Biochemistry: Fatty Acid Metabolism
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Questions and Answers

Which ketone body is produced when acetoacetate spontaneously decarboxylates?

  • Beta-Hydroxybutyrate
  • Acetone (correct)
  • Acetoacetate
  • Acetyl-CoA

    • What is the primary enzyme responsible for the carboxylation of acetyl-CoA to form malonyl-CoA in fatty acid synthesis?

  • Enoyl-ACP Reductase
  • Beta-Ketoacyl-ACP Reductase
  • Acetyl-CoA Carboxylase (correct)
  • Malonyl-Acyl Transferase

    • During fatty acid biosynthesis, how many carbons are added during each cycle of elongation?

  • 1 carbon
  • 4 carbons
  • 3 carbons
  • 2 carbons (correct)

    • In the fatty acid synthase complex, which of the following activities is NOT part of the complex?

    <p>Malonyl-CoA Synthase</p> Signup and view all the answers

    In which cellular compartment does fatty acid synthesis occur?

    <p>Cytoplasm</p> Signup and view all the answers

    Which of the following is a key distinguishing factor of peroxisomal β-oxidation compared to mitochondrial β-oxidation?

    <p>Shortens very long chain fatty acids</p> Signup and view all the answers

    What is the initial product formed when two molecules of acetyl-CoA condense during ketone body formation?

    <p>Acetoacetyl-CoA</p> Signup and view all the answers

    What is the main fate of excess acetyl-CoA produced during β-oxidation under normal conditions?

    <p>Entry into the citric acid cycle</p> Signup and view all the answers

    What is the direct precursor for the synthesis of ceramide in sphingolipid metabolism?

    <p>Sphinganine</p> Signup and view all the answers

    Which enzyme is responsible for the degradation of glucocerebrosides?

    <p>𝛽-Glucosidase</p> Signup and view all the answers

    In sphingomyelin metabolism, what molecular component is ceramide esterified to?

    <p>Phosphorylcholine</p> Signup and view all the answers

    What condition results from the defective function of sphingomyelinase?

    <p>Niemann-Pick Syndrome</p> Signup and view all the answers

    Which enzyme is responsible for degrading galactocerebrosides?

    <p>𝛽-galactosidase</p> Signup and view all the answers

    What occurs at the end of seven cycles of fatty acid synthesis involving Malonyl-CoA?

    <p>Palmitate is hydrolyzed from ACP</p> Signup and view all the answers

    Which enzyme is identified as the key regulatory enzyme in fatty acid synthesis?

    <p>Acetyl-CoA Carboxylase</p> Signup and view all the answers

    What effect does palmitoyl-CoA have on ACC1?

    <p>It inhibits ACC1 by promoting depolymerization</p> Signup and view all the answers

    Which of the following accurately describes the source of the two-carbon units in fatty acid elongation?

    <p>Malonyl-CoA provides the two-carbon units</p> Signup and view all the answers

    Identify the two processes that occur closely together in the endoplasmic reticulum.

    <p>Fatty acid elongation and desaturation</p> Signup and view all the answers

    How does insulin affect ACC1?

    <p>It enhances its polymerization</p> Signup and view all the answers

    What characterizes the intermediates found in fatty acid synthesis compared to beta-oxidation?

    <p>Fatty acid synthesis intermediates are linked to acyl carrier protein</p> Signup and view all the answers

    What is a critical difference in the location of fatty acid synthesis and beta-oxidation?

    <p>FA synthesis occurs in the cytoplasm, while beta-oxidation occurs in mitochondria and peroxisomes</p> Signup and view all the answers

    Which statement about the relationship between fatty acid synthesis and beta-oxidation is correct?

    <p>Fatty acid synthesis resembles the reverse process of beta-oxidation</p> Signup and view all the answers

    What is a unique characteristic of desaturases involved in fatty acid synthesis?

    <p>They are specific for particular positions of double bonds</p> Signup and view all the answers

    What is the main substrate for glyceroneogenesis?

    <p>Malate</p> Signup and view all the answers

    What is required for the formation of Acyl-CoA from fatty acids?

    <p>ATP and Coenzyme-A</p> Signup and view all the answers

    Which enzyme catalyzes the formation of acylcarnitine from Acyl-CoA?

    <p>Carnitine acyltransferase-1</p> Signup and view all the answers

    What is the yield of ATP from the complete oxidation of Palmitoyl-CoA?

    <p>108 ATP</p> Signup and view all the answers

    Which type of fatty acids requires an additional enzyme for degradation due to cis double bonds?

    <p>Unsaturated fatty acids</p> Signup and view all the answers

    How many carbon atoms are removed in each cycle of beta-oxidation?

    <p>2 carbons</p> Signup and view all the answers

    What is the primary function of fatty acid binding proteins in lipolysis?

    <p>Transport fatty acids in blood to various tissues</p> Signup and view all the answers

    During fatty acid transport into mitochondria, what role does carnitine play?

