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I have all the content. Now I'll compile a thorough, well-organized study guide.

πŸ“š Study Guide: Amino Acid Degradation & Synthesis

Source: Lippincott's Biochemistry, Chapter 20

1. Overview

  • Catabolism begins with removal of the Ξ±-amino group β†’ carbon skeletons enter metabolism as one of 7 key intermediates:
    • Pyruvate, Oxaloacetate, Ξ±-Ketoglutarate, Fumarate, Succinyl-CoA, Acetyl-CoA, Acetoacetate
  • End products are used for energy (TCA cycle) or biosynthesis (glucose, lipids)
  • Nonessential AAs: synthesized from metabolic intermediates or other AAs
  • Essential AAs: must come from diet

2. Glucogenic vs. Ketogenic AAs

CategoryCarbon skeleton yieldsExamples
GlucogenicGlucose precursors (pyruvate, Ξ±-KG, OAA, fumarate, succinyl-CoA)Alanine, Aspartate, Glutamate, etc.
KetogenicKetone bodies (acetyl-CoA, acetoacetate)Leucine, Lysine
BothBoth glucose and ketone bodiesIsoleucine, Tryptophan, Phenylalanine, Tyrosine
High-yield: Leucine & Lysine are the ONLY exclusively ketogenic AAs (cannot form glucose)

3. Specific Catabolic Pathways

β†’ Oxaloacetate

AAPathwayClinical note
Asparagine→ Aspartate → OxaloacetateAsparaginase depletes plasma asparagine - used in leukemia treatment
AspartateTransaminated β†’ Oxaloacetate

β†’ Ξ±-Ketoglutarate

AAPathwayClinical note
Glutamine→ Glutamate → α-KG (via glutaminase & GDH)
Proline, Arginine, Histidine→ all convert to Glutamate → α-KG
Histidine→ FIGlu → Glutamate (folate-dependent)FIGlu test detects folate deficiency

β†’ Pyruvate

AAKey steps
AlanineTransaminated β†’ Pyruvate; major gluconeogenic AA
Serineβ†’ Glycine + N⁡,N¹⁰-methylene-THF; or β†’ Pyruvate via serine dehydratase
Glycine↔ Serine; also β†’ COβ‚‚ + NH₃; β†’ Glyoxylate β†’ Oxalate
Cystine→ Cysteine → Desulfuration → Pyruvate; sulfate → PAPS (activated sulfur donor)
Threonine→ Pyruvate OR → α-Ketobutyrate → Succinyl-CoA
⚠️ Glycine β†’ Oxalate pathway: excess oxalate causes kidney stones in Primary Oxaluria Type 1

β†’ Fumarate (+ Acetoacetate) - Glucogenic AND Ketogenic

  • Phenylalanine β†’ Tyrosine (via phenylalanine hydroxylase + BHβ‚„) β†’ Fumarate & Acetoacetate
  • Disorders: PKU, Alkaptonuria, Albinism

β†’ Succinyl-CoA via Methionine

StepDetail
Methionine β†’ SAMMajor methyl donor in the body
SAM β†’ SAH β†’ Homocysteine
Homocysteine β†’ Methionine (remethylation)Requires folate + B₁₂
Homocysteine β†’ Transsulfuration β†’ Cysteine + Ξ±-Ketobutyrate β†’ Propionyl-CoA β†’ Succinyl-CoARequires Vitamin B6

4. Homocysteine & Vascular Disease ⚠️

  • ↑ Homocysteine β†’ endothelial damage, inflammation, thrombosis risk
  • Causes of ↑ Homocysteine: deficiency in folate, B₆, B₁₂
  • Associated with neural tube defects
  • Classic Homocystinuria (CBS deficiency) β†’ premature vascular events

5. AAs β†’ Succinyl-CoA, Acetyl-CoA, Acetoacetyl-CoA

β†’ Succinyl-CoA (Glucogenic)

