20 Standard Amino Acids
Overview of the proteinogenic amino acids and their key properties
Glycine
Smallest, no chiral center
Alanine
Simple methyl group
Valine
Branched chain
Leucine
Branched chain
Isoleucine
Branched chain
Proline
Creates kinks in proteins
Phenylalanine
Aromatic ring
Tryptophan
Largest, aromatic
Methionine
Contains sulfur
Serine
Hydroxyl group
Threonine
Hydroxyl group
Cysteine
Forms disulfide bonds
Tyrosine
Aromatic with OH
Asparagine
Amide group
Glutamine
Amide group
Aspartic Acid
Carboxyl group (acidic)
Glutamic Acid
Carboxyl group (acidic)
Lysine
Amino group (basic)
Arginine
Guanidinium group (basic)
Histidine
Imidazole ring (basic)
Polarity Classification
Nutritional Classification
Essential (9)
Must be obtained from diet: His, Ile, Leu, Lys, Met, Phe, Thr, Trp, Val
Conditional (3)
Essential under certain conditions: Arg, Gln, Tyr
Non-Essential (8)
Synthesized in the body from metabolic intermediates
Metabolic Fate: Glucogenic vs Ketogenic
When amino acids are catabolized, their carbon skeletons enter metabolism at different points, determining whether they can be converted to glucose or ketone bodies.
Glucogenic (14 AAs)
Can be converted to glucose via gluconeogenesis
Ala, Arg, Asn, Asp
Cys, Gln, Glu, Gly
His, Met, Pro, Ser
Thr, Val
Yield: Pyruvate or Oxaloacetate
Both (4 AAs)
Can form both glucose and ketones
Ile, Phe
Trp, Tyr
Enter at multiple metabolic points
Ketogenic (2 AAs)
Only form ketone bodies, cannot form glucose
Leu, Lys
Yield: Acetyl-CoA or Acetoacetyl-CoA
Entry Points into Energy Metabolism
After deamination or transamination, amino acid carbon skeletons enter central metabolism at various points.
| Metabolic Intermediate | Amino Acids | Pathway |
|---|---|---|
| Pyruvate | Ala, Cys, Gly, Ser, Thr, Trp | Glycolysis / Gluconeogenesis |
| Acetyl-CoA | Ile, Leu, Lys, Phe, Trp, Tyr | TCA Cycle / Ketogenesis |
| α-Ketoglutarate | Arg, Gln, Glu, His, Pro | TCA Cycle |
| Succinyl-CoA | Ile, Met, Val | TCA Cycle |
| Fumarate | Phe, Tyr | TCA Cycle |
| Oxaloacetate | Asn, Asp | TCA Cycle / Gluconeogenesis |
Special Metabolic Roles
Glutamate & Glutamine
Central to nitrogen metabolism
- Primary nitrogen donors for synthesis
- Ammonia assimilation via glutamate dehydrogenase
- Glutamine transports ammonia safely in blood
Methionine & SAM
Methylation reactions
- Methionine → SAM (S-adenosylmethionine)
- SAM is universal methyl donor
- Links to cysteine synthesis pathway
Branched-Chain AAs
Val, Ile, Leu (BCAAs)
- Metabolized primarily in muscle
- Important for protein synthesis signals
- Leucine is purely ketogenic
Aromatic AAs
Phe, Tyr, Trp
- Precursors to neurotransmitters
- Tyr from Phe (conditionally essential)
- Trp → Serotonin, Melatonin