Glycogen and Pentose Phosphate Pathway: Key Concepts for Biochemistry

What is glycogen?

A highly branched homopolymer of glucose present in all tissues.

Where are the largest stores of glycogen found?

In the liver and muscle.

What is the primary function of liver glycogen?

To break down glycogen and release glucose into the blood for energy.

How does muscle glycogen function?

It is mobilized to provide energy for muscle contraction.

What type of glycosidic bonds does glycogen contain?

α1-4 and α1-6 glycosidic bonds.

What enzyme breaks down glycogen?

Glycogen phosphorylase.

What does glycogen phosphorylase release during glycogen breakdown?

Glucose 1-phosphate.

What is the role of phosphoglucomutase?

It converts glucose 1-phosphate to glucose 6-phosphate.

What is the function of debranching enzymes in glycogen metabolism?

They help cleave glycogen near branch points and remove α1-6 linked glucose.

What is the role of liver glucose 6-phosphatase?

It converts glucose 6-phosphate to free glucose for transport.

How is muscle phosphorylase regulated?

By intracellular energy charge, with AMP stabilizing the active R state.

What is the default form of liver phosphorylase?

The a form in the R state, always active.

What negatively regulates liver phosphorylase?

Glucose, facilitating the transition from R state to T state.

What activates phosphorylase kinase?

Phosphorylation and Ca2+ binding.

What hormones stimulate glycogen degradation?

Glucagon and epinephrine.

What is the role of cAMP in glycogen metabolism?

It activates protein kinase A, which phosphorylates and activates phosphorylase kinase.

How is glycogen degradation turned off?

By GTPase activity of G proteins, phosphodiesterase converting cAMP to AMP, and protein phosphatase 1 removing phosphoryl groups.

What is UDP-glucose?

A glucose donor synthesized from glucose 1-phosphate and UTP.

What does glycogen synthase do?

Transfers a glucose moiety from UDP-glucose to form an α-1,4-glycosidic bond.

What is required for glycogen synthase to function?

An oligosaccharide of glucose residues as a primer.

What enzyme synthesizes the primer for glycogen synthase?

Glycogenin.

How does the branching enzyme function?

It cleaves an α-1,4-linkage and synthesizes an α-1,6-linkage to create branches.

How many ATP molecules are required to incorporate one glucose into glycogen?

Two molecules of ATP.

What effect does insulin have on glycogen synthesis?

It stimulates glycogen synthesis by activating protein kinases and promoting glucose transporter insertion.

What is the role of protein phosphatase 1 (PP1) in glycogen metabolism?

It shifts metabolism from degradation to synthesis by removing phosphoryl groups.

What happens to PP1 activity in muscle when GM is phosphorylated?

PP1 dissociates from glycogen, decreasing its activity.

What is the role of the second regulatory subunit in PP1?

It binds to and further inhibits PP1 when phosphorylated.

How does glucose affect phosphorylase in the liver?

Glucose converts phosphorylase from R state to T state, leading to the dissociation of PP1 and activation of glycogen synthesis.

What happens to glycogen metabolism when blood glucose levels are high?

Glycogen degradation is inhibited, and glycogen synthesis is stimulated.

What is the main product of the pentose phosphate pathway?

NADPH and pentose phosphate from glucose 6-phosphate.

Why is NADPH important for non-photosynthetic organisms?

It is the only pathway to generate NADPH, which is crucial for biosynthetic reactions.

What is the function of NADP+ in anabolism?

NADP+ acts as an electron carrier, receiving electrons and H+ to become reduced.

What is the rate-limiting step in the pentose phosphate pathway?

The conversion of glucose 6-phosphate to 6-phosphoglucono-δ-lactone by glucose 6-phosphate dehydrogenase.

What are the two main modes of producing pentose phosphate?

Oxidation of glucose-6-phosphate and conversion from glycolysis intermediates.

What is the significance of ribose 5-phosphate in cells?

It is essential for DNA replication during cell division.

What condition is caused by glucose 6-phosphate dehydrogenase deficiency?

It can lead to drug-induced hemolytic anemia.

How does glutathione protect red blood cells?

It eliminates reactive oxygen species and maintains the normal structure of hemoglobin.

What are the five classes of lipids?

1. Free fatty acids, 2. Triacylglycerols, 3. Phospholipids, 4. Glycolipids, 5. Steroids.

What is the primary function of triacylglycerols?

They serve as the storage form of fatty acids.

What distinguishes saturated fatty acids from unsaturated fatty acids?

Saturated fatty acids have only C-C single bonds, while unsaturated fatty acids contain at least one C=C double bond.