    <p>It is an acyl group carrier molecule</p> Signup and view all the answers

    Which of the following is true regarding the oxidation of odd-chain fatty acids?

    <p>They yield Acetyl-CoA and propionyl-CoA</p> Signup and view all the answers

    Which enzyme is NOT involved in the beta-oxidation of fatty acids?

    <p>Carnitine acyltransferase-1</p> Signup and view all the answers

    Which enzyme is responsible for hydrolyzing the ester bond at C1 of glycerol?

    <p>PLA1</p> Signup and view all the answers

    Which phospholipid acts as a signal for macrophages to engulf cells?

    <p>Phosphatidylserine</p> Signup and view all the answers

    What role does diacylglycerol play in the synthesis of phosphatidylethanolamine and phosphatidylcholine?

    <p>It is a precursor that reacts with CDP-ethanolamine and CDP-choline.</p> Signup and view all the answers

    Phospholipid turnover involves which of the following processes?

    <p>Rapid replacement of fatty acids.</p> Signup and view all the answers

    What is the primary function of phosphatidylcholine in biological membranes?

    <p>Serving as a surfactant.</p> Signup and view all the answers

    Which phospholipase hydrolyzes both ester bonds at C1 and C2 of glycerol?

    <p>PLB</p> Signup and view all the answers

    The synthesis of phosphatidylserine primarily involves which initial substrate?

    <p>Ethanolamine</p> Signup and view all the answers

    Which of the following statements about phospholipid degradation by phospholipases is true?

    <p>PLC cleaves the phosphodiester bond between glycerol and phosphate.</p> Signup and view all the answers

    What is the main component that contributes to 25% of all phospholipids in humans?

    <p>Phosphatidylethanolamine</p> Signup and view all the answers

    Which of the following best describes the relationship between CTP and phospholipid synthesis?

    <p>CTP is consumed to convert phospho-ethanolamine to CDP-ethanolamine.</p> Signup and view all the answers

    What is the primary function of glutamine synthetase in nitrogen metabolism?

    <p>Facilitates the conversion of glutamate to glutamine</p> Signup and view all the answers

    Which pathway is primarily responsible for the synthesis of glutamate?

    <p>Transamination with α-ketoglutarate</p> Signup and view all the answers

    Which of the following amino acids is considered essential?

    <p>Leucine</p> Signup and view all the answers

    When nitrogen uptake exceeds nitrogen excretion, what condition is indicated?

    <p>Positive nitrogen balance</p> Signup and view all the answers

    Which reaction is primarily involved in the transfer of amino groups during amino acid biosynthesis?

    <p>Transamination reactions</p> Signup and view all the answers

    What is the role of pyridoxal-5'-phosphate (PLP) in transamination reactions?

    <p>It binds the amino acid and facilitates its transfer</p> Signup and view all the answers

    Which two amino acid pairs are most important for nitrogen disposal in the urea cycle?

    <p>Oxaloacetate and aspartate</p> Signup and view all the answers

    What distinguishes essential amino acids from nonessential ones?

    <p>Essential amino acids must be obtained through diet.</p> Signup and view all the answers

    Which mechanism describes the interaction of PLP with the donor amino acid during transamination?

    <p>Ping-pong mechanism</p> Signup and view all the answers

    What is the primary function of glutamine synthetase in nitrogen metabolism?

    <p>Conversion of glutamate to glutamine</p> Signup and view all the answers

    Which of the following pathways leads to the synthesis of glutamate?

    <p>Ammonium assimilation via glutamine synthase</p> Signup and view all the answers

    Which amino acid is considered nonessential due to its ability to be synthesized by the body?

    <p>Alanine</p> Signup and view all the answers

    What is described as a positive nitrogen balance in organisms?

    <p>Nitrogen intake exceeds nitrogen excretion</p> Signup and view all the answers

    Which enzyme is responsible for catalyzing the transamination reaction for amino acid synthesis?

    <p>Transaminases</p> Signup and view all the answers

    Which of the following amino acids is classified as essential and must be obtained through diet?

    <p>Valine</p> Signup and view all the answers

    What biochemical process is directly associated with nitrogen assimilation?

    <p>Incorporation of inorganic nitrogen into organic compounds</p> Signup and view all the answers

    In the context of amino acid production, what is the role of glutamate dehydrogenase?

    <p>Catalyzing the deamination of glutamate to alpha-ketoglutarate</p> Signup and view all the answers

    Which compound is essential for the nitrogen fixation process carried out by the nitrogenase complex?

    <p>ATP</p> Signup and view all the answers

    Which of the following is true regarding the activity of the nitrogenase complex?

    <p>It involves two proteins: dinitrogenase and dinitrogenase reductase</p> Signup and view all the answers

    What is the primary function of glutamine synthetase in nitrogen metabolism?

    <p>Catalyzes the formation of glutamine from glutamate and ammonium</p> Signup and view all the answers

    Which of the following describes the role of glutamate synthase in amino acid biosynthesis?

