Biochem Exam 4 SG
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Stony Brook University**We aren't endorsed by this school
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BIO 361
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Biology
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Dec 17, 2024
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6
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Lecture 31: Amino Acid DegradationEssential vs. Nonessential Amino Acids●Essential amino acids: Cannot be synthesized by the body; must be obtained throughdiet.●Nonessential amino acids: Can be synthesized by the body.Essential vs. Non-Essential Amino AcidsTypeExamplesEssentialLeucine, Lysine, Methionine, ValineNon-EssentialAlanine, Glutamate, SerineConditionallyEssentialArginine, Cysteine (during growth/illness)Carbon Skeleton Recycling●Carbon skeletons are reused and fed back into energy-producing pathways to makeenergy.○Occurs through conversion to keto acids.Catabolism of Amino Acids●Process:○Amino acids are broken down by enzymes.○Enzymes produce different products depending on the amino acid.●Glucogenic Amino Acids(18):○Converted to intermediates of the citric acid cycle.●Ketogenic Amino Acids(7):○Converted to acetyl-CoA and/or acetoacetyl-CoA.○Examples:■Lysine and leucine are solely ketogenic.■Threonine is both glucogenic and ketogenic because it produces bothpyruvate and acetyl-CoA.
Ketone Bodies●Definition:○Three water-soluble compounds produced as by-products of fatty acidbreakdown in the liver and kidney:■Acetone, acetoacetic acid, beta-hydroxybutyric acid.■Acetoacetic acid and beta-hydroxybutyric acid are the main ketonebodies.●Function:○Transported from the liver to tissues.○Acetoacetate and beta-hydroxybutyrate are reconverted to acetyl-CoA for energyproduction via the Krebs cycle.●Ketosis:○Excess ketone body accumulation.○Occurs when:■Glycogen is depleted.■Fats and some amino acids are used for energy.○Most ketone bodies are derived from fatty acid breakdown but can be producedfrom amino acids under certain conditions.●Additional Functions:○Pass through the blood-brain barrier.○In the brain:■Converted into acetyl-CoA to extend lipid acyl chains.■Used for lipid synthesis in the brain.●Formation:○Two acetyl-CoA molecules lose their coenzyme A and form acetoacetate.○Acetoacetyl-CoA can also lose its coenzyme to form acetoacetate.○Beta-hydroxybutyrate is a reduced form of acetoacetate (hydrogen added to thecarbonyl oxygen).Cofactors in Amino Acid Catabolism●Tetrahydrofolate (H4 Folate):○Derived from vitamin B9 (folate).○Transfers one-carbon groups in reactions.●S-Adenosylmethionine (adoMet):○Most common cofactor for methyl transfer.○Contains methionine and a sulfur group linked to adenosine.●Biotin:○Derived from vitamin B7 (biotin).○Transfers CO2 (or bicarbonate).●Pyridoxal-5’-Phosphate (PLP):○Derived from vitamin B6 (pyridoxine).
○Catalyzes transamination reactions.Biotin as a Cofactor●Important in one-carbon transfer reactions:○Transfers CO2 via two enzymes:■Biotin carboxylase: Catalyzes attachment of CO2 to biotin using ATP.■Carboxyltransferase: Transfers CO2 to acetyl-CoA, forming malonyl-CoA.Tetrahydrofolate (THF) as a Cofactor●Transfers one-carbon groups in various oxidation states:○CH3 (methyl), CH2OH (methylene), CHO (formyl).●Carbon source:○Serine: Converts to glycine, transferring carbon to THF.○Formate: Captured by N10-formyl-THF synthetase, forming N10-formyl-THF.S-Adenosylmethionine (adoMet)●Preferred cofactor for methyl transfer (CH3).●Synthesized from ATP and methionine.●Methyl group transfer regenerates adoMet.End Products of Amino Acid Degradation●Ketogenic Amino Acids:○Leucine and lysine produce acetyl-CoA or acetoacetyl-CoA.●Glucogenic Amino Acids:○Pyruvate, alpha-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate.Summary TableCofactorFunctionAssociated ReactionsSource
PLP (B6)Transamination,DecarboxylationGlutamate ↔α-Ketoglutarate,Histidine → HistamineVitamin B6(pyridoxine)Biotin (B7)Carboxylation (CO₂transfer)Propionyl-CoA →Methylmalonyl-CoAVitamin B7(biotin)THF (B9)One-carbon unittransferSerine ↔ Glycine,MethioninebiosynthesisVitamin B9(folate)adoMet(SAM)MethylationreactionsNorepinephrine →Epinephrine,Methionine cycleMethionine +ATP1.Classification of Amino Acids: Glucogenic vs. KetogenicTypeAmino AcidsGlucogenicOnlyAlanine, Asparagine, Glutamate, Serine, ValineKetogenic OnlyLeucine, LysineBothIsoleucine, Phenylalanine, Tyrosine,TryptophanDegradation Pathways●To Pyruvate:○Alanine, tryptophan, threonine, glycine, and serine.●To Oxaloacetate:○Asparagine (converted to aspartate) and aspartate.
●To Alpha-Ketoglutarate:○Histidine, proline, glutamine, arginine.●Branched-Chain Amino Acids:○Leucine, isoleucine, valine.●To Succinyl-CoA:○Methionine, isoleucine, threonine, valine.Genetic Disorders in Amino Acid Catabolism●Phenylketonuria (PKU):○Mutation in phenylalanine hydroxylase leads to phenylpyruvate accumulation.○Causes intellectual deficits; managed with dietary restrictions.●Maple Syrup Urine Disease (MSUD):○Deficiency in branched-chain alpha-keto acid dehydrogenase.○Causes neurological issues; treated with a low-protein diet.Amino Acids as Precursors●Tryptophan:○Converted to serotonin, niacin, and indoleacetate.●Glutamate:○Converted to gamma-aminobutyric acid (GABA).●Histidine:○Converted to histamine.●Tyrosine:○Converted to melanin, dopamine, epinephrine.Key Tables to Memorize1. Pathway Summary:End ProductAmino Acids InvolvedPyruvateAla, Cys, Gly, Ser, Thr, TrpSuccinyl-CoAIle, Met, Thr, ValFumaratePhe, TyrAcetyl-CoALeu, Lys, Phe, Trp, Tyr
2. Genetic Disorders:ConditionEnzyme DefectSymptomsPKUPhenylalanine hydroxylaseIntellectual disabilityAlkaptonuriaHomogentisatedioxygenaseDark urine, arthritis