Chapter 14 - Glycolysis, Gluconeogenesis & Pentose Phosphate Pathway Flashcards
What defines a carbohydrate in biochemistry?
Organic molecules with aldehydes or ketones and multiple hydroxyl groups.
How are monosaccharides classified?
As simple sugars with one polyhydroxy aldehyde or ketone unit.
What is the main difference between monosaccharides and disaccharides?
Monosaccharides have a single unit, while disaccharides consist of two linked monosaccharide units.
What are glycosidic bonds?
Covalent bonds between monosaccharides in oligosaccharides and polysaccharides.
What characterizes a reducing sugar?
A sugar that can donate electrons in a redox reaction, often due to a free aldehyde group.
How do aldoses differ from ketoses?
Aldoses have a carbonyl group at the end of the chain; ketoses have it elsewhere.
What is an example of a common aldose?
Glucose.
What is a common example of a ketose?
Fructose.
Describe pyranoses and furanoses.
Pyranoses are six-membered rings; furanoses are five-membered rings in monosaccharides.
What process leads to the formation of hemiacetals and hemiketals?
Reaction of alcohol with aldehydes (hemiacetals) or ketones (hemiketals).
What is mutarotation in carbohydrates?
The interconversion between α and β anomers in solution.
What distinguishes D- and L-forms in monosaccharides?
The orientation of the hydroxyl group on the reference carbon farthest from the carbonyl group.
What are epimers?
Sugars that differ only in the configuration around one carbon atom.
What are anomers?
Isomers of a monosaccharide that differ at the hemiacetal or hemiketal carbon.
How do polysaccharides like starch and glycogen serve in biological systems?
As energy storage molecules; starch in plants, glycogen in animals.
What is the primary structural polysaccharide in plants?
Cellulose.
What is chitin, and where is it found?
A structural polysaccharide in fungal cell walls and arthropod exoskeletons.
What function do glycosaminoglycans serve?
They provide structural support and hydration in the extracellular matrix.
What is a proteoglycan?
A protein core with covalently attached glycosaminoglycan chains, important in the ECM.
How are glycolipids significant in cell membranes?
They help with cell recognition and communication.
What is an example of a hexose derivative?
Glucose-6-phosphate, used in cellular metabolism.
How does the Benedict’s test identify reducing sugars?
By forming a red precipitate when reducing sugars react with Cu²⁺.
What is the structure of starch?
Composed of amylose (linear) and amylopectin (branched) glucose polymers.
Describe the branching in glycogen.
Extensively branched with α(1→6) linkages every 8-12 residues.
How does cellulose differ structurally from starch?
It consists of β(1→4) linked glucose, forming straight, rigid chains.
What role do lectins play in glycobiology?
They bind specific carbohydrate moieties, facilitating cell recognition.
How does mass spectrometry help in carbohydrate analysis?
It determines the mass and structure of carbohydrate molecules.
What information does NMR spectroscopy provide in glycomics?
Details on the structure, conformation, and dynamics of carbohydrates.
What are glycoproteins and their function?
Proteins with attached carbohydrate chains, involved in immune response and cell signaling.
How do glycolipids participate in cell signaling?
Through oligosaccharide chains that interact with receptors in the cell membrane.
What is the glycocalyx?
A carbohydrate-rich layer on cell surfaces involved in cell recognition.
Describe the structure of amylose.
Amylose is a linear polymer of D-glucose with α(1→4) linkages.
What structural role does cellulose play in plants?
It provides rigidity and strength to plant cell walls.
What is a lactone, and how is it formed in carbohydrates?
A lactone is a cyclic ester formed from aldonic or uronic acids.
How do phosphorylated sugars function in cells?
They trap sugars inside cells due to lack of transporters for phosphorylated forms.
What is chondroitin sulfate?
A glycosaminoglycan in cartilage providing structure and resistance to compression.
How does keratan sulfate function in the extracellular matrix?
It contributes to tissue hydration and resilience.
What is the main difference between heparan sulfate and heparin?
Heparin is more highly sulfated and used primarily for anticoagulation.
What are the roles of selectins in cell interactions?
They mediate cell adhesion, especially in immune responses.
What is the role of integrins in glycobiology?
Integrins help cells anchor to the extracellular matrix and facilitate signaling.
How does glycosylation affect protein function?
It can impact protein folding, stability, and recognition by other molecules.
What is the difference between O-linked and N-linked glycosylation?
O-linked attaches to serine/threonine residues; N-linked attaches to asparagine residues.
How does the structure of hyaluronic acid benefit the extracellular matrix?
Its large size and negative charge enable it to retain water and provide cushioning.
What is the “sugar code” in glycobiology?
The information encoded by specific oligosaccharide structures on cell surfaces.
How do enzymes recognize stereoisomers of sugars?
Enzymes are stereospecific, often recognizing only one isomer, such as D-glucose.
What structural feature differentiates α and β anomers?
The orientation of the hydroxyl group on the anomeric carbon.
How does tautomerization affect ketoses?
It allows ketoses to rearrange and act as reducing sugars under certain conditions.
What is a glycosphingolipid?
A glycolipid with a sphingosine backbone, important in neural tissue.
What are the primary bonds in peptidoglycan structure?
β(1→4) linkages between N-acetylglucosamine and N-acetylmuramic acid.
What is an aggrecan, and where is it found?
A large proteoglycan in cartilage that binds to hyaluronan, providing compressive strength.
How do hydrogen bonds influence polysaccharide structures?
They stabilize folding and contribute to rigidity in structural polysaccharides like cellulose.
How are oligosaccharides typically analyzed?
Using methods like mass spectrometry and NMR to determine sequence and structure.
What is the role of the conserved RGD sequence in integrins?