    <p>It utilizes glutamine as a nitrogen donor to produce glutamate</p> Signup and view all the answers

    Which amino acids must be obtained from the diet due to the inability of humans to synthesize them?

    <p>Methionine, leucine, and histidine</p> Signup and view all the answers

    What is the primary nitrogen source for amino acid synthesis in plants and prokaryotes?

    <p>Ammonium and nitrate sources</p> Signup and view all the answers

    In eukaryotes, which pathway primarily utilizes glutamate dehydrogenase for nitrogen excretion?

    <p>Reversible conversion of alpha-ketoglutarate to ammonia</p> Signup and view all the answers

    Which component is essential for the reaction catalyzed by glutamate synthase?

    <p>NADPH</p> Signup and view all the answers

    What distinguishes essential amino acids from nonessential amino acids?

    <p>Essential amino acids cannot be produced by the human body</p> Signup and view all the answers

    What type of reaction does glutamine synthetase mediate?

    <p>Formation of glutamine from glutamate and ammonium</p> Signup and view all the answers

    Why is the reaction of glutamate dehydrogenase primarily running in reverse in eukaryotes?

    <p>To excrete excess nitrogen as ammonia</p> Signup and view all the answers

    What role does ATP play in the reaction catalyzed by glutamine synthetase?

    <p>It activates glutamate for subsequent nitrogen transfer</p> Signup and view all the answers

    Which of the following nitrogen-containing molecules is least reactive?

    <p>N2</p> Signup and view all the answers

    What is the primary role of the nitrogenase complex?

    <p>Reduction of atmospheric nitrogen to ammonia</p> Signup and view all the answers

    Which enzyme is responsible for transferring electrons from NADH to the dinitrogenase in the nitrogenase complex?

    <p>Dinitrogenase reductase</p> Signup and view all the answers

    Which of the following statements accurately describes nitrogen fixation?

    <p>Only some prokaryotes can perform it.</p> Signup and view all the answers

    What defines positive nitrogen balance in a biological system?

    <p>Nitrogen intake exceeds nitrogen excretion.</p> Signup and view all the answers

    Which of the following amino acids is classified as nonessential?

    <p>Glutamate</p> Signup and view all the answers

    What is the net result of the nitrogen fixation reaction as described?

    <p>1 N2 is converted into 2 NH3.</p> Signup and view all the answers

    Which of the following processes involves the incorporation of inorganic nitrogen into organic molecules?

    <p>Nitrogen assimilation</p> Signup and view all the answers

    What is a characteristic of the speed of nitrogen fixation by the nitrogenase complex?

    <p>It operates very slowly, at about 6 reactions per second.</p> Signup and view all the answers

    Which term best describes the overall reaction catalyzed by the nitrogenase complex?

    <p>Endothermic and requires significant energy</p> Signup and view all the answers

    Which process is required to incorporate nitrogen from nitrate?

    <p>Reduction to ammonium</p> Signup and view all the answers

    What is the primary function of glutamine synthetase?

    <p>To catalyze the conversion of glutamate to glutamine</p> Signup and view all the answers

    Which enzyme is responsible for the reductive amination of a-ketoglutarate?

    <p>Glutamate synthetase</p> Signup and view all the answers

    Which condition describes the predominant reaction direction of glutamate dehydrogenase in eukaryotes?

    <p>Excretion of nitrogen</p> Signup and view all the answers

    How do essential and nonessential amino acids differ for animals?

    <p>Essential amino acids cannot be synthesized by animals.</p> Signup and view all the answers

    What is required for the reaction catalyzed by glutamate synthetase?

    <p>NADPH</p> Signup and view all the answers

    Which biochemical pathway leads to the production of glutamate?

    <p>Transamination</p> Signup and view all the answers

    What primarily characterizes how organic nitrogen moves through ecosystems?

    <p>As amino acids</p> Signup and view all the answers

    Which substance is produced as a byproduct during the synthesis of glutamate from a-ketoglutarate?

    <p>NADP+</p> Signup and view all the answers

    Which amino acids can humans not synthesize biosynthetically?

    <p>Methionine, Valine, Tryptophan</p> Signup and view all the answers

    What occurs during a positive nitrogen balance?

    <p>Children experience growth.</p> Signup and view all the answers

    Which amino acid is most frequently the donor in transamination reactions?

    <p>Glutamate</p> Signup and view all the answers

    What is a primary function of pyridoxal-5'-phosphate (PLP) in transamination reactions?

    <p>It forms a Schiff base with donor amino acids.</p> Signup and view all the answers

    Which of the following statements correctly describes transamination reactions?

    <p>They are catalyzed by aminotransferases.</p> Signup and view all the answers

    What is a characteristic of branched chain amino acids (BCAAs) in amino acid metabolism?

    <p>They facilitate nitrogen transfer from the liver to other tissues.</p> Signup and view all the answers

    Which reaction is crucial for the disposal of nitrogen through the urea cycle?

