Which three enzymes are the irreversible regulatory steps of glycolysis?
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Hexokinase, Phosphofructokinase-1 (PFK-1), and Pyruvate kinase (PK).
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Which three enzymes are the irreversible regulatory steps of glycolysis?
Hexokinase, Phosphofructokinase-1 (PFK-1), and Pyruvate kinase (PK).
Which enzyme is the rate-limiting (committed) step of glycolysis?
Phosphofructokinase-1 (PFK-1) is the rate-limiting (committed) step and the pacemaker of glycolysis.
Name two allosteric activators of PFK-1.
AMP and Fructose-2,6-bisphosphate (F-2,6-BP) activate PFK-1.
Name two inhibitors of PFK-1.
ATP and citrate inhibit PFK-1.
Compare hexokinase and glucokinase: which has higher affinity for glucose?
Hexokinase has a high affinity for glucose (low Km); glucokinase has a low affinity (high Km).
Where are hexokinase and glucokinase predominantly located?
Hexokinase: in all tissues. Glucokinase: in the liver and pancreatic β-cells.
How does glucose-6-phosphate affect hexokinase and glucokinase?
Glucose-6-phosphate is an allosteric inhibitor of hexokinase but has no effect on glucokinase.
Which hormone induces glucokinase expression?
Insulin induces glucokinase expression; glucagon represses it.
How is pyruvate kinase regulated?
Pyruvate kinase is regulated allosterically and by phosphorylation; it is activated by F-1,6-bisphosphate and inhibited by ATP and high energy state (citrate).
What is the overall sequence of major enzymes from glucose to pyruvate?
Major enzymes include Hexokinase → Phosphohexose isomerase → PFK-1 → Aldolase → Triose phosphate isomerase → Glyceraldehyde-3-phosphate dehydrogenase → Phosphoglycerate kinase → Phosphoglycerate mutase → Enolase → Pyruvate kinase.
How does insulin affect glycolysis at the hormonal level?
Insulin (stimulated by carbohydrate feeding/hyperglycemia) activates glycolysis by inducing synthesis of key glycolytic enzymes and promoting dephosphorylation/activation of enzymes.
How does glucagon affect glycolysis?
Glucagon (stimulated by fasting/hypoglycemia) inactivates glycolysis by repressing synthesis of glycolytic enzymes and promoting phosphorylation/inactivation of key enzymes via cAMP signaling.
Describe the effect of epinephrine on glycolysis regulation.
Epinephrine activates adenylate cyclase → ↑cAMP → protein kinase A, which can inactivate glycolytic enzymes (through phosphorylation) in some tissues; insulin opposes this by activating phosphodiesterase.
Which second messenger does glucagon/epinephrine use to modulate enzyme activity?
They use cAMP (via activation of adenylate cyclase) to activate protein kinase and alter enzyme activity.
What are the primary sources of pyruvate in the cell?
Glucose (glycolysis), some amino acids (e.g., alanine), glycerol, and malic acid (malate) are sources of pyruvate.
What are the main fates of pyruvate?
Pyruvate can be converted to lactate, oxaloacetate (for gluconeogenesis), or acetyl-CoA (to enter the citric acid cycle).
How does arsenate affect glycolysis and ATP yield?
Arsenate uncouples substrate-level phosphorylation by forming unstable 1-arseno-3-phosphoglycerate, which bypasses ATP generation at that step — reducing ATP yield (in the presence of arsenate glycolysis can yield less or zero net ATP under anaerobic conditions).
Does arsenate directly inhibit an enzyme?
No — arsenate is not a direct enzyme inhibitor; it acts as an uncoupler of oxidation and phosphorylation affecting ATP formation.
Which enzymes are inhibited by mercury and iodoacetate, and how?
Mercury and iodoacetate inhibit glyceraldehyde-3-phosphate dehydrogenase (GAPDH) by reacting with its thiol (–SH) group.
How does fluoride affect glycolysis?
Fluoride inhibits enolase by forming Mg-fluoride, which removes Mg2+ needed for enolase activity.
What is the consequence of arsenite (and mercury) on the pyruvate dehydrogenase complex?
Arsenite and mercury bind SH groups and inhibit the pyruvate dehydrogenase complex, causing accumulation of pyruvate → increased lactate formation → lactic acidosis.
