MN09 - Fructose and Galactose

What is the primary function of liver glycogen?

Liver glycogen maintains blood glucose levels during fasting by releasing glucose into the bloodstream.

What percentage of liver weight is glycogen?

Approximately 10% of liver weight is glycogen.

What is the primary function of muscle glycogen?

Muscle glycogen provides glucose for ATP production during muscle contraction.

What percentage of muscle weight is glycogen?

Approximately 1-2% of muscle weight is glycogen.

What is the key functional difference between liver and muscle glycogen?

Liver glycogen maintains blood glucose for the body, whereas muscle glycogen is used locally for energy in muscle cells.

What molecule serves as the glucose donor for glycogen synthesis?

UDP-glucose serves as the activated glucose donor for glycogen synthesis.

How is UDP-glucose produced?

UDP-glucose is produced from glucose-1-phosphate and UTP by the enzyme UDP-glucose pyrophosphorylase.

What is the rate-limiting enzyme of glycogen synthesis?

Glycogen synthase is the rate-limiting enzyme of glycogen synthesis.

What type of bond does glycogen synthase form?

Glycogen synthase forms α(1→4) glycosidic bonds between glucose molecules.

What is glycogenin and what is its role?

Glycogenin is a primer protein that initiates glycogen synthesis by autoglycosylating a tyrosine residue.

How many glucose residues does glycogenin add to initiate glycogen formation?

Glycogenin adds approximately 4-8 glucose residues to form the initial glycogen primer.

Where is glycogenin located in the glycogen molecule?

Glycogenin remains at the core of the glycogen particle.

What enzyme forms branches in glycogen?

The branching enzyme (amylo-α(1→4)→α(1→6) transglycosylase) forms branches in glycogen.

How does the branching enzyme work?

The branching enzyme transfers a block of 6-8 glucose residues from a chain to form an α(1→6) glycosidic bond at a branch point.

What is the rate-limiting enzyme of glycogenolysis?

Glycogen phosphorylase is the rate-limiting enzyme of glycogenolysis.

What bond does glycogen phosphorylase cleave?

Glycogen phosphorylase cleaves α(1→4) glycosidic bonds.

What product is released by glycogen phosphorylase?

Glycogen phosphorylase releases glucose-1-phosphate.

How close to a branch point does glycogen phosphorylase stop?

Glycogen phosphorylase stops 4 residues before a branch point.

What enzyme is responsible for debranching glycogen?

The debranching enzyme, which has two activities, is responsible for removing branches in glycogen.

What are the two activities of the debranching enzyme?

The debranching enzyme has transferase activity and α(1→6) glucosidase activity.

What does the transferase activity of the debranching enzyme do?

It transfers 3 glucose residues from a branch to a nearby chain.

What does the α(1→6) glucosidase activity do?

It cleaves the α(1→6) bond and releases free glucose at the branch point.

What happens to glucose-6-phosphate in the liver after glycogen breakdown?

In the liver, glucose-6-phosphate is converted to free glucose by glucose-6-phosphatase and released into the blood.

What enzyme allows the liver to release free glucose?

Glucose-6-phosphatase, located in the endoplasmic reticulum, allows the liver to release glucose.

What happens to glucose-6-phosphate in muscle after glycogen breakdown?

In muscle, glucose-6-phosphate enters glycolysis to produce ATP.

Why can muscle not release free glucose?

Muscle lacks glucose-6-phosphatase, so it cannot convert glucose-6-phosphate into free glucose.

What is the active form of glycogen phosphorylase?

The active form of glycogen phosphorylase is the phosphorylated form.

What is the inactive form of glycogen phosphorylase?

The inactive form of glycogen phosphorylase is the dephosphorylated form.

What is the active form of glycogen synthase?

The active form of glycogen synthase is the dephosphorylated form.

What is the inactive form of glycogen synthase?

The inactive form of glycogen synthase is the phosphorylated form.

How do glucagon and epinephrine affect glycogen metabolism?

Glucagon and epinephrine increase cAMP levels, activating protein kinase A (PKA), which promotes glycogen breakdown and inhibits glycogen synthesis.

What does protein kinase A (PKA) do in glycogen regulation?

PKA activates phosphorylase kinase (activating glycogen phosphorylase) and inhibits glycogen synthase.

How does insulin affect glycogen metabolism?

Insulin decreases cAMP and activates protein phosphatase 1, promoting glycogen synthesis and inhibiting glycogen breakdown.

What enzyme does insulin activate to promote glycogen synthesis?

Insulin activates protein phosphatase 1 (PP1), which dephosphorylates enzymes.

What is the effect of glucose-6-phosphate on glycogen metabolism?

Glucose-6-phosphate activates glycogen synthase and inhibits glycogen phosphorylase.

What is the effect of glucose on glycogen phosphorylase in the liver?