    <p>Transamination with glutamate.</p> Signup and view all the answers

    What type of mechanism does the transamination reaction employ?

    <p>Ping-pong mechanism.</p> Signup and view all the answers

    Which cofactor is required for the synthesis and degradation of amino acids involving transaminases?

    <p>Pyridoxal-5'-phosphate</p> Signup and view all the answers

    Which pair of amino acids is essential for the glucose-alanine cycle?

    <p>Pyruvate/alanine</p> Signup and view all the answers

    What metabolic process occurs when negative nitrogen balance is evident?

    <p>Potential malnutrition.</p> Signup and view all the answers

    What condition results when nitrogen uptake is less than nitrogen excretion?

    <p>Negative nitrogen balance</p> Signup and view all the answers

    Which amino acid pair is primarily involved in the urea cycle for nitrogen disposal?

    <p>Aspartate / Oxaloacetate</p> Signup and view all the answers

    What is NOT a function of pyridoxal-5'-phosphate (PLP) in transamination reactions?

    <p>Acting as a coenzyme for amino acid transport</p> Signup and view all the answers

    Which metabolic mechanism aids in transporting nitrogen from the liver to other tissues?

    <p>Branched chain amino acids (BCAA)</p> Signup and view all the answers

    What is the primary source of nitrogen required for biosynthetic reactions?

    <p>Dietary amino acids</p> Signup and view all the answers

    What type of reaction is characterized as reversible and involves the synthesis and degradation of amino acids?

    <p>Transamination</p> Signup and view all the answers

    What cofactor is essential for the transfer of one carbon groups in amino acid metabolism?

    <p>THF or SAM</p> Signup and view all the answers

    Which amino acid is most commonly the donor in transamination reactions?

    <p>Glutamate</p> Signup and view all the answers

    What describes the mechanism by which PLP transfers NH2 groups to acceptor keto acids during transamination?

    <p>Ping-pong mechanism</p> Signup and view all the answers

    What mechanism primarily involves breaking bonds to generate new amino acids from keto acids?

    <p>Transamination</p> Signup and view all the answers

    Which enzyme is responsible for the initial step in the nitrogen fixation process?

    <p>Dinitrogenase</p> Signup and view all the answers

    What essential function does nitrogen assimilation serve in organisms?

    <p>Incorporating inorganic nitrogen into organic molecules</p> Signup and view all the answers

    How many electrons are required in total for the nitrogen fixation reaction to occur?

    <p>8 electrons</p> Signup and view all the answers

    Which of the following compounds is not directly involved in the nitrogen fixation reaction mediated by the nitrogenase complex?

    <p>Nitrate (NO3-)</p> Signup and view all the answers

    What distinguishes essential amino acids from nonessential amino acids?

    <p>Essential amino acids cannot be synthesized by the body.</p> Signup and view all the answers

    Which reaction is catalyzed by transaminases/aminotransferases?

    <p>Transfer of amino groups between amino acids and alpha-keto acids</p> Signup and view all the answers

    What characterizes a positive nitrogen balance in an organism?

    <p>Nitrogen intake exceeds excretion</p> Signup and view all the answers

    What is the significance of the nitrogenase complex having a slow reaction rate of 6 per second?

    <p>It highlights the complexity of the nitrogen fixation process.</p> Signup and view all the answers

    What does the term 'nitrogen balance' specifically refer to in biological systems?

    <p>The difference between nitrogen intake and nitrogen excretion.</p> Signup and view all the answers

    What must happen to nitrate before it can be incorporated into organic nitrogen molecules?

    <p>It must be reduced to ammonium.</p> Signup and view all the answers

    Which enzyme is responsible for the nitrogen donation during the synthesis of glutamate from a-ketoglutarate?

    <p>Glutamate synthase</p> Signup and view all the answers

    What is the role of NADPH in the reaction catalyzed by glutamate synthase?

    <p>It serves as a reducing agent in the formation of glutamate.</p> Signup and view all the answers

    Which statement best describes the ability of animals to synthesize amino acids?

    <p>Animals can synthesize some but require certain essential amino acids from their diet.</p> Signup and view all the answers

    What is the primary fate of amino groups transferred from glutamate in amino acid biosynthesis?

    <p>They are involved in the synthesis of other amino acids.</p> Signup and view all the answers

    What does glutamate dehydrogenase primarily generate in eukaryotes?

    <p>NH4+ for nitrogen excretion</p> Signup and view all the answers

    What distinguishes the capabilities of glutamine synthetase in prokaryotes and plants from those in animals?

    <p>Animals lack the enzyme glutamine synthetase entirely.</p> Signup and view all the answers

    What is the significance of ATP in the reaction catalyzed by glutamine synthetase?

    <p>It provides energy for the incorporation of ammonium.</p> Signup and view all the answers

    Which process is primarily responsible for the synthesis of nonessential amino acids in animals?

