Biology Reference
In-Depth Information
TABLE 14.3
The Four Types of ATP-utilizing Primary Active
Transport Systems.
ATP-utilizing primary
active transport systems
Example
Na
þ
/K
þ
ATPase
Ca
2
þ
pump
H
þ
acid pump
P-Type
F-Type
Mitochondrial ATP synthase
Chloroplast ATP synthase
V-Type
Vacuolar ATPase
ABC
(ATP binding
cassette transporter)
Many
Primary Active Transport
Primary active transport is also called direct active transport or uniport. It involves
using energy (usually ATP)
to directly pump a solute against
its electrochemical
gradient.
The most studied example of primary active transport is the plasma membrane Na
þ
/K
þ
ATPase discussed below. Other familiar examples of primary active transport are the redox
H
þ
-gradient generating system of mitochondria, the light-driven H
þ
-gradient generating
system of photosynthetic thylakoid membranes and the ATP-driven acid (H
þ
) pump found
in the epithelial lining of the stomach. There are 4 basic types of ATP-utilizing primary active
transport systems (
Table 14.3
).
Na
D
/K
D
ATPase
A crucial active transport protein is the plasma membrane-bound Na
þ
/K
þ
ATPase.
This single enzyme accounts for one-third of human energy expenditure and is often
referred to as the 'pacemaker for metabolism'. As a result, the Na
þ
/K
þ
ATPase has
been extensively studied for more than fifty years. The enzyme was discovered in
1957 by Jens Skou (
Figure 14.14
) who, 40 years later, was awarded the 1997 Nobel Prize
in Chemistry. As is often the case in biochemistry, a serendipitous discovery of a natural
product from the jungles of Africa has been instrumental in unraveling the enzyme's
mechanism of action. The compound is ouabain, a cardiac glucoside, first discovered
in a poison added to the tip of Somali tribesmen's hunting arrows (
Figure 14.15
). In
fact the name ouabain comes from the Somali word waabaayo which means 'arrow
poison'. The sources of ouabain are ripe seeds and bark of certain African plants and
it is potent enough to kill a hippopotamus with a single arrow. For decades after its
