Fundamental question on biochem metabolism

[Daitong]

Hi,

So I'm a bit confused on the fundamentals of metabolism

Mainly, I don't seem to understand gluconeogenesis- why are we breaking down glucose through glycolysis only for it to be remade into glucose through gluconeogenesis? Or does gluconeogenesis occur through different precursors other than from glycolysis (b/c otherwise this just seems like a cyclical, useless cycle).

This probably comes off as an idiotic question, but any help would be greatly appreciated.

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[DrPicard]

Gluconeogenesis is a process carried out by the liver to provide glucose to other tissues. Tissues then use glycolysis to burn this glucose. The liver uses energy from FA beta oxidation to run gluconeogenesis.

Basically during a fasting state adipose releases FFAs --> liver --> beta oxidation --> powers gluconeogenesis both through enzyme inhibition/activation and provision of energy to be inserted into glucose --> glucose gets thrown into the blood --> tissues take it up and use it via glycolysis.

If you look at the activators and inhibitors of glycolysis, you'll notice there are some difference between the liver and other tissues, which line up with what I said above.

 

[Pholston]

For metabolism questions first think which state you are in-- Fed state or Fasting state (~90 minutes after a meal) since they are not happening simultaneouly.

In Fed state, you have enough Glucose available so you are using it and storing the surplus as Glycogen/TG etc (for fasting state) with the help of Insulin.

In Fasting state, you need to break down Glycogen & make Glucose (Gluconeogenesis) since RBCs exclusively can only use Glucose. You also break down TG (beta oxidation) to provide energy for Gluconeogenesis and Ketone bodies for use by Kidney, Brain etc All this is done by stress hormones (Glucagon/Cortisol/Epi). At a later stage you also start breaking down Proteins to provide substrate for Gluconeogenesis (e.g Alanine).

This is, of course, in a nutshell as there are many intermediate steps as well as substrates derived from TG & AAs.