Biology Reference
In-Depth Information
2
OUTSIDE OF CELL
Transport protein
shifts to alternative
conformation
3
1
Glucose binds
to binding site
open to outside
Glucose is
released to the
inside and
protein returns
to its original
conformation
O
O
O
Glucose
T
2
O
INSIDE OF CELL
T
1
T
1
Glucose
Glucose transporter
(GluTt)
FIGURE 14.6
Glucose facilitated diffusion transporter GLUT-1
[10]
.
channels, all of which provide essential membrane-associated functions including setting
and shaping action potentials and hormone secretion:
1. Calcium-activated potassium channel.
2. Inwardly rectifying potassium channel.
3. Tandem pore domain potassium channel.
4. Voltage-gated potassium channel.
Potassium channels are composed of four protein subunits that can be the same (homote-
tramer) or closely related (heterotetramer). All potassium channel subunits have a distinctive
pore-loop structure that sits at the top of the channel and is responsible for potassium selec-
tivity
[12]
. This is often referred to as a selectivity or filter loop. The selectivity filter strips the
waters of hydration from the potassium ion. Further down the structure is a 10
˚
diameter
trans-membrane, water filled central channel that conducts potassium across the membrane.
Elucidating the three-dimensional structure of this important integral membrane protein by
X-ray crystallography (
Figure 14.7
)
[12]
was a seminal accomplishment in the field of
membrane biophysics. For this work, from 1998, Rod MacKinnon of Rockefeller University
(
Figure 14.8
) was awarded the 2003 Nobel Prize in Chemistry. Until the potassium channel
work, just obtaining the structure of non-water soluble proteins was next to impossible.
MacKinnon's work not only elucidated the structure of the potassium channel but also its
molecular mechanism. It has served as a blueprint for determining the structure of other
membrane proteins and has greatly stimulated interest in the field.
Sodium Channel
In some ways Na
þ
channels
[13]
parallel the action of K
þ
channels. They are both facili-
tated diffusion carriers that conduct the cation down the ion's electrochemical gradient. In
excitable cells such as neurons, myocytes, and some glia, Na
þ
channels are responsible for