AAPathway
Valine & Isoleucineβ†’ Propionyl-CoA β†’ Succinyl-CoA (requires biotin & B₁₂)
Threonine→ α-Ketobutyrate → Propionyl-CoA → Succinyl-CoA

β†’ Acetyl-CoA / Acetoacetyl-CoA (Ketogenic)

AAProductsCategory
LeucineAcetyl-CoA + AcetoacetateExclusively ketogenic
IsoleucineAcetyl-CoA + Propionyl-CoABoth glucogenic & ketogenic
LysineAcetoacetyl-CoAExclusively ketogenic (no transamination)
TryptophanAlanine + Acetoacetyl-CoABoth glucogenic & ketogenic
Phenylalanine/Tyrosinealso yield acetoacetateBoth

6. Branched-Chain Amino Acids (BCAAs) πŸ”Ί

BCAAs = Valine, Leucine, Isoleucine - catabolized in muscle (not liver)
StepEnzymeCofactors
TransaminationBranched-chain aminotransferaseVitamin B₆
Oxidative DecarboxylationBranched-chain α-keto acid dehydrogenaseTPP, lipoic acid, FAD, NAD⁺, CoA
DehydrogenationForms Ξ±,Ξ²-unsaturated acyl-CoA
End products:
  • Valine β†’ Succinyl-CoA (glucogenic)
  • Leucine β†’ Acetoacetate + Acetyl-CoA (ketogenic)
  • Isoleucine β†’ Acetyl-CoA + Succinyl-CoA (both)
⚠️ MSUD (Maple Syrup Urine Disease): deficiency in branched-chain Ξ±-keto acid dehydrogenase Isovaleryl-CoA dehydrogenase deficiency β†’ "sweaty feet" odor

7. Folic Acid & One-Carbon Metabolism

  • One-carbon units: formyl, methenyl, methylene, methyl - transferred via THF (Tetrahydrofolate)
  • THF = active form of folic acid, synthesized by dihydrofolate reductase (uses 2 NADPH)
  • THF transfers 1C units at N5, N10, or both
  • Key in DNA synthesis (purines, TMP)
  • Folate deficiency β†’ Megaloblastic anemia
  • Biotin = 1C carrier for COβ‚‚ (not part of the one-carbon pool)

8. Biosynthesis of Nonessential Amino Acids

Via Transamination from Ξ±-Keto Acids

AA synthesizedFrom
Alanine← Pyruvate
Aspartate← Oxaloacetate
Glutamate← Ξ±-Ketoglutarate (also via glutamate dehydrogenase)

Via Amidation (ATP-dependent)

AA synthesizedReaction
Glutamine← Glutamate + NH₃ (glutamine synthetase); ammonia transport form
Asparagine← Aspartate + NH₃ (asparagine synthetase); uses glutamine as NH₃ donor

Special Pathways

AASource
ProlineFrom glutamate β†’ cyclization & reduction
SerineFrom 3-phosphoglycerate OR from glycine via serine hydroxymethyltransferase + N⁡,N¹⁰-methylene-THF
GlycineFrom serine (reverse of serine synthesis)
CysteineFrom methionine (via homocysteine) + serine; requires adequate dietary methionine
TyrosineFrom phenylalanine via phenylalanine hydroxylase; needs BHβ‚„ (regenerated by dihydropteridine reductase)

9. Inborn Errors of Amino Acid Metabolism

Phenylketonuria (PKU) ⭐ Most Clinically Relevant

FeatureDetail
Defect↓ Phenylalanine hydroxylase (or BHβ‚„/dihydropteridine reductase deficiency)
Prevalence1 in 15,000 births
Biochemistry↑ Phenylalanine, ↓ Tyrosine β†’ toxic: phenylpyruvate, phenyllactate, phenylacetate
CNS effectsIntellectual disability, seizures, microcephaly, failure to thrive
Urine odorMusty/mousey (phenylketones)
Other signsHypopigmentation (light skin, hair, eyes - ↓ melanin)
ScreeningUniversal newborn heel prick after 24-48 h of protein feeding
TreatmentLow-phenylalanine diet (start within 7-10 days), supplement tyrosine, avoid aspartame
Maternal PKUHigh maternal Phe = teratogenic β†’ fetal microcephaly, heart defects

