MCAT BIO/BIOCHEM
Glycolysis and Krebs Cycle Flashcards
36 glycolysis and Krebs cycle terms the MCAT expects you to define, not just recognise
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All 36 MCAT BIO/BIOCHEM Glycolysis and Krebs Cycle flashcards
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- Acetyl-CoA
- A two-carbon acetyl group bonded to coenzyme A through a high-energy thioester; it is the common entry fuel that carbohydrates, fatty acids, and some amino acids all converge on.For example: Beta-oxidation of a single fat molecule releases many of these two-carbon units.
- Aconitase
- The Krebs enzyme that isomerises citrate into isocitrate by removing then re-adding water, repositioning a hydroxyl group so the molecule can next be oxidised and decarboxylated.For example: Fluoroacetate poisoning blocks this enzyme, stalling the entire cycle.
- Aldolase
- The glycolytic enzyme that cleaves six-carbon fructose-1,6-bisphosphate into two interconvertible three-carbon sugars: glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.For example: It belongs to the lyase class, breaking a carbon-carbon bond without adding water.
- Alpha-Ketoglutarate
- The five-carbon Krebs intermediate formed after the first carbon dioxide is lost; it also leaves the cycle to be transaminated into the amino acid glutamate.For example: This exit is a key link between carbohydrate and amino-acid metabolism.
- Alpha-Ketoglutarate Dehydrogenase
- The Krebs enzyme running the second oxidative decarboxylation, converting alpha-ketoglutarate into succinyl-CoA while releasing carbon dioxide and NADH. It resembles the pyruvate dehydrogenase complex mechanically.For example: Like that complex, it depends on thiamine pyrophosphate and lipoic acid.
- Amphibolic Pathway
- A pathway that serves both catabolism and anabolism at once; the Krebs cycle qualifies because it both oxidises fuel for energy and supplies carbon skeletons for biosynthesis.For example: Oxaloacetate leaving for gluconeogenesis and alpha-ketoglutarate leaving for glutamate are both anabolic exits.
- Anaplerotic Reaction
- A reaction that refills a depleted Krebs cycle intermediate so oxidation can continue; the main example is pyruvate carboxylase adding carbon dioxide to pyruvate to make oxaloacetate.For example: It becomes essential when oxaloacetate is drained off for glucose synthesis.
- Citrate
- The six-carbon tricarboxylic acid that gives the cycle its alternative name; when it accumulates and spills into the cytoplasm it signals abundance and inhibits phosphofructokinase-1.For example: Cytoplasmic citrate also supplies the acetyl-CoA used for fatty-acid synthesis.
- Citrate Synthase
- The enzyme opening the Krebs cycle by condensing an acetyl group with oxaloacetate; its pace is throttled by ATP, NADH, and its own product citrate when energy is plentiful.For example: It commits carbon to full oxidation rather than to storage as fat or glycogen.
- Cori Cycle
- The interorgan loop in which lactate made by anaerobic muscle travels to the liver, is rebuilt into glucose by gluconeogenesis, and returns to the muscle to be used again.For example: It shifts the metabolic cost of a sprint from the muscle onto the liver.
- Dihydroxyacetone Phosphate (DHAP)
- The three-carbon ketose produced alongside glyceraldehyde-3-phosphate by aldolase; it cannot proceed through glycolysis until triose phosphate isomerase rearranges it into glyceraldehyde-3-phosphate.For example: In the liver it can instead be diverted toward triglyceride synthesis.
- Energy-Investment Phase
- The first five steps of glycolysis, which spend two ATP to phosphorylate and destabilise glucose before any energy is harvested. Also called the preparatory phase.For example: Hexokinase and phosphofructokinase-1 each consume one ATP during this stretch.
- Energy-Payoff Phase
- The last five steps of glycolysis, where two three-carbon sugars are oxidised to pyruvate, producing four ATP and two NADH so the pathway nets a positive energy return.For example: The GAPDH and pyruvate kinase steps each run twice here, once per triose molecule.
- FADH2
- A reduced flavin electron carrier that holds electrons at a lower energy than NADH, so each molecule yields fewer ATP when it donates them to the electron transport chain.For example: The succinate dehydrogenase step produces exactly one per Krebs turn.
- Gluconeogenesis
- The pathway that builds new glucose from non-carbohydrate precursors such as lactate, glycerol, and amino acids, largely reversing glycolysis but bypassing its three irreversible steps with distinct enzymes.For example: During an overnight fast the liver uses it to keep blood sugar steady.
- Glyceraldehyde-3-Phosphate (G3P)
- The three-carbon sugar that is the only glycolytic intermediate to continue into the payoff phase; dihydroxyacetone phosphate must be converted into it first.For example: Because DHAP is funnelled into it, each glucose ultimately yields two of these molecules.
- Glyceraldehyde-3-Phosphate Dehydrogenase (GAPDH)
- The only glycolytic step that reduces NAD+ to NADH, oxidising glyceraldehyde-3-phosphate and attaching inorganic phosphate to form the high-energy compound 1,3-bisphosphoglycerate.For example: Arsenate poisons this step by mimicking phosphate, uncoupling it from ATP capture.
- Glycolysis
- The ten-step cytoplasmic pathway that partially oxidises one glucose into two pyruvate, netting two ATP and two NADH, and functioning with or without oxygen present.For example: A sprinting muscle leans on it to make ATP quickly before oxygen delivery can catch up.