It facilitates binding with ECM components, like fibronectin and collagen.
What happens during the formation of an acetal or ketal in carbohydrates?
A second alcohol group adds to a hemiacetal or hemiketal, forming a stable linkage.
What structural role do glycosaminoglycans play in the ECM?
They form a gel-like matrix that supports cells and allows nutrient diffusion.
What reaction does phosphohexose isomerase catalyze in glycolysis?
Converts glucose-6-phosphate to fructose-6-phosphate.
Describe the role of aldolase in glycolysis.
It cleaves fructose-1,6-bisphosphate into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.
What is the role of triose phosphate isomerase in glycolysis?
Converts dihydroxyacetone phosphate to glyceraldehyde-3-phosphate.
What is the energy yield from glycolysis per molecule of glucose?
2 ATP (net) and 2 NADH.
How does glucokinase differ from hexokinase?
Glucokinase is specific to the liver, has a higher Km for glucose, and is not inhibited by glucose-6-phosphate.
Which enzymes bypass the irreversible steps of glycolysis in gluconeogenesis?
Pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase.
Why is gluconeogenesis considered energetically expensive?
It requires 6 high-energy molecules (4 ATP and 2 GTP) per glucose synthesized.
How is pyruvate converted to phosphoenolpyruvate (PEP) in gluconeogenesis?
Through pyruvate carboxylase converting pyruvate to oxaloacetate, then phosphoenolpyruvate carboxykinase converting oxaloacetate to PEP.
What are the key roles of NADPH produced in the pentose phosphate pathway?
NADPH is crucial for biosynthesis of fatty acids and cholesterol and for reducing oxidative stress.
What reaction does lactonase catalyze in the pentose phosphate pathway?
Converts 6-phosphoglucono-δ-lactone to 6-phosphogluconate.
What is the function of 6-phosphogluconate dehydrogenase in the pentose phosphate pathway?
Catalyzes the oxidative decarboxylation of 6-phosphogluconate to ribulose-5-phosphate, generating NADPH.
What does transaldolase do in the non-oxidative phase of the pentose phosphate pathway?
Transfers a three-carbon unit to form fructose-6-phosphate and erythrose-4-phosphate.
Which phase of the pentose phosphate pathway is responsible for producing ribose-5-phosphate?
The oxidative phase.
What is the role of xylulose-5-phosphate in carbohydrate metabolism?
It activates phosphoprotein phosphatase 2A, increasing fructose 2,6-bisphosphate levels and promoting glycolysis.
Why is fructose metabolism particularly active in the liver?
The liver has fructokinase, which converts fructose directly to fructose-1-phosphate, bypassing hexokinase.
What effect does high ATP concentration have on phosphofructokinase-1 (PFK-1)?
High ATP levels inhibit PFK-1, slowing glycolysis when cellular energy is sufficient.
How does AMP regulate fructose-1,6-bisphosphatase (FBPase-1) in gluconeogenesis?
High AMP inhibits FBPase-1, slowing gluconeogenesis under low-energy conditions.
Describe how insulin affects fructose 2,6-bisphosphate levels.
Insulin increases fructose 2,6-bisphosphate levels, promoting glycolysis and inhibiting gluconeogenesis.
What is the role of pyruvate decarboxylase in ethanol fermentation?
Catalyzes the decarboxylation of pyruvate to acetaldehyde, the first step in ethanol fermentation.
What happens to lactate produced in muscles during anaerobic respiration?
It is transported to the liver, where it can be converted back to glucose in gluconeogenesis.
Why is glycerol an important gluconeogenic substrate?
It can be converted to dihydroxyacetone phosphate and enter gluconeogenesis.
How does pyruvate carboxylase contribute to gluconeogenesis?
It converts pyruvate to oxaloacetate in the mitochondria, a crucial step in bypassing pyruvate kinase.
What cofactor is essential for pyruvate carboxylase activity?
Biotin, which carries CO₂ for carboxylation of pyruvate.
Explain the biochemical basis of lactose intolerance.
Due to the absence of lactase, lactose cannot be broken down, leading to gastrointestinal symptoms.
How does phosphoenolpyruvate carboxykinase (PEPCK) regulate gluconeogenesis?
It converts oxaloacetate to PEP, a rate-limiting step in gluconeogenesis.
What is the consequence of glucose-6-phosphatase deficiency?
Impaired glucose release from the liver, causing glycogen storage diseases.
What is the impact of G6PD deficiency on red blood cells?
Reduced NADPH production compromises the cell’s ability to neutralize oxidative stress, leading to hemolysis.
How does alcohol consumption affect gluconeogenesis?
It produces excess NADH, which inhibits gluconeogenesis, potentially causing hypoglycemia.
What is the function of sedoheptulose-7-phosphate in the pentose phosphate pathway?
It is an intermediate formed by transketolase reactions in the non-oxidative phase.
Why is glycolysis considered a “substrate-level phosphorylation” process?
ATP is produced directly through enzyme-catalyzed reactions without the need for an electron transport chain.
Describe the role of thiamine pyrophosphate (TPP) in fermentation.
TPP stabilizes the carbanion intermediate in pyruvate decarboxylase reaction during ethanol fermentation.
Why is gluconeogenesis critical during fasting?
It supplies glucose to organs that depend on it, like the brain and red blood cells.
How does glucose-6-phosphate serve as a metabolic branch point?
It can enter glycolysis, gluconeogenesis, or the pentose phosphate pathway based on cellular needs.
What role does lactate dehydrogenase play in glycolysis under anaerobic conditions?
It reduces pyruvate to lactate, regenerating NAD+ for glycolysis.
Why is acetyl-CoA an allosteric activator of pyruvate carboxylase?
It signals high energy availability, promoting gluconeogenesis.