Maple Syrup Urine Disease (MSUD)

FeatureDetail
Defect↓ Branched-chain Ξ±-keto acid dehydrogenase
AAs affectedLeucine, Isoleucine, Valine
SymptomsPoor feeding, vomiting, acidosis, maple syrup-smelling urine
Classic formSevere, neonatal onset, lethal if untreated
TreatmentLifelong restricted BCAA diet; thiamine in responsive cases

Albinism

FeatureDetail
Defect↓ Tyrosinase (blocks tyrosine β†’ melanin)
FeaturesHypopigmentation (skin, hair, eyes), photophobia, ↓ visual acuity
Risk↑ Skin cancer
TypesAutosomal recessive (most common); dominant/X-linked forms exist
Most severeTyrosinase-negative oculocutaneous albinism

Homocystinuria

FeatureDetail
Defect↓ Cystathionine Ξ²-synthase (CBS)
Biochemistry↑ Homocysteine & methionine, ↓ Cysteine
ClinicalEctopia lentis, skeletal abnormalities, thrombi, osteoporosis
B6-responsive formMilder symptoms
Treatment↓ Methionine diet, supplement B6, B12, folate

Alkaptonuria

FeatureDetail
Defect↓ Homogentisic acid oxidase (in tyrosine catabolism)
TriadDark urine (homogentisic aciduria) + Ochronosis (black connective tissue) + Arthritis (large joints)
Early signDiaper staining in infancy
OnsetUsually asymptomatic until ~40 years
Treatment↓ Phenylalanine & tyrosine intake; manage arthritis

10. Quick-Reference Summary Tables

Vitamin Cofactors in AA Metabolism

VitaminRole
B6 (PLP)Transamination (all AAs); BCAA catabolism; transsulfuration
B12Remethylation of homocysteine; propionyl-CoA β†’ succinyl-CoA
Folate (THF)One-carbon transfers; remethylation of homocysteine
BiotinCOβ‚‚ carboxylation (propionyl-CoA β†’ methylmalonyl-CoA)
BHβ‚„Phenylalanine hydroxylase coenzyme

Disease Summary - "Which AA, Which Enzyme"

DiseaseDeficient EnzymeKey AAUrine Finding
PKUPhenylalanine hydroxylasePhe ↑Phenylketones (musty odor)
MSUDBCAA Ξ±-keto acid dehydrogenaseLeu, Ile, Val ↑Maple syrup odor
HomocystinuriaCystathionine Ξ²-synthaseHomocysteine ↑↑ Homocysteine
AlkaptonuriaHomogentisic acid oxidaseTyr catabolite ↑Darkens on standing
AlbinismTyrosinaseTyr blocked-
Primary Oxaluria IAlanine-glyoxylate aminotransferaseGly β†’ Oxalate↑ Oxalate (kidney stones)

11. High-Yield Exam Mnemonics

  • "Leu & Lys = Pure Keto" - only two exclusively ketogenic AAs
  • "BCAAs in Muscle" - Valine, Leucine, Isoleucine catabolized in muscle, not liver
  • "SAM I AM" - SAM = S-Adenosylmethionine = major methyl donor
  • "Folate + B12 = Recycle Homocysteine" - deficiency β†’ hyperhomocysteinemia
  • "Musty Mouse = PKU" - urine/body odor of phenylketones
  • "Maple Syrup = MSUD" - sweet urine odor from BCAA accumulation
  • "Alkaptonuria Triad" - Dark urine + Ochronosis + Arthritis
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