Saturday, 24th March 2012
Pores and transporters
Arguably the defining feature of a cell is its membrane, which determines what is part of a cell and what isn't. The membrane is vital for creating spaces in which the distribution of chemicals are not in equilibrium. In particular, the DNA, RNA and proteins must be at a concentration much higher than found outside of a cell. A disequilibrium is achieved by selectively allowing chemicals to cross the membrane.
A pore is a protein that allows a substance to passively diffuse across a membrane. We can consider a pore to be an enzyme that converts a chemical inside the cells into the same chemical outside of the cell. For example, an A-pore, catalyses the reaction:
Acell ⇌ Asolution
In these pore "reactions" the equilbrium constant is always 1, i.e. equilbrium is reached when the concentration of the chemical inside the cell equals the concentraion outside.
Despite the fact that pore only allow passive diffusion of a substrate, they can still be used to concentrate metabolites within a cell. For example, the cell below has an A-pore, an AD-pore, and the enzyme ADase.
If we put this cell containing no metabolites in a solution of AD, then the AD will diffuse in and be broken down by ADase into A and D. Since, it has an A-pore, substrate A will diffuse out of the cell, but substrate D cannot, so is concentrated. Since ADase strongly favours the AD hydrolysis reaction, equilibrium is only reached when D is very, very concentrated.
In the same way that enzymes can be coupled together, as described on the previous page, pores can also be coupled, creating transporters. For example, we can create an A-D symporter by combining an A-pore with a D-pore:
Acell + Dcell ⇌ Asolution + Dsolution
By combining an A-pore with a D-pore in reverse directions, we can create an antiporter:
Acell + Dsolution ⇌ Asolution + Dcell
Furthermore, since we treat pore as a type of enzyme, we can couple a pore to an enzyme to create an active transporter. For example, by coupling an ADase to a B-pore, we create an ADase-driven B-pump, which can use the favourable AD hydrolysis reaction to pump B into the cell against a concentration gradient:
AD + Bsolution ⇌ A + D + Bcell