    <p>Transamination reactions</p> Signup and view all the answers

    Which compound must be present to facilitate the direct amination of a-ketoglutarate?

    <p>NADH</p> Signup and view all the answers

    Study Notes

    Phytanic Acid

    • Phytanic acid is derived from phytol, a component of chlorophyll found in green vegetables.
    • Phytanic acid is oxidized in peroxisomes.

    Beta-oxidation in Peroxisomes

    • Peroxisomes are organelles that specialize in the shortening of very long-chain fatty acids.
    • Peroxisomes oxidize very long chain fatty acids into medium chain fatty acids.
    • The process halts at octanoyl-CoA.
    • Peroxisomal beta-oxidation does not generate ATP.
    • The initial reaction produces hydrogen peroxide (H2O2), which is neutralized by catalase to form water.

    Beta-oxidation in Peroxisome vs. Mitochondria

    • Peroxisomes carry out the initial stages of the oxidation of very long-chain fatty acids.
    • Mitochondria complete the oxidation of medium-chain fatty acids produced by peroxisomes.

    Ketogenesis and Ketone Bodies

    • Ketogenesis is the process of producing ketone bodies.
    • Ketone bodies include acetoacetate, beta-hydroxybutyrate, and acetone.
    • Ketone bodies are synthesized primarily in the liver from excess acetyl-CoA.
    • Under normal conditions, only small amounts of excess acetyl-CoA are produced, so ketone body production is minimal.

    Ketosis

    • Ketosis is a state in which acetone is produced.
    • Ketosis occurs most significantly during prolonged starvation and uncontrolled diabetes.
    • Acetoacetate spontaneously decarboxylates to form acetone.

    Ketone Body Formation

    • Ketone body formation involves a series of reactions:
      • Two acetyl-CoA molecules condense to form acetoacetyl-CoA.
      • Acetoacetyl-CoA condenses with another acetyl-CoA to form HMG-CoA.
      • HMG-CoA lyase hydrolyzes HMG-CoA into acetoacetate.
      • Acetoacetate spontaneously decarboxylates to form acetone.
      • Acetoacetate can also be reversibly reduced to beta-hydroxybutyrate.
    • Heart and skeletal muscle use ketone bodies for energy.
    • Brain uses ketone bodies as an energy source during prolonged starvation.

    Conversion of Ketone Bodies to Acetyl-CoA

    • Ketone bodies can be converted back into acetyl-CoA.
    • Beta-hydroxybutyrate is converted to acetyl-CoA in the mitochondrial matrix of the liver.

    Fatty Acid Biosynthesis

    • Fatty acid synthesis takes place in the cytoplasm.
    • Intermediates are covalently linked to acyl carrier protein (ACP).
    • ACP is a fatty acid activator in fatty acid biosynthesis.
    • Acetyl-CoA and malonyl-CoA are converted into Acetyl-ACP and Malonyl-ACP, respectively, for fatty acid synthesis.
    • The process involves a four-step repeating cycle: condensation, reduction, dehydration, and reduction.
    • Each cycle extends the fatty acid chain by two carbons.

    Fatty Acid Synthase (FAS)

    • Fatty acid synthase is a multi-enzyme complex composed of distinct enzyme activities:
      • Malonyl-acyl transferase (MAT)
      • Ketosynthase (KS)
      • Keto reductase (KR)
      • Dehydratase (DH)
      • Enoyl reductase (ER)
    • FAS is an X-shaped homodimer, meaning it consists of two identical polypeptide chains.
    • Each polypeptide chain contains seven catalytic domains and an ACP.
    • FAS synthesizes two fatty acids simultaneously.

    Fatty Acid Biosynthesis (Phases 1 and 2)

    • Fatty acid biosynthesis proceeds in two phases:
      • Phase 1: Synthesis of Malonyl-CoA
      • Phase 2: Sequential addition of two-carbon units to synthesize palmitic acid (16:0)
    • Acetyl-ACP and Malonyl-ACP are the substrates for fatty acid biosynthesis.

    Fatty Acid Biosynthesis (Phase 1: Malonyl-CoA formation)

    • Acetyl-CoA carboxylase (ACC1) carboxylates acetyl-CoA to synthesize malonyl-CoA.
    • This reaction is irreversible and is the rate-limiting step in fatty acid biosynthesis.
    • Malonyl-CoA is activated by malonyl-acetyl transferase to form malonyl-ACP.

    Acetyl-ACP Formation

    • Acetyl-CoA reacts with ACP-SH to form acetyl-ACP (activated acetate).
    • Acetyl-ACP is transferred to the ketosynthase (KS) unit of the fatty acid synthase complex.

