How Fructose and Glucose are Stored

[Carl Fredrik Ahl]

Hi,

I think I have heard that the liver can only store fructose and the muscles can only use  glucose (to make glucagon). Is this true? What happens then if the muscle needs energy and you only consume fructose? Will no glucagon get stored in the muscles then? I have also heard that fructose can get converted to glucose. Can someone please explain? I am confused.

[BabcockHall]

Almost nothing of what you wrote is true.  Liver stores glycogen and can make glucose from it.  Glucagon is a hormone, and is not the same thing as glycogen.  Glucose and fructose have a complex relationship metabolically.  Do you have a copy of Nelson and Cox's biochemistry textbook or something equivalent?

[SamIam]

Fructose metabolism in muscle differs little than that of glucose. Hexokinase witch converts glucose to G6P on entry into muscle cells, also phosphorylates fructose yielding F6P.

[Carl Fredrik Ahl]

6 hours ago, BabcockHall said:

Almost nothing of what you wrote is true.  Liver stores glycogen and can make glucose from it.  Glucagon is a hormone, and is not the same thing as glycogen.  Glucose and fructose have a complex relationship metabolically.  Do you have a copy of Nelson and Cox's biochemistry textbook or something equivalent?

Thx for the answer. No I do not.

6 hours ago, SamIam said:

Fructose metabolism in muscle differs little than that of glucose. Hexokinase witch converts glucose to G6P on entry into muscle cells, also phosphorylates fructose yielding F6P.

Thanks for the answer.

[BabcockHall]

16 hours ago, SamIam said:

Fructose metabolism in muscle differs little than that of glucose. Hexokinase witch converts glucose to G6P on entry into muscle cells, also phosphorylates fructose yielding F6P.

I do not disagree, but in liver fructokinase converts fructose into fructose 1-phosphate.  IIRC the regulatory mechanisms governing hexokinase and phosphofructokinase are thus bypassed and glycolysis is entered into later on in the pathway.  @OP, this is why having a good biochemistry textbook is helpful.

Edited October 20, 2018 by BabcockHall

[SamIam]

I was referring to muscle not the liver.                                                                                                                         Liver contains little hexokinase; rather it contains glucokinase. Fructose metabolism in liver must therefore differ from that in muscle. In fact, liver converts fructose to glycolytic intermediates through a pathway that involves 6 enzymes.                                                                       Fructokinase catalyzes the phosphorylation of fructose by ATP at C(1) to form fructose-1-phosphate..                                                                  Class 1 aldolase has several isoenzymic forms. Muscle contains Type A aldolase. Liver, however, contains Type B aldolase, which uses froctose-1-phosphate as a substrate.                                                                                                                                                                                       Direct phosphorylation of glyceraldehyde by ATP through the action of glyceraldehyde kinase forms the glycolytic intermediate glyceral dehyde-3-phosphate.                                                                                                                                                                                                                           I am tired of typing the Last three can be explained by someone else. 

, 

1 hour ago, BabcockHall said:

I do not disagree, but in liver fructokinase converts fructose into fructose 1-phosphate.  IIRC the regulatory mechanisms governing hexokinase and phosphofructokinase are thus bypassed and glycolysis is entered into later on in the pathway.  @OP, this is why having a good biochemistry textbook is helpful.

 

Edited October 20, 2018 by SamIam

[BabcockHall]

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4145298/

"Normal Roles for Dietary Fructose in Carbohydrate Metabolism"

"The liver is the major site for fructose metabolism."