ch 18, 19
CH 18
18.1
Pyruvate dehydrogenase complex connects glycolysis and citric acid cycle.
Glycolysis occurs in cytoplasm; citric acid cycle occurs in mitochondria.
Pyruvate is converted to acetyl CoA in mitochondrial matrix via oxidative decarboxylation.
Links glycolysis to citric acid cycle, committing carbon to oxidation or fatty acid synthesis.
Reaction produces NADH, indicating electron transfer potential for citric acid cycle.
Pyruvate dehydrogenase complex consists of three enzymes with distinct active sites:
Pyruvate dehydrogenase (TPP prosthetic group) - catalyzes decarboxylation.
Dihydrolipoyl transacetylase (Lipoamide prosthetic group) - transfers acetyl group to CoA.
Dihydrolipoyl dehydrogenase (FAD prosthetic group) - regenerates oxidized lipoamide.
Reaction occurs in three steps:
Decarboxylation to hydroxyethyl-TPP (rate-limiting step).
Oxidation, forming acetyl–lipoamide and thioester bond.
Formation of acetyl CoA from acetyl–lipoamide.
The flexible lipoamide arm facilitates substrate transfer between active sites.
Intermediates remain bound, enhancing reaction efficiency and minimizing side reactions.
18.2
The pyruvate dehydrogenase complex is stringently regulated by allosteric interactions and covalent modifications.
Formation of acetyl CoA from pyruvate is irreversible, preventing conversion back to glucose.
Acetyl CoA commits carbon from glucose for two fates:
Oxidation via the citric acid cycle for energy generation.
Incorporation into fatty acids, as acetyl CoA is crucial for fatty acid synthesis.
High concentrations of reaction products inhibit the pyruvate dehydrogenase complex:
Acetyl CoA inhibits the transacetylase component by direct binding.
NADH inhibits dihydrolipoyl dehydrogenase, signaling energy needs are met.
Covalent modification via phosphorylation regulates the complex:
Pyruvate dehydrogenase kinase phosphorylates and inactivates the complex.
Pyruvate dehydrogenase phosphatase removes the phosphoryl group to activate the complex.
Energy charge influences activity:
High acetyl CoA/CoA and ATP/ADP ratios stimulate kinase, leading to deactivation.
Increased ADP and pyruvate levels during muscle contraction activate the dehydrogenase by inhibiting the kinase.
Hormonal regulation occurs in tissues:
In the liver, epinephrine activates the phosphatase pathway.
Insulin stimulates phosphatase in tissues capable of fatty acid synthesis.
Defective regulation can lead to lactic acidosis due to persistent pyruvate conversion to lactate instead of acetyl CoA.
Cancer cells often exhibit enhanced pyruvate dehydrogenase kinase activity, promoting aerobic glycolysis (Warburg effect).
Thiamine pyrophosphate deficiency, a key coenzyme, can result in
disorders like beriberi, affecting neurological and cardiovascular systems.
CH 19
19.1
The citric acid cycle consists of two stages: introduction of acetyl and regeneration of oxaloacetate.
The cycle begins with a two-carbon acetyl unit condensing with four-carbon oxaloacetate to form six-carbon citrate.
Citrate is oxidized, releasing high-energy electrons and leading to the formation of a four-carbon compound.
Two carbons leave the cycle as carbon dioxide while regenerating oxaloacetate for another cycle.
The cycle produces high-energy electron carriers NADH and one molecule of ATP.
The citric acid cycle itself does not generate significant ATP nor requires oxygen directly.
High-energy electrons from NADH are used in oxidative phosphorylation to produce up to nine ATP molecules.
The cycle is a key component of cellular respiration, facilitating the capture of high-energy electrons from carbon fuels to generate energy efficiently.
19.2
The citric acid cycle oxidizes two carbon atoms to gather energy-rich electrons.
Primary catabolic purpose: energy extraction from acetyl CoA.
Initial condensation of four-carbon oxaloacetate with two-carbon acetyl unit yields six-carbon citrate.
Citrate loses two carbons in oxidative decarboxylation, producing a four-carbon molecule and two NADH.
Citrate Synthase catalyzes formation of citrate from oxaloacetate and acetyl CoA.
Hydrolysis of citryl CoA thioester drives citrate synthesis.
Mammalian citrate synthase operates as a dimer; binding order is oxaloacetate first, then acetyl CoA.
Enzyme undergoes conformational changes to create binding sites, preventing wasteful side reactions.
Citrate is isomerized to isocitrate for optimal structure for subsequent reactions.