    Fatty Acid Biosynthesis (Phase 2)

    • The seven steps in phase 2 are:
      1. Condensation of acetyl and malonyl groups to form acetoacetyl-ACP.
      2. Reduction of the beta-carbonyl group to form an alcohol.
      3. Dehydration of the alcohol to form a carbon-carbon double bond.
      4. Reduction of the double bond to form a saturated four-carbon acyl group.
      5. Transfer of the acyl group from ACP to the SH group of beta-ketoacyl synthase (KS).
      6. Condensation of the acyl group with another ACP-linked malonyl group, extending the chain by two carbons.
      7. Hydrolysis of palmitate from ACP by thioesterase (TE).

    Repeated Cycles for Elongation

    • After the first cycle, a four-carbon chain is associated with the ACP arm.
    • This four-carbon chain is transferred back to the KS arm, and a new malonyl-CoA is introduced on the ACP arm.
    • Each subsequent cycle extends the acyl chain by two carbons.
    • After seven cycles, a 16-carbon fatty acid (palmitoyl-ACP) is attached to the ACP arm.
    • Palmitoyl-ACP is hydrolyzed to release free palmitate.

    Net Reaction for Palmitate Synthesis

    • 8 Acetyl-CoA + 14 NADPH + 17 H+ + 7 ATP → Palmitate + 14 NADP+ + 7 ADP + 7 Pi + 8 CoASH + 6 H2O

    Acetyl-CoA Carboxylase (ACC)

    • Acetyl-CoA carboxylase (ACC1) is a key enzyme in fatty acid synthesis.
    • ACC1 catalyzes the rate-limiting step in fatty acid synthesis.
    • ACC exists in active and inactive forms:
      • Active form: Polymerized, dephosphorylated dimer.
      • Inactive form: Phosphorylated monomer.

    Allosteric Regulation of ACC1

    • Activators:
      • Citrate: A feedforward activator that promotes polymerization of ACC1.
    • Inhibitors:
      • Palmitoyl-CoA: The end product of fatty acid synthesis, which depolymerizes ACC1 and inhibits activity.

    Hormonal Regulation of ACC1

    • Insulin: Dephosphorylates and activates ACC1.
    • Epinephrine and glucagon: Phosphorylate and inhibit ACC1.

    Fatty Acid Elongation and Desaturation

    • Fatty acid elongation and desaturation are closely integrated processes.
    • Both processes occur in the endoplasmic reticulum (ER).
    • Palmitic acid is first activated to palmitoyl-CoA for elongation and desaturation.
    • For elongation, malonyl-CoA provides a two-carbon unit.
    • Elongases are enzymes that synthesize longer chain fatty acids.
    • Desaturases are enzymes that synthesize unsaturated fatty acids.
    • Mammals lack the enzymes to introduce a double bond beyond carbon 9.
    • Fatty acids containing double bonds beyond carbon 9 are supplied by the diet (essential fatty acids, EFAs).
    • Desaturases are specific for the position of the double bond.

    Nitrogen in Biological Systems

    • Nitrogen is an essential element for life.
    • Nitrogen exists in different forms - atmospheric nitrogen (N2), ammonia (NH3), ammonium (NH4+), nitrite (NO2-), and nitrate (NO3-).
    • Nitrogen gas in the atmosphere is inert and unavailable for use by organisms.
    • Nitrogen fixation is the process of converting atmospheric nitrogen into a usable form – ammonia.
    • Nitrogen fixation is carried out by some prokaryotes, which possess the nitrogenase enzyme complex.

    Nitrogenase Complex

    • The nitrogenase complex is a key enzyme responsible for nitrogen fixation.
    • It consists of two proteins: dinitrogenase and dinitrogenase reductase, both containing iron.
    • Dinitrogenase reductase transfers electrons from NADH to dinitrogenase.
    • Dinitrogenase then reduces N2 to ammonia (NH3).
    • This process requires a significant amount of energy.

    Nitrogen Assimilation

    • Nitrogen assimilation is the process of incorporating inorganic nitrogen into organic molecules, primarily amino acids.
    • Glutamine synthetase catalyzes the first step in nitrogen assimilation.
    • It converts glutamate and ammonia into glutamine using ATP.

    Glutamate Synthase

    • Glutamine acts as a nitrogen donor for the reductive amination of α-ketoglutarate, catalyzed by glutamate synthase, which is present only in prokaryotes and plants.
    • Glutamate synthase requires NADPH as a cofactor.

    Direct Amination of α-Ketoglutarate

    • Glutamate dehydrogenase catalyzes the direct amination of α-ketoglutarate, forming glutamate.
    • This reaction is reversible but primarily runs in reverse in eukaryotes to generate ammonia for nitrogen excretion.
    • It plays a minor role in nitrogen assimilation.

    Essential and Nonessential Amino Acids

    • Plants and prokaryotes can synthesize all twenty amino acids.
    • Animals can synthesize only nonessential amino acids and must obtain essential amino acids from their diet.

    Nitrogen Balance

    • Nitrogen balance refers to the equilibrium between nitrogen intake and excretion.
    • Positive nitrogen balance: Intake exceeds excretion, often occurring in children during growth.
    • Negative nitrogen balance: Excretion exceeds intake, caused by malnutrition or disease.

