins Isotonic 123I exposed on the outer membrane surface (i.e., proteins A and B). If membrane vesicles are (b) first incubated in a hypotonic medium to Lactop oxidaseO o make them permeable to LP and (c) then transferred to an isotonic solution containing 15I but no external LB proteins exposed on the inner membrane surface (i.e., proteins B and C) become labeled.Explanation / Answer
From the given figure, we know that exposed regions of membrane proteins can be labeled with 125I by the lactoperoxidase (LP) reaction. Similarly, carbohydrate side chains of membrane glycoproteins can be labeled with 3H by oxidation of galactose groups with galactose oxidase (GO) followed by reduction with tritiated borohydride (3HBH4). Both LP and GO are too large to penetrate into the interior of an intact cell.
Iodination by radioactive (heavier isotope) Iodine is carried out by the enzyme more generally at Tyrosine residues and less typically at Histidine residues. Experimentally, iodination selectivity of this enzyme for tyrosine residue number 74 in cytochrome c further adds to this explanation.
ANSWER Hence a) Some of the membrane proteins are associated with the outer phospholipid layer of the plasma membrane and protrude out from the membrane sufficiently to allow exposed tyrosine groups to be labeled by the LP reaction.
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