Amino Acids
| Definition | Organic compounds with amino and carboxyl groups |
|---|---|