    Amino Acid Metabolism

    • Amino acids are the primary source of nitrogen for biosynthesis.
    • Dietary amino acids may not be available in the optimal proportions required, and metabolic mechanisms are needed to balance and utilize them.
    • Branched-chain amino acids (BCAAs) play a vital role in transporting nitrogen from the liver to other tissues.
    • Amino acids are transported across cell membranes by specific proteins.

    Overview of Amino Acid and Nucleotide Biosynthesis

    • Several key types of reactions are involved:
      • Transamination reactions catalyzed by aminotransferases using pyridoxal phosphate (PLP) as a cofactor.
      • Transfer of one-carbon groups using tetrahydrofolate (THF) or S-adenosylmethionine (SAM) cofactors.
      • Transfer of amino groups from glutamine.

    Transamination Reactions

    • They are reversible reactions involved in both synthesis and degradation of amino acids.
    • Aminotransferases are numerous in eukaryotes, present in both cytoplasm and mitochondria.
    • They specifically recognize the donor amino acid and acceptor keto acid.
    • Glutamate is commonly the donor amino acid.
    • The α-ketoglutarate/glutamate pair and the oxaloacetate/aspartate pair are crucial.

    Pyridoxal-5'-Phosphate (PLP)

    • PLP is a cofactor for transamination reactions, acting as the active form of Vitamin B6.
    • PLP binds to the active site of transaminases.
    • It plays a key role in various reactions, including transamination, racemization, and decarboxylation.

    PLP Schiff Base Formation

    • Transamination involves the formation of a Schiff base between PLP and the donor amino acid.
    • This Schiff base is crucial for the transfer of the amino group.
    • The reaction mechanism involves a series of steps, including bond breaking and formation, leading to the transfer of the amino group from the donor to the acceptor.
    • This process is also known as a ping-pong or double displacement mechanism.

    Why is Nitrogen Important?

    • Nitrogen is an essential nutrient for all forms of life, making up approximately 78% of the air we breathe.
    • We cannot directly utilize nitrogen gas (N2) as it is inert.
    • Nitrogen fixation refers to the process of converting atmospheric nitrogen into usable forms.

    Nitrogen Fixation

    • Performed exclusively by certain prokaryotes (bacteria).
    • Nitrogenase enzyme complex is responsible for N2 reduction to ammonia (NH3).
    • The nitrogenase complex consists of two proteins: dinitrogenase and dinitrogenase reductase.
    • Dinitrogenase reductase transfers electrons from NADH to dinitrogenase.
    • Dinitrogenase then catalyzes the reduction of N2 to NH3.
    • The reaction is energetically demanding, requiring 8 electrons, 16 ATP, and 10 protons.

    Nitrogen Assimilation

    • The process of incorporating inorganic nitrogen (NH4+ or NO3-) into organic molecules.
    • Primarily involves the synthesis of amino acids.
    • Glutamine synthetase catalyzes the incorporation of ammonia into glutamate to form glutamine.
    • Glutamate synthase, found in prokaryotes and plants, utilizes glutamine as a nitrogen donor to reduce α-ketoglutarate to glutamate, requiring NADPH.
    • Glutamate dehydrogenase catalyzes the direct amination of α-ketoglutarate to form glutamate.

    Amino Acid Biosynthesis

    • Plants and prokaryotes are capable of synthesizing all 20 amino acids.
    • Animals can only synthesize a subset of amino acids, known as nonessential amino acids.
    • Essential amino acids must be obtained from the diet.

    Nitrogen Balance

    • Nitrogen balance refers to the equilibrium between nitrogen intake and excretion.
    • Positive nitrogen balance occurs when intake exceeds excretion, commonly seen in growing children.
    • Negative nitrogen balance occurs when excretion exceeds intake, potentially leading to malnutrition.

    Amino Acid Metabolism

    • Amino acids serve as the primary nitrogen source for biosynthetic reactions.
    • Dietary amino acids may not be present in optimal proportions for our needs.
    • Branched-chain amino acids (BCAAs) play a crucial role in transporting nitrogen from the liver to other tissues.

    Overview of Reactions

    • Transamination: Transfer of amino groups catalyzed by pyridoxal phosphate-dependent enzymes (aminotransferases or transaminases).
    • One-carbon group transfer: Utilizing tetrahydrofolate (THF) or S-adenosylmethionine (SAM) cofactors.
    • Glutamine-dependent amino group transfer: Utilizing glutamine as a nitrogen donor.

    Transamination Reactions

    • Reversible reactions involved in both amino acid synthesis and degradation.
    • Aminotransferases are abundant enzymes in eukaryotes, found in both cytoplasm and mitochondria.
    • Glutamate serves as a common donor amino acid in transamination reactions.
    • Pyridoxal-5’-phosphate (PLP) acts as a coenzyme for transaminases.