- Hexokinase
- The low-Km first enzyme of glycolysis, product-inhibited by glucose-6-phosphate; its high-Km liver isozyme glucokinase engages only when blood glucose is abundant.For example: Glucokinase lets the liver ramp up glucose uptake after a carbohydrate-rich meal.
- Isocitrate Dehydrogenase
- The Krebs enzyme catalysing the first oxidative decarboxylation, releasing carbon dioxide and reducing NAD+; it is allosterically stimulated by ADP and calcium and inhibited by ATP and NADH.For example: A surge of ADP during exercise accelerates it and speeds the whole cycle.
- Krebs Cycle
- The eight-step mitochondrial pathway that fully oxidises an acetyl group to two carbon dioxide molecules while harvesting reduced electron carriers. Also known as the citric acid or tricarboxylic acid cycle.For example: Its intermediates are also borrowed away to build amino acids and haem.
- Lactate Fermentation
- The anaerobic reduction of pyruvate to lactate by lactate dehydrogenase, whose real purpose is to reoxidise NADH back to NAD+ so glycolysis can keep producing ATP without oxygen.For example: Red blood cells, which have no mitochondria, depend on it permanently.
- Link Reaction
- The oxidative decarboxylation of pyruvate to acetyl-CoA that bridges glycolysis and the Krebs cycle, releasing one carbon dioxide and one NADH per pyruvate. Also called the transition reaction.For example: For a single glucose it runs twice, feeding two acetyl-CoA into the cycle.
- NADH
- The principal reduced electron carrier of respiration, formed when NAD+ accepts two electrons and a proton; it later delivers them to the electron transport chain to drive ATP synthesis.For example: Glycolysis, the link reaction, and the Krebs cycle together make ten per glucose.
- Oxaloacetate
- The four-carbon acceptor that begins and ends each Krebs turn; because it is regenerated rather than consumed, tiny amounts can support the oxidation of large amounts of acetyl-CoA.For example: It can also be pulled away to start gluconeogenesis, which then requires topping up.
- Oxidative Phosphorylation
- The oxygen-dependent process where electrons from NADH and FADH2 pass down the electron transport chain, pumping protons whose backflow through ATP synthase makes most of a cell's ATP.For example: It is where the NADH and FADH2 banked by glycolysis and the Krebs cycle finally pay out.
- Phosphoenolpyruvate (PEP)
- The glycolytic intermediate holding the highest phosphate-transfer potential of any biological molecule; giving up its phosphate to ADP is what drives the final, irreversible ATP-forming step.For example: Enolase creates it from 2-phosphoglycerate one step earlier in the pathway.
- Phosphofructokinase-1 (PFK-1)
- The main regulatory enzyme of glycolysis, allosterically switched off by ATP and citrate and switched on by AMP and fructose-2,6-bisphosphate to match sugar breakdown to energy demand.For example: Rising citrate from a full Krebs cycle signals plenty and slows this enzyme down.
- Pyruvate
- The three-carbon end product of glycolysis that sits at a metabolic crossroads: it can be oxidised to acetyl-CoA, reduced to lactate, or carboxylated to oxaloacetate.For example: In yeast it is instead decarboxylated on the route toward ethanol.
- Pyruvate Dehydrogenase Complex
- A large three-enzyme assembly in the mitochondrial matrix that irreversibly decarboxylates and oxidises pyruvate, joining the remaining two carbons to coenzyme A. It is inhibited by its products NADH and acetyl-CoA.For example: It requires five cofactors, including thiamine pyrophosphate derived from vitamin B1.
- Pyruvate Kinase
- The enzyme catalysing the last, irreversible step of glycolysis, transferring phosphoenolpyruvate's phosphate to ADP to yield pyruvate and ATP. It is activated by fructose-1,6-bisphosphate.For example: Feed-forward activation by fructose-1,6-bisphosphate speeds the exit once the pathway has committed.
- Substrate-Level Phosphorylation
- The direct handoff of a phosphate group from a high-energy substrate onto ADP or GDP by an enzyme, forming ATP or GTP with no electron transport chain and no oxygen.For example: The phosphoglycerate kinase and pyruvate kinase steps of glycolysis both work this way.
- Succinate Dehydrogenase
- The only Krebs enzyme embedded in the inner mitochondrial membrane, and the single enzyme shared with the electron transport chain, where it acts as Complex II and reduces FAD to FADH2.For example: It oxidises succinate to fumarate while feeding electrons straight into the chain.
- Succinyl-CoA Synthetase
- The only Krebs enzyme performing substrate-level phosphorylation, cleaving the thioester bond of succinyl-CoA to make succinate and capture the released energy as one GTP.For example: The GTP it forms is readily converted to ATP by nucleoside diphosphate kinase.
- Triose Phosphate Isomerase
- The isomerase that interconverts dihydroxyacetone phosphate and glyceraldehyde-3-phosphate so both halves of the cleaved sugar enter the payoff phase. It is a catalytically perfect, diffusion-limited enzyme.For example: Its near-perfect speed means the DHAP-to-G3P conversion never bottlenecks glycolysis.
- Warburg Effect
- The tendency of many cancer cells to rely on glycolysis and lactate production even when oxygen is available, favouring fast ATP and biosynthetic intermediates over maximal energy yield.For example: PET scans exploit it, lighting up tumours by their heavy glucose uptake.