Biology Reference
In-Depth Information
CHAPTER
7
Membrane Sugars
OUTLINE
A. Introduction
107
D. GPI-Anchored Proteins
114
B. Glycolipids
111
Summary
115
C. Glycoproteins
112
References
115
A. INTRODUCTION
In addition to lipids and proteins, a third major component exists, primarily on the outer
surface of the cell plasma membrane. This component is a collection of very water-soluble
carbohydrates (sugars) [1
4] . The word 'carbohydrate' is a contraction of 'hydrated carbon',
(C H 2 O) n , the empirical formulation of most sugars. For example, the most common sugar,
glucose, is C 6 H 12 O 6 or C 6 (H 2 O) 6 , and is thus a 'hydrated carbon' ( Figure 7.1 ).
Unlike membrane lipids and proteins, carbohydrates have no hydrophobic segments and
so are never found integrated into the non-polar interior of membranes. Instead, they sit on
the membrane surface, where they are favorably surrounded by water. Their location,
almost exclusively on the outer surface of the plasma membrane, strongly indicates that
their main function is to interact with the cell's external environment. It is well established
that, like lipids, carbohydrates have no enzymatic functions and so must influence
membrane biochemistry through indirect methods. General roles for membrane carbohy-
drates include: biosynthetic sorting, protecting membrane proteins from proteolysis, assist-
ing in protein folding (serves as chaperones), enhancing protein stability, roles in cell
identification, recognition and cell
e
cell adhesion, providing immunological properties
(they are antigens), and serving as receptors. Carbohydrates clearly have a large impact
on plasma membrane function.
e
 
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