A text I am reading says

"GLUT2 is a low-affinity transporter in hepatocytes and pancreatic cells. After a meal, blood travels through the hepatic portal vein and GLUT 2 captures excess glucose for storage.

When concentration drops below Km, much of the remainder bypasses the liver and enters circulation. The Km of GLUT2 is quite high so the liver will pick up glucose in proportion to its concentration in the blood".

I know Km is a measure indirectly of binding affinity.

Rate also is based on concentration so at a higher concentration the rate is higher.

But the passage seems to be saying that at low Km, most of the glucose will not be transported, while at high glucose concentrations, more will be transported.

But i thought Km was a constant so I am confused.

  • $\begingroup$ Km is a constant which follows the unit of substrate concentration.It has a constant value. "Km of GLUT 2 is quite high" which implies that it's substrate affinity is low (inversely related). When the substrate concentration drops below that required for binding with GLUT 2 (Km value) obviously there would be bypass and no uptake by liver. $\endgroup$ – user 33690 May 12 '18 at 11:54
  • $\begingroup$ It would help if you would either copy the text acurately or provide a link. You are obviously a non-native English speaker as you have left out the articles, and probably other things besides. I assume that the sentence actually reads or means "When the concentration of glucose drops below the Km of GLUT2…" Km has units of concentration, so there is no question of the Km changing, it is the concentration of glucose that is changing, and its relationship to the Km of GLUT2 that determines whether this transporter acts on it, in the way described in any text. $\endgroup$ – David May 12 '18 at 20:52
  • $\begingroup$ I'm voting to close this question as off-topic because it is based on a misreading of a text. $\endgroup$ – David May 12 '18 at 20:54
  • $\begingroup$ Agree with @David - you're misreading... Km is the concentration that the transporter works at one-half the maximal rate (velocity) of transport. There's nothing in what you type that says anything about changing the Km (Km is a property of the GLUT2 transporter). $\endgroup$ – Vance L Albaugh May 13 '18 at 19:11

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