    Pyridoxal-5’-Phosphate (PLP)

    • PLP is a coenzyme derived from vitamin B6.
    • PLP facilitates transamination reactions by forming Schiff bases with amino acids.
    • PLP also serves as a coenzyme for racemization and decarboxylation reactions.

    Schiff Base Formation

    • PLP forms a Schiff base with the donor amino acid, which is essential for the transamination process.

    Nitrogen Importance

    • Nitrogen is a key element for life, present in many essential molecules like amino acids and nucleic acids.
    • Air is composed of ~78% nitrogen gas (N2).
    • N2 is inert (stable/nonreactive) and cannot be directly used by most organisms.

    Nitrogen Fixation

    • Nitrogen fixation is the process of converting atmospheric N2 into a usable form, usually ammonia (NH3).
    • This process requires significant energy and can only be carried out by certain prokaryotes (nitrogen-fixing bacteria).
    • The nitrogenase complex is a key enzyme involved in nitrogen fixation.

    Nitrogenase Complex

    • The nitrogenase complex is composed of two proteins: dinitrogenase and dinitrogenase reductase.
    • These proteins contain iron and are crucial for the reduction of N2 to NH3.
    • The dinitrogenase reductase transfers electrons from NADH to dinitrogenase.
    • Dinitrogenase utilizes these electrons to reduce N2.
    • The process is relatively slow, with only about 6 reactions per second.

    Nitrogen Assimilation

    • Nitrogen assimilation is the incorporation of inorganic nitrogen (like NH4+ or NO3-) into organic molecules.
    • It primarily involves the incorporation of nitrogen into amino acids.
    • Glutamine synthetase is a key enzyme catalyzing the incorporation of ammonia (NH4+) into glutamate to form glutamine.
    • This reaction requires ATP.
    • If NO3- is the nitrogen source, it must be reduced to NH4+ first.

    Glutamate Synthase

    • Glutamine acts as a nitrogen donor in the reductive amination of α-ketoglutarate.
    • This reaction is catalyzed by glutamate synthase, which is found in plants and prokaryotes but not animals.
    • Glutamate synthase requires NADPH.

    Direct Amination of α-Ketoglutarate

    • Glutamate dehydrogenase catalyzes the direct amination of α-ketoglutarate to form glutamate.
    • This reaction is reversible, but in eukaryotes, it mainly runs in reverse to generate NH4+ for nitrogen excretion.

    Essential and Nonessential Amino Acids

    • Plants and prokaryotes can synthesize all 20 amino acids.
    • Animals can only synthesize nonessential amino acids, requiring essential amino acids from their diet.
    • Humans cannot synthesize three amino acids biosynthetically (histidine, isoleucine, and lysine) but they can be produced as byproducts of other biochemical pathways.

    Nitrogen Balance

    • Nitrogen balance occurs when nitrogen uptake equals nitrogen excretion.
    • Positive nitrogen balance indicates nitrogen uptake exceeding excretion, often seen during growth periods.
    • Negative nitrogen balance indicates nitrogen uptake less than excretion, potentially leading to malnutrition.

    Amino Acid Metabolism

    • Amino acids are the primary source of nitrogen for biosynthetic reactions.
    • Dietary amino acids may not be available in the proportions needed, requiring metabolic regulation.
    • Branched-chain amino acids (BCAAs) play a role in transporting nitrogen from the liver to other tissues.
    • Specific transport proteins facilitate amino acid movement across cell membranes.

    Overview of Reactions in Amino Acid and Nucleotide Biosynthesis

    • Transamination reactions: catalyzed by pyridoxal phosphate-containing enzymes (aminotransferases or transaminases) and involve the transfer of amino groups.
    • One-carbon group transfers: utilize THF or SAM cofactors.
    • Glutamine-dependent amino group transfers: involve transfer of amino groups from glutamine.

    Transamination Reactions

    • Reversible reactions involved in both the synthesis and degradation of amino acids.
    • Aminotransferases are numerous in eukaryotes, present in both cytoplasm and mitochondria.
    • Glutamate is often a donor amino acid, forming the α-ketoglutarate/glutamate pair.
    • The oxaloacetate/aspartate pair is essential for the urea cycle (nitrogen disposal).
    • The pyruvate/alanine pair is crucial for the glucose-alanine cycle (gluconeogenesis).

    Pyridoxal-5'-Phosphate (PLP)

    • PLP is a coenzyme required for transamination reactions.
    • It acts as a vitamin B6 derivative and binds to the active site of transaminases.
    • PLP is also involved in racemization and decarboxylation reactions.

    PLP Forms a Schiff Base

    • PLP forms a Schiff base with the donor amino acid, which is crucial for transamination.
    • The Schiff base formation and subsequent breakage lead to the transfer of the amino group from the donor amino acid to the acceptor keto acid.

    Aminotransferases – Mechanism

    • Aminotransferases utilize a ping-pong or double displacement mechanism.
    • During the process, the amino group is temporarily attached to PLP before being transferred to the acceptor keto acid.

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