Glucose inhibits glycogen phosphorylase in the liver.

What is the effect of AMP on glycogen phosphorylase in muscle?

AMP activates glycogen phosphorylase in muscle, signaling low energy.

What is the effect of ATP on glycogen phosphorylase?

ATP inhibits glycogen phosphorylase, signaling high energy.

What is the role of calcium (Ca²⁺) in glycogen metabolism?

Calcium activates phosphorylase kinase via calmodulin, promoting glycogen breakdown during muscle contraction.

How does glycogen regulation differ between liver and muscle?

Liver responds to glucagon and maintains blood glucose, while muscle lacks glucagon receptors and responds to AMP, calcium, and epinephrine for its own energy needs.

What is the polyol pathway?

The polyol pathway converts glucose to sorbitol via aldose reductase and then sorbitol to fructose via sorbitol dehydrogenase.

What enzyme converts glucose to sorbitol?

Aldose reductase converts glucose to sorbitol.

What enzyme converts sorbitol to fructose?

Sorbitol dehydrogenase converts sorbitol to fructose.

Why does sorbitol accumulation cause damage?

Sorbitol accumulation causes osmotic stress, leading to cell swelling and damage.

Which tissues are most affected by sorbitol accumulation?

The lens, nerves, and kidneys are most affected by sorbitol accumulation.

When is the polyol pathway most active?

The polyol pathway is most active during hyperglycemia, such as in diabetes.

What is the first step in galactose metabolism?

Galactose is phosphorylated to galactose-1-phosphate by galactokinase.

What enzyme converts galactose-1-phosphate to UDP-galactose?

Galactose-1-phosphate uridyltransferase (GALT) converts it using UDP-glucose.

What enzyme converts UDP-galactose to UDP-glucose?

UDP-galactose epimerase converts UDP-galactose to UDP-glucose.

What are the main uses of galactose in the body?

Galactose is used for glycogen synthesis and for making glycoproteins, lactose, and proteoglycans.

What are the two phases of the pentose phosphate pathway?

The pentose phosphate pathway has an oxidative phase and a non-oxidative phase.

What is produced in the oxidative phase of the pentose phosphate pathway?

The oxidative phase produces NADPH and ribulose-5-phosphate.

What is produced in the non-oxidative phase of the pentose phosphate pathway?

The non-oxidative phase produces ribose-5-phosphate and glycolysis intermediates.

Is the oxidative phase reversible or irreversible?

The oxidative phase is irreversible.

Is the non-oxidative phase reversible or irreversible?

The non-oxidative phase is reversible.

Where does the pentose phosphate pathway occur?

The pentose phosphate pathway occurs in the cytosol.

Which tissues have high pentose phosphate pathway activity?

The liver, adipose tissue, adrenal cortex, gonads, and red blood cells have high activity.

What is the rate-limiting enzyme of the pentose phosphate pathway?

Glucose-6-phosphate dehydrogenase (G6PD) is the rate-limiting enzyme.

How is G6PD regulated?

G6PD is inhibited by high levels of NADPH.

How does the non-oxidative phase connect to glycolysis and gluconeogenesis?

It converts sugar intermediates into glycolytic or gluconeogenic intermediates using transketolase and transaldolase.

What enzymes are used in the non-oxidative phase of the PPP?

Transketolase and transaldolase catalyze reactions in the non-oxidative phase.

What are the roles of NADPH in the body?

NADPH is used for reductive biosynthesis, detoxification, antioxidant defense, and nitric oxide synthesis.

How is NADPH used in fatty acid and cholesterol synthesis?

NADPH provides reducing power for biosynthesis of fatty acids and cholesterol.

How does NADPH contribute to detoxification?

NADPH provides electrons for cytochrome P450 enzymes in detoxification reactions.

How does NADPH help neutralize reactive oxygen species?

NADPH regenerates reduced glutathione, which detoxifies reactive oxygen species.

What role does NADPH play in nitric oxide synthesis?

NADPH provides reducing equivalents for nitric oxide synthase.

What is G6PD deficiency?

G6PD deficiency is an X-linked disorder that impairs NADPH production and leads to hemolytic anemia.

Why are red blood cells especially affected by G6PD deficiency?

Red blood cells rely solely on the pentose phosphate pathway for NADPH production.

What triggers hemolytic crises in G6PD deficiency?

Oxidative stress from drugs, infections, or certain foods can trigger hemolysis.

Which drugs can trigger hemolysis in G6PD deficiency?

Drugs such as sulfonamides and primaquine can trigger hemolysis.

What food can trigger hemolysis in G6PD deficiency?

Fava beans can trigger hemolysis, a condition known as favism.

What is the underlying cause of hemolysis in G6PD deficiency?

Lack of NADPH leads to inability to detoxify reactive oxygen species, causing red blood cell damage.