19-CARBOHYDRATE METABOLISM – GLUCONEOGENESIS – Dr. Block

Question 1

What steps in the glycolysis pathway cannot be reversed for use in gluconeogenesis?

Answer 1

Three steps in the synthesis pathway prevent reversal into the glycolysis pathway.

○ glucose → glucose-6-phosphate (via hexokinase/glucokinase)
○ fructose 6-phosphate → fructose 1,6-bisphosphate (via PFK-1)
○ phosphoenol pyruvate (PEP) → pyruvate (via pyruvate kinase)

Question 2

What two types of approaches are used to get around barriers encountered in reversing the glycolytic pathway?

Answer 2

○ enzymes specific to the gluconeogenesis are used
○ individual reactions in the gluconeogenesis pathway take place not only in the cytosol, but some also take place inside the mitochondria and endoplasmic reticulum separated from the cytosolic reactions

Question 3

What subcellular organelles are involved in gluconeogenesis?

Answer 3

The mitochondria and the endoplasmic reticulum are cellular compartments that are used by gluconeogenesis. This separates it from glycolysis, which occurs exclusively in the cytosol.

Question 4

What is the malate shuttle, and where does it take place?

Answer 4

○ the malate shuttle takes malate from mitochondria → cytosol, and also allows for the shuttling of NADH to the cytosol as well

After pyruvate is converted to oxaloacetate by pyruvate carboxylase, it must be transported out to the cytosol. The mitochondrial membrane does not allow oxaloacetate to pass through its membrane. So, it must be converted to malate, by malate dehydrogenase. Malate can then be transported across the membrane to the cytosol. The cytosolic malate dehydrogenase enzyme will convert malate back into oxaloacetate. This “shuttle” not only can transport oxaloacetate across the membrane, but NADH as well. The mitochondrial malate dehydrogenase uses up mitochondrial NADH for the conversion. The cytosolic malate dehydrogenase will produce NADH in the reverse reaction, replenishing cytosolic NADH. (Aspartate Amino Transferase can be used as an alternative method for transporting oxaloacetate when cytosolic NADH is high.)

Question 5

How does glucose-6-phosphate get converted to glucose?

Answer 5

The conversion of G-6-phosphate occurs within the lumen of the endoplasmic reticulum. There is a series of transporter proteins that plays an integral role in the conversion to glucose. First, T1 translocase will pump G-6-Phophate into the lumen of the ER. The phosphatase for converting G-6-Phosphate into Glucose lies within the lumen. After dephosphorylation, the disassociated Pi is removed from the lumen into the cytosol by T2 translocase. T2 requires Ca2+ to pump out Pi. A separate channel allows Ca2+ to flow into the lumen of the ER. Then the final product, glucose, is pumped out of the ER to the cytosol by T3 translocase.

Question 6

Which enzyme used in gluconeogenesis is both inside mitochondria and outside in the cytosol?

Answer 6

Malate dehydrogenase

Question 7

What tissues are capable of carrying out all steps of gluconeogenesis?

Answer 7

○ liver

○ kidney (during starvation)

Question 8

What is the rate limiting step of gluconeogenesis? Where does this reaction occur?

Answer 8

Pyruvate –> Oxaloacetate (OAA) via Pyruvate carboxylase

This reaction occurs in the mitochondria

Question 9

In what step of gluconeogensis can see the effects of an allosteric effector? What is this allosteric effector?

Answer 9

There is coordinate control over gluconeogenesis at the fructose 1,6 bisphosphatase step (versus glycolysis at the phosphofructokinase-1 step) by varying the concentration of the allosteric effector fructose 2,6-bisphosphate.

Question 10

Where does malate dehydrogenase exist?

Answer 10

Malate dehydrogenase exists both in the cytosol and the mitochondrial matrix.

Question 11

What organ/tissue shows a marked increase in gluconeogenesis late in starvation?

Answer 11

kidney

Question 12

Which organ/tissue is solely dependent on glucose as fuel?

Answer 12

red blood cells

Question 13

What compound(s) is/are positive allosteric effector(s) for pyruvate carboxylase?

Answer 13

Acetyl CoA

Question 14

In glucagon SYNTHESIS, what is the major product of phosphoglucomutase?

Answer 14

glucose 1-phosphate (unlike gluconeogenesis [the breakdown of glycogen, where glucose 6-phosphate is the main product)

Question 15

In gluconeogenesis pathway, the pyruvate carboxylase reaction takes place in the…?

Answer 15

mitochondria