Isomerization involves dehydration followed by hydration using aconitase enzyme.
Isocitrate undergoes oxidative decarboxylation to form alpha-ketoglutarate, generating NADH.
The conversion of isocitrate to succinyl CoA involves another oxidative decarboxylation step, analogous to pyruvate dehydrogenase reactions.
Overall, two carbon atoms enter the cycle, and two carbon atoms are oxidized, yielding two NADH.
19.3
The citric acid cycle regenerates oxaloacetate through a series of reactions starting and ending with four-carbon molecules.
Energy is harvested in the form of high-energy electron carriers and one molecule of ATP.
Succinyl CoA, an energy-rich thioester compound, is produced in the cycle.
Cleavage of the thioester from succinyl CoA drives the synthesis of citrate from oxaloacetate and a two-carbon fragment.
Succinyl CoA synthetase catalyzes a reaction yielding high phosphoryl-transfer potential, coupled to ADP phosphorylation.
Two enzyme forms exist in mammals: one for ADP, common in cells involved in respiration, and another for GDP, prevalent in liver cells performing anabolic reactions.
The enzyme mechanism involves displacement of coenzyme A, formation of succinyl phosphate, and ATP generation via substrate-level phosphorylation.
Succinate is oxidized to regenerate oxaloacetate, comprising three steps: oxidation, hydration, and a second oxidation.
Succinate dehydrogenase catalyzes the first oxidation step, utilizing FAD as the hydrogen acceptor.
Oxidation and hydration reactions continue until malate is oxidized to regenerate oxaloacetate, with NADH serving as the hydrogen acceptor in the final step.
The cycle produces high-energy electrons in the form of NADH, one molecule of ATP, and carbon dioxide.
Each complete cycle generates three NADH, one FADH2, and one ATP, yielding a net total of 10 ATP through electron transport chain processes.
19.4
The citric acid cycle (CAC) is the final pathway for aerobic oxidation of fuel molecules and is crucial for biosynthesis.
The cycle's entry and rate are controlled at multiple stages, primarily by enzymes such as isocitrate dehydrogenase and dehydrogenase.
Pyruvate dehydrogenase links glycolysis to the CAC, converting glucose-derived pyruvate to acetyl CoA, which can also come from fat breakdown.
Regulation occurs primarily through the concentrations of ATP and NADH, with isocitrate dehydrogenase activated by ADP and inhibited by NADH and ATP.
Dehydrogenase catalyzes a rate-limiting step and is inhibited by succinyl CoA and NADH as well as high ATP levels.
Isocitrate and dehydrogenase control points integrate the CAC with other pathways, affecting metabolic processes.
Citrate accumulation signals phosphofructokinase to halt glycolysis and serves as a source of acetyl CoA for fatty acid synthesis.
The CAC also serves as a source of biosynthetic precursors, with intermediates utilized for the formation of amino acids, porphyrins, and other compounds.
Oxaloacetate needs replenishment, especially during high energy charge states, as it's essential for the CAC to function efficiently.
The enzyme pyruvate carboxylase catalyzes the formation of oxaloacetate from pyruvate, an important anaplerotic reaction.
Defects in citric acid cycle enzymes can contribute to cancer, highlighting the cycle's role not only in energy production but in metabolic regulation as well.
19.5
Acetyl CoA primarily undergoes oxidation in the citric acid cycle and cannot be converted back to glucose in most organisms due to two decarboxylation steps.
Plants and some microorganisms can convert acetyl CoA into glucose through the glyoxylate cycle.
The glyoxylate cycle resembles the citric acid cycle but bypasses the two decarboxylation steps, allowing for glucose synthesis.
In the glyoxylate cycle, two molecules of acetyl CoA enter per turn compared to one in the citric acid cycle.
The cycle begins with the condensation of acetyl CoA and oxaloacetate to form citrate, which is isomerized to isocitrate.
Instead of decarboxylating, isocitrate is cleaved into succinate and glyoxylate by isocitrate lyase.
Oxaloacetate is regenerated from glyoxylate with the involvement of malate and malate synthase.
The glyoxylate cycle enables plants to utilize acetate for growth, especially in oil-rich seeds like sunflowers, cucumbers, and castor beans.
Succinate produced in this cycle can be converted into carbohydrates through the citric acid cycle and gluconeogenesis, which support seedling growth until photosynthesis begins.
This cycle provides metabolic flexibility, allowing organisms to use acetyl CoA as a precursor for glucose and other biomolecules.