What is the effect of trans-unsaturated fats on health?

They may trigger inflammatory pathways and increase LDL cholesterol levels.

How are carbon atoms numbered in fatty acids?

Starting from the carboxyl terminal carbon atom, with carbons 2 and 3 referred to as α and β, respectively.

What is the role of omega-3 fatty acids?

They may protect against cardiovascular disease and are precursors to important hormones.

What factors influence the properties of fatty acids?

Chain length, cis/trans configuration, and degree of unsaturation.

What is the general structure of fatty acids?

Chains of hydrocarbons with a carboxylic acid (-COOH) at one end and a methyl group (-CH3) at the other.

What is the significance of the pentose phosphate pathway in nucleotide biosynthesis?

It provides ribose 5-phosphate, which is crucial for the synthesis of nucleic acids.

What happens during the oxidative phase of the pentose phosphate pathway?

Glucose 6-phosphate is oxidized to produce NADPH and ribulose 5-phosphate.

What is the net result of the non-oxidative reactions in the pentose phosphate pathway?

Conversion of 3 pentoses into 2 hexoses and 1 triose.

What is the relationship between the pentose phosphate pathway and glycolysis?

They are interconnected, with intermediates from glycolysis being used in the pentose phosphate pathway.

What are the four distinct modes of the pentose phosphate pathway?

1. More ribose 5-phosphate than NADPH, 2. Balanced needs, 3. More NADPH than ribose 5-phosphate, 4. Both NADPH and ATP required.

What is the impact of glucose 6-phosphate on the oxidative phase of the pentose phosphate pathway?

It can be inhibited when ribose 5-phosphate needs exceed NADPH needs.

How does the pentose phosphate pathway contribute to cellular signaling?

It produces NADPH, which is used in various signaling pathways and biosynthetic processes.

What enhances the fluidity of fatty acids?

Short chain length and the presence of cis double bonds.

Why do cis-unsaturated fatty acids have lower melting points?

Due to less van der Waals interactions.

What are triacylglycerols?

Storage fatty acids formed by the esterification of three fatty acids to one molecule of glycerol.

How much energy does one gram of triacylglycerol store compared to glycogen?

More than six times the energy of one gram of glycogen.

What is saponification?

The hydrolysis of triacylglycerols by alkali to produce glycerol and soap.

What are the four components of phospholipids?

Fatty acids, a platform (glycerol or sphingosine), a phosphate, and an alcohol.

What are phosphoglycerides?

Phospholipids with a glycerol platform.

What are glycolipids?

Carbohydrate-containing lipids found on the extracellular surface of the cell membrane for cell-cell recognition.

What is the structure of steroid lipids?

Built on a tetracyclic platform consisting of three cyclohexane rings and a cyclopentane ring fused together.

What is the role of cholesterol in membranes?

Maintains membrane fluidity and is a precursor to steroid hormones.

What are membrane anchors?

Hydrophobic groups covalently attached to proteins that tether them to the membrane.

What role do bile acids play in lipid digestion?

They make lipid droplets more accessible to lipases.

What is a chylomicron?

Ultra low-density lipoproteins (ULDL) formed in intestinal cells for cholesterol transport.

What are the three stages of fatty acid degradation?

1. Lipolysis (degradation of triacylglycerols), 2. Activation of fatty acids to form acyl-CoA, 3. Degradation of fatty acids to produce acetyl-CoA.

What hormones stimulate lipolysis?

Epinephrine and glucagon.

What is the function of acyl-CoA synthetase?

Activates fatty acids by attaching them to coenzyme A.

What is the process of transporting fatty acids into mitochondria?

The activated fatty acid is transferred from CoA to Carnitine and then transported into the mitochondria.

What is β-oxidation?

The process of degrading fatty acids to produce acetyl-CoA.

What is the ATP yield from one palmitate fatty acid?

106 ATPs.

Why can't animals convert fatty acids into glucose?

Fats are converted into acetyl-CoA, which cannot lead to the net synthesis of glucose.

What happens during the metabolism of unsaturated fatty acids?

They require isomerase and reductase for β-oxidation depending on the number of double bonds.

What is produced from the β-oxidation of fatty acids with odd numbers of carbons?

Propionyl CoA.

What are ketone bodies and how are they synthesized?

Acetoacetate, 3-hydroxybutyrate, and acetone synthesized from 2 acetyl CoA in liver mitochondria.

What is diabetic ketosis?

Excess production of ketone bodies leading to acidosis in blood due to lack of insulin.

How does starvation affect fuel metabolism?

After several days, the brain begins to use ketone bodies as fuel, reducing protein degradation.