A vital role in supporting successful
functioning of biological
cells is played by membrane channels called antiporters. These channel
proteins utilize the concentration gradient of one type of species
to move another type of species in the opposite direction and against
their concentration gradient. It is believed that antiporters operate
via alternating conformational transitions that expose these proteins
to different sides of the membrane, and that only thermodynamics controls
the activation of these channels. Here we explicitly investigate a
chemical-kinetic model of antiporters to argue that there are additional
kinetic constraints that need to be satisfied for these channels to
be operational. This implies that kinetics and not thermodynamics
governs the functioning of antiporters. In addition, the efficiency
of antiporters is analyzed and the most optimal operating conditions
are discussed. Our theoretical analysis clarifies some important aspects
of the molecular mechanisms of biological membrane transport.
Polypeptides present remarkable selectivity challenges for chemical methods. Amino groups are ubiquitous in polypeptide structure, yet few paradigms exist for reactivity and selectivity in arylation of amine groups. This communication...
Successful functioning of biological cells relies on efficient translocation of different materials across cellular membranes. An important part of this transportation system is membrane channels that are known as antiporters and symporters. They exploit the energy stored as a trans-membrane gradient of one type of molecules to transport the other types of molecules against their gradients. For symporters, the directions of both fluxes for driving and driven species coincide, while for antiporters, the fluxes move in opposite directions. There are surprising experimental observations that despite differing only by the direction of transport fluxes, the molecular mechanisms of translocation adopted by antiporters and symporters seem to be drastically different. We present chemical-kinetic models to quantitatively investigate this phenomenon. Our theoretical approach allows us to explain why antiporters mostly utilize a single-site transportation when only one molecule of any type might be associated with the channel. At the same time, the transport in symporters requires two molecules of different types to be simultaneously associated with the channel. In addition, we investigate the kinetic constraints and efficiency of symporters and compare them with the same properties of antiporters. Our theoretical analysis clarifies some important physical–chemical features of cellular trans-membrane transport.
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