Glucose Carrier Proteins in Cell Membranes

Question

I'm using Campbell's Biology textbook, and it states that certain carrier proteins transport glucose across the cell membrane much faster than would occur normally. It states that the "glucose transporter" is an extremely selective protein, which doesn't accept even fructose, which is an isomer of glucose. However, another hexose, galactose, appears to me to be even more similar to glucose- it's just that one of the H-OH pairs is flipped. In comparison, though fructose is indeed an isomer, it's a ketose, but glucose and galactose are both aldoses. Given this, I'm pretty sure galactose is more similar to fructose.

My question is: is the carrier protein specific enough to carry only glucose and not even galactose? If it isn't and it's able to carry galactose, what would happen if galactose was transported into the cell in greater quantities?

Note: I'm a bio beginner, so please have some patience if my questions are nonsensical or my facts are incorrect, any corrections would be welcome.

Answer 1

First thing's first: there are many different kinds of glucose transporters (genes), even within the same organism. These genes share partial sequence identity in mammals, and are part of the SLC2A, or "GLUT" gene family. In yeast, there exist glucose- and galactose-specific transporters.

Often, not always, a glucose transporter type may be tissue specific. GLUT3 is expressed in neurons, in the placenta, and elsewhere. Compare this to the expression of GLUT1, found in red blood cells, and GLUT4, found in adipose and skeletal muscle tissue.

Each kind of transporter has various affinities for glucose (or other sugars for that matter), and various levels of permeability for similar molecules. This is what is called the specificity of the transporter. The rate at which sugars can pass through the pore of the transporter is called the frequency. Together, these properties define the function of a hexose transporter, and hence why there are different kinds with site-specific expression across the body.

In terms of regulation: GLUT1 is dynamically regulated based on available glucose (up-regulated during shortage and down-regulated during excess). This is not the case with GLUT4; instead, GLUT4 is insulin-regulated, so that muscles and fat tissues can uptake glucose for storage, which is a hormone-controlled process.

Here is a great webpage to help you understand the characteristics of hexose transporters, especially with regard to galactose, fructose and glucose.