Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

24 Chapter 2
surface glycoprotein
glycolipids (gangliosides)
EXTERIOR
polar head group
integral
membrane
protein
PHOSPHOLIPID
BILAYER
non-polar fatty acids
INTERIOR
membrane-associated
protein
membrane-anchored
protein
Figure 2.22 Membrane proteins. Black dots represent monosaccharide units attached to glycolipids
and glycoproteins.
(2) intrinsic or integral membrane proteins which are buried in the membrane
bilayer.
Why should some enzymes be membrane associated Many biological
processes involve passage of either a molecule or a ‘signal’ across biological
membranes, and these processes are often mediated by membrane proteins.
These membrane processes have very important cellular functions such as cell–
cell signalling, response to external stimuli, transport of essential nutrients and
export of cellular products. In many cases these membrane proteins have an
associated catalytic activity and are therefore enzymes.
Intrinsic membrane proteins which completely span the membrane bilayer
often possess multiple transmembrane a-helices containing exclusively hydrophobic
or non-polar amino acid side chains which interact favourably with the
hydrophobic environment of the lipid bilayer. The structure of bacteriorhodopsin,
a light-harvesting protein found in photosynthetic bacteria, is shown in
Figure 2.23.
2.10 Glycoproteins
A signiWcant number of proteins found in animal and plant cells contain an
additional structural feature attached covalently to the polypeptide backbone
of the protein: they are glycosylated by attachment of carbohydrates. The
attached carbohydates can be monosaccharides such as glucose, or complex
oligosaccharides. The glycoproteins are usually membrane proteins residing in