Biology Reference
In-Depth Information
CHAPTER
6
M embrane Protein s
OUTLINE
A. Introduction
85
a. Endo and Ecto Proteins
94
b. Trans-membrane Proteins
94
B. The Amino Acids
86
c. Prenylated Lipid-Anchored
Proteins
C. How Many Membrane Protein Types
are there?
102
91
d. GPI-Anchored Proteins
103
I. Peripheral Proteins
91
Summary
103
II. Amphitropic Proteins
93
III. Integral Proteins
93
References
104
A. INTRODUCTION
Membranes are composed primarily of lipids (Chapters 4 and 5) and proteins ( Chapter 6 )
with a variable amount of carbohydrates (Chapter 7) attached to the surface [1] . By number,
lipids are the major component of all membranes where they exceed proteins by ~40 to 1 up
to ~200 to 1 and are responsible for the basic membrane structure and environment. Lipids,
however, do not directly express any biochemical catalytic activity. The amount of proteins in
a membrane varies over a wide range depending on how biochemically active the membrane
is. It has been estimated that more than half of all proteins have some association with
a membrane. At the low extreme is the myelin sheath whose primary function is to insulate
nerve cells. Only ~20% of the myelin sheath by weight is protein. Myelin lipids outnumber
the proteins by ~200 to 1. At the other extreme is the mitochondrial inner membrane that
supports many complex functions including electron transport, oxidative phosphorylation
and numerous trans-membrane transport systems. By weight this membrane is ~75%
protein. But even here lipids outnumber the proteins by more than 40 to 1. Since different
types of membranes have very different functions, but have similar basic physical properties,
 
Search WWH ::




Custom Search