glucose absorption

[raghda]

Hi everyone
sorry I don't know where I put my question
someone tell me how sodium- dependent glucose transporter SGLT binds to glucose to carry through the plasma membrane to the cytoplasm of cells

and I need textbook of Biochemistry ,which one?!

[kotoreru]

Sorry, can't help on the glucose part but I can recommend a textbook - Biochemistry by Lubert Stryer.

[MrMistery]

Lehninger Principles of Biochemistry is also good.

Here is how the glucose-sodium transporter works.
You are at the outside of the cell. one glucose molecule and two sodium molecules bind onto the outside of the cell.in this moment, the glucose transporter changes its conformation so that it exposes both the sodium molecules and the glucose molecules on the cytoplasmic side of the membrane. now, because of the concentration gradient, the sodium molecules and the glucose molecule come off the membrane. But now, the protein has all 3 binding sites exposed. in this state, it changes to a 3d conformation with the binding sites on the outside, ready to transport another glucose molecule by the aid of the sodium molecules.

[raghda]

this means that energy is required to pump the sodium out off the cell ,so it's considered active transport method . isn't it?

[MrMistery]

yes it is. Biochemistry books will define two types of active transport:
1. primary active transport - uses the energy from the hydrolysis of ATP directly to pump a substance against its concentration gradient - the Na/K pump is an example
2. secondary active transport - uses energy under the form of a gradient. It is then necessary to create such a gradient by primary active transport - the glucose/Na simporter is an example

However, in the case of the Na/glucose simporter, ATP is NOT used to generate the sodium gradient. there is already a lot of sodium on the outside, which comes from your food.

[raghda]

I wonder if the 2 types of active transport occur in the same time ?
for me, NO as you said

in the case of the Na/glucose simporter, ATP is NOT used to generate the sodium gradient. there is already a lot of sodium on the outside, which comes from your food.

so it depends on the amount of sodium isn't it?

[Jammerz]

The Na/glucose symporter uses the sodium gradient created and maintained by the Na/K pump, which is a primary active transporter and uses ATP.

[raghda]

hello people!
I and my collages have serious argument on the number of sodium molecules is it one or two ?

[Jammerz]

Number of sodium molecules in what?

[raghda]

the sodium molecules which bind with SGLT

[lara]

it is done by another Na K pump

[MrMistery]

@raghda
Both you and your peers are correct.
There are 2 isoforms of the SGTL.
SGTL1 is located in the small intestine and in a very small proportion in the nephron proximat tubule. It binds 2 Na molecules for every glucose molecule. Since this one is the example in most biochemistry books, some lazy professors will tell you that this is the case for all SGTL - which is wrong
SGTL2 is located in the proximal tubule of the nephron, where it handles something like 99% of the glucose transport. It binds 1 molecule of Na for every glucose molecule.

Regards,
Andrew