1 - Gluconeogenesis

What is gluconeogenesis?

The synthesis of "new" glucose from three or four carbon precursors like lactate, pyruvate, glycerol, and oxaloacetate, primarily occurring in the liver.

1 - What is the first step in gluconeogenesis when starting from lactate?

Lactate is converted to pyruvate by Lactate Dehydrogenase in the cytosol.

2 - After pyruvate is formed, what is the next step in gluconeogenesis?

Pyruvate is transported into the mitochondria and converted to oxaloacetate by Pyruvate Carboxylase (requires biotin).

3 - What happens to oxaloacetate in the mitochondria?

It is converted to malate by mitochondrial Malate Dehydrogenase to allow transport into the cytosol.

4 - What happens to malate once it enters the cytosol?

Malate is converted back to oxaloacetate by cytosolic Malate Dehydrogenase.

5 - How is oxaloacetate converted to phosphoenolpyruvate (PEP)?

By cytosolic Phosphoenolpyruvate Carboxykinase (PEPCK) using GTP.

6 - What happens to PEP in gluconeogenesis?

PEP is converted through a series of reversible glycolytic steps to fructose-1,6-bisphosphate.

7 - Which enzyme converts fructose-1,6-bisphosphate to fructose-6-phosphate?

Fructose-1,6-bisphosphatase, bypassing the irreversible phosphofructokinase-1 step of glycolysis.

8 - What enzyme converts fructose-6-phosphate to glucose-6-phosphate?

Phosphohexose Isomerase (reversible glycolytic enzyme).

9 - What is the final step in gluconeogenesis?

Glucose-6-phosphatase converts glucose-6-phosphate to free glucose, primarily in the liver and kidney.

Under what metabolic conditions does the organism perform gluconeogenesis?

The body tries to maintain a glucose concentration of about 1 mg/mL in the blood.

Entry of glycerol into gluconeogenesis uses triacylglycerol metabolism.

What tissues / organs are capable of performing gluconeogenesis?

Tissues: Red blood cell, nervous tissue, adrenal medulla, testis/ovaries Organs: Liver, kidney, to a lesser extent in small intestine, skeletal muscle, adipose Precursors: Lactate, pyruvate, oxaloacetate, and glycerol

Why do these tissues perform gluconeogenesis?

To maintain blood glucose levels to supply dependent tissues.

What is the fate of the glucose synthesized by these tissues?

Meets the glucose needs of tissues.

Cite two reasons why gluconeogenesis is not the simple reverse of glycolysis.

1. There is no Le-Chalier’s Principle to drive reaction forward/backward

2. There are 4 additional enzymes that bypass reversible steps

Compare glycolysis and gluconeogenesis.

1. Glycolysis:

   1. Irreversible reaction enzymes: pyruvate kinase, phosphofructokinase, hexokinase

   2. 7 of the 10 steps are reversible

2. Gluconeogenesis:

   1. No reversible reactions

What enzymes are different?

Glycolysis: Hexokinase, phosphofructokinase, pyruvate kinase

Why are additional enzymes necessary / involved in gluconeogenesis?

To bypass the reversible reactions that are present in glycolysis.

What are the additional enzymes of gluconeogenesis?

lactate dehydrogenase, pyruvate carboxylase, PEP carboxykinase

Why are there control points for gluconeogenesis?

Because gluconeogenesis is energy expensive!

Which enzymes exclusive to gluconeogenesis are allosteric enzymes?

Pyruvate carboxylase

Fructose 1,6 bisphosphate

What are the positive allosteric effectors of Pyruvate carboxylase and

Fructose 1,6 bisphosphate?

Activated by Acetyl-CoA and ATP.

What are the negative allosteric effectors of Pyruvate carboxylase and

Fructose 1,6 bisphosphate?

Inhibited by AMP and Fructose-2,6-bisphosphate.

How is gluconeogenesis and glycolysis reciprocally controlled?

Insulin – inhibits synthesis of phosphoenolpyruvate carboxykinase, fructose 1,6 bisphosphate, glucose 6 phosphate.

Glucagon – stimulates synthesis of phosphoenolpyruvate carboxykinase, fructose 1,6 bisphosphate, glucose 6 phosphate.

What are the three irreversible steps of glycolysis that are bypassed in gluconeogenesis?

Steps catalyzed by Pyruvate Kinase, Phosphofructokinase-1, and Hexokinase.

What enzyme bypasses Phosphofructokinase-1?

ructose-1,6-bisphosphatase.

What enzyme bypasses Hexokinase?

Glucose-6-phosphatase.

How many high-energy phosphate bonds are required to synthesize one glucose molecule from lactate or pyruvate?

Six (4 ATP and 2 GTP).