Glucose has many hydroxyl groups which makes it very hydrophilic. There are two types
of transporters. The GLUTs are all sodium and ATP-independent and are found all
throughout the body in all cell types. The other type of glucose transporter is known as
the sodium-dependent glucose transporters or the SGLTs. These are sodium-dependent
and require ATP. They can be found in the intestine, renal tubules as well as the blood
brain barrier. We have five very important glucose transporters in the body known as
GLUT1, GLUT2, GLUT3, GLUT4, GLUT5. GLUT1 transporters are found in the blood
(when RBCs are erythrocytes). They’re also found in the blood brain barrier and we even
have some in the heart. The red blood cells use GLUT1 heavily to uptake glucose. The
main important key point for GLUT1 is that they are insulin independent. Red Blood
Cells never need insulin to uptake glucose so they always use glucose even when insulin
is not present. GLUT2 transporters are found in the liver, the pancreas and in the small
intestine as well. These are also insulin independent and they have a high Km which
means they have a very low affinity for glucose. That means that the liver, the pancreas
and small intestine only uptake glucose through GLUT2 when glucose concentrations are
very high. A lot of times the liver won’t take up any glucose, it’ll let the rest of the body
take glucose for other usage. The liver will uptake glucose through GLUT2 when glucose
concentrations are very high. It’ll uptake glucose to store it as glycogen. The pancreas
does this as well with beta-cells. Beta-cells will uptake glucose when glucose
concentrations are relatively high. Which means that beta-cells uptake glucose and then
the beta cells will release insulin to compensate for the high glucose level. The next
glucose transporter is GLUT3 which found in the brain in the neurons as well as the
sperm. It is insulin independent and the key point with GLUT3 is that it has a low Km
giving it a high affinity for glucose (always saturated with glucose). This tells us that the
brain and the neurons in the brain always take up glucose which is done with a high
affinity. If there’s any glucose present at all they will make sure that they take up the
glucose. The brain makes sure that it takes up its required energy substrates. It makes
sure it maintains its metabolism at a constant state regardless of what’s going on in the
rest of the body. GLUT4 are found in the skeletal muscle, adipose tissue, and the heart. It
is also good to recall that the heart has GLUT1 (insulin independency). GLUT4
outnumbers GLUT1 in the heart by a 3 to1 ratio, which makes GLUT4 very significant
for heart metabolism. It is also good to note that GLUT4 is insulin dependent so when
insulin is released, it allows the translocation and the incorporation of GLUT4 into the
cell membrane of skeletal muscle adipose tissue in the heart to allow those organs to
uptake glucose. This is why insulin allows uptake of glucose because it acts through
GLUT4. GLUT4 has a moderate Km giving it a moderate affinity for glucose. GLUT5
transporters are found in the enterocytes of the intestinal epithelium. They’re particularly
on the luminal side so they face the lumen of the small intestines and they are insulin
independent as well. They are important for fructose transport (fructose uptake through
GLUT5, fructose is always taken up by GLUT5). SGLT1 are found in the enterocytes of
the intestinal epithelium (as well as the luminal side facing the lumen of the small
intestine). SGLT1 is Insulin-independent and ATP/Na-dependent as well as important for
glucose absorption. In SGLT2 is found in the proximal tubule of a nephron. The nephron
is the functional unit of the kidney. SGLT2s are also insulin independent and ATP and
sodium dependent and very important for glucose retention. This prevents the loss of
glucose in our urine and can come into play big time when we start talking about diabetes
when levels of glucose are so high that these transporters become saturated and you
actually lose some of your glucose in your urine.