23. Which one of the following does NOT constitute the lipid moiety in lipid-linked membrane
proteins?
(A) Palmitic acid
(B) Stearic acid
(C) Farnesyl groups
(D) Myristic acid
Palmitic acid, myristic acid, and farnesyl groups commonly form the lipid moieties in lipid-linked membrane proteins, while stearic acid does not. Understanding these distinctions clarifies protein anchoring mechanisms in cell biology.
Correct Answer
B) Stearic acid stands out as the lipid that does NOT constitute the lipid moiety in lipid-linked membrane proteins.
Lipid Anchoring Basics
Lipid-linked membrane proteins gain membrane association through covalent attachment of specific hydrophobic lipids to amino acid residues. These modifications, known as prenylation or fatty acylation, target proteins to specific membrane domains for signaling and trafficking roles. Common types include N-myristoylation, S-palmitoylation, and prenylation with farnesyl or geranylgeranyl groups.
Option Breakdown
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Palmitic acid (A): A 16-carbon saturated fatty acid (C16:0) attaches via thioester bonds to cysteine residues in S-palmitoylation, enabling reversible membrane anchoring in proteins like Ras and G-proteins.
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Stearic acid (B): This 18-carbon saturated fatty acid (C18:0) rarely participates in protein lipidation; enzymes favor shorter chains like palmitate or myristate, excluding stearate from standard lipid anchors.
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Farnesyl groups (C): A 15-carbon isoprenoid from prenylation attaches to cysteine in a CaaX motif, permanently anchoring proteins like Ras GTPases to membranes.
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Myristic acid (D): A 14-carbon saturated fatty acid (C14:0) covalently links to N-terminal glycine via amide bonds in N-myristoylation, stabilizing proteins such as Src kinases.
Biological Significance
These lipid moieties dictate protein localization, with palmitate and myristate providing dynamic acyl anchors and farnesyl ensuring stable insertion. Dysregulation links to cancers, as seen in hyperactive lipidated oncoproteins.
