20 Standard Amino Acids
Sorted by essentiality, then alphabetically within each group. Structures show the full molecule — backbone plus each unique R group. Arginine appears at the junction: it is essential for most animals but non-essential for healthy adult humans.
Essential Amino Acids — 9 (+1 conditional)
Cannot be synthesized by the body · must come from dietary protein
Histidine
Imidazole ring (basic)
Isoleucine
Branched chain
Leucine
Branched chain
Lysine
Amino group (basic)
Methionine
Contains sulfur
Phenylalanine
Aromatic ring
Threonine
Hydroxyl group
Tryptophan
Largest, aromatic
Valine
Branched chain
Arginine
Guanidinium group (basic)
Non-essential Amino Acids — 10 (+1 conditional)
Synthesized endogenously · not required in diet for healthy adults
Arginine
Guanidinium group (basic)
Alanine
Simple methyl group
Asparagine
Amide group
Aspartic Acid
Carboxyl group (acidic)
Cysteine
Forms disulfide bonds
Glutamic Acid
Carboxyl group (acidic)
Glutamine
Amide group
Glycine
Smallest, no chiral center
Proline
Creates kinks in proteins
Serine
Hydroxyl group
Tyrosine
Aromatic with OH
Polarity Classification
Nutritional Classification
Essential (9)
Must be obtained from diet: His, Ile, Leu, Lys, Met, Phe, Thr, Trp, Val
Conditional — humans vs. animals (1)
Arginine: essential for most animals; non-essential for healthy adult humans
Non-Essential (10)
Synthesized in the body (Ala, Asn, Asp, Cys, Gln, Glu, Gly, Pro, Ser, Tyr)
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 synthesized from Phe (conditionally essential)
- Trp → Serotonin, Melatonin