From Cellular Function to Industrial Products
special type of active transport process for some substrates - the so-called group translocation - the
substrate is converted to an impermeable derivative as soon as it crosses the cell membrane. The
different types of active transport are illustrated in Fig. 2.3.
NAD* + H,0 ADP + P
ADP + P
Elll-P Sugar-P Elll
P P P
2.3 Different active transport processes.
A. Primary active transport illustrated by the oxidative phosphorylation. In the oxidation of NADH to
NAD* electrons are donated to oxygen via the electron transport chain (not shown in full detail). When
the electrons are transported through the electron transport chain protons are pumped across the
membrane (the cytosolic membrane in prokaryotes and the inner mitochondrial
eukaryotes). The protons are pumped against a proton and electrochemical gradient and the Gibbs free
energy gained in the oxidation of NADH is hereby transferred to a concentration and electrochemical
gradient. The proton gradient can be used to generate high-energy phosphate bonds in the form of ATP
when the protons slide down the concentration gradient via the enzyme F
Fr ATPase, i.e., the Gibbs free
energy is reconverted from a concentration and electrochemical gradient to high-energy chemical bonds.
The stoichiometry in the process, i.e., the number of protons pumped in the electron transport chain and
the number of protons involved in the F
F,-ATPase depends on the cell type. There is also not a fixed
stoichiometric relationship between oxidation of NADH and ATP synthesis since the proton gradient
may be used for other purposes, e.g., proton symport.
B. Secondary active transport, where the transport of one compound, e.g. a substrate that is to be
transported into the cells, is associated with the transport of another compound, typically protons, that
are transported down a concentration gradient. In order to maintain the proton gradient primary active
transport of protons is required by an ATPase, which is a primary active transporter.
C. Transport by group translocation where the transport is associated with a conversion of the substrate
to another compound. The most well-known group translocation process is the phosphotransferase
system in bacteria. Here sugars are phosphorylated upon transport, and the phosphate group is donated
by phosphoenolpyruvate (PEP), which is converted to pyruvate. The phosphate group is transferred from
PEP to the sugar via several proteins, some of which are specific for the individual sugar and some of
which are used in the translocation of different sugars.