Which products inhibit the pyruvate dehydrogenase (PDH) complex?
Acetyl-CoA and NADH + H+ (the products of PDH) inhibit the PDH complex by product feedback.
How is the PDH complex regulated by covalent modification?
PDH is regulated by phosphorylation (inactivation) and dephosphorylation (activation). Insulin promotes dephosphorylation → activation, while glucagon/epinephrine promote phosphorylation → inactivation.
Which enzyme converts pyruvate to lactate?
Lactate dehydrogenase (LDH) converts pyruvate → lactate while oxidizing NADH to NAD+.
Which intermediate links glyceraldehyde-3-phosphate oxidation to ATP generation and requires NAD+?
Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) converts glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, requiring NAD+ → NADH+H+.
What happens to 1,3-bisphosphoglycerate in glycolysis?
1,3-bisphosphoglycerate is converted to 3-phosphoglycerate by phosphoglycerate kinase, generating ATP from ADP (substrate-level phosphorylation).
Which enzyme converts phosphoenolpyruvate (PEP) to pyruvate?
Pyruvate kinase converts PEP → pyruvate, producing ATP.
Which molecule activates pyruvate kinase through feed-forward regulation?
Fructose-1,6-bisphosphate (F-1,6-BP) activates pyruvate kinase (feed-forward activation).
How does feeding (high carbohydrate) change metabolic hormone signaling relevant to glycolysis?
Feeding/hyperglycemia → ↑Insulin → promotes glycolysis and lipid synthesis by activating enzymes and inducing their synthesis.
How does fasting (low carbohydrate) change metabolic hormone signaling relevant to glycolysis?
Fasting/hypoglycemia → ↑Glucagon (and epinephrine) → promotes gluconeogenesis and lipid breakdown, and represses glycolytic enzyme synthesis.
What effect does insulin have on the pyruvate dehydrogenase complex?
Insulin activates PDH by promoting its dephosphorylation, which increases conversion of pyruvate to acetyl‑CoA.
What effect do glucagon and epinephrine have on the PDH complex?
Glucagon/epinephrine promote phosphorylation → inactivation of the PDH complex, reducing conversion of pyruvate to acetyl‑CoA.
Name a reversible step enzyme in glycolysis that interconverts triose phosphates.
Triose phosphate isomerase (TPI) interconverts dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (GAP).
Which glycolytic enzyme requires Mg2+ as a cofactor and is inhibited by fluoride?
Enolase requires Mg2+ and is inhibited by fluoride (as Mg‑fluoride).
Why does inhibition of GAPDH reduce ATP production in glycolysis?
GAPDH catalyzes the oxidation step that produces 1,3‑bisphosphoglycerate, which is the substrate for substrate-level ATP generation by phosphoglycerate kinase; inhibition blocks this ATP-producing sequence.
Name one amino acid that can be converted to pyruvate.
Alanine can be transaminated to form pyruvate.
What is the effect of product accumulation (acetyl‑CoA, NADH) on PDH activity?
Accumulation of acetyl‑CoA and NADH + H+ inhibits PDH by feedback inhibition.
How does adenylate cyclase/phosphodiesterase interplay affect glycolysis under hormonal control?
Glucagon/epinephrine activate adenylate cyclase → ↑cAMP → protein kinase A → phosphorylation of enzymes (often inactivating them). Insulin activates phosphodiesterase → lowers cAMP → favors enzyme dephosphorylation/activation.
Which enzyme generates NADH in glycolysis?
Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) generates NADH + H+ when converting GAP to 1,3‑bisphosphoglycerate.
What is the metabolic link reaction between glycolysis and the citric acid cycle?
Conversion of pyruvate → acetyl‑CoA by the pyruvate dehydrogenase complex (PDH) links glycolysis to the citric acid cycle.
Which glycolytic enzyme catalyzes a substrate-level phosphorylation producing ATP from ADP?
Phosphoglycerate kinase catalyzes substrate-level phosphorylation converting 1,3‑bisphosphoglycerate → 3‑phosphoglycerate, producing ATP.
What happens to pyruvate during anaerobic conditions in muscle?
Under anaerobic conditions, pyruvate is converted to lactate by LDH, regenerating NAD+ for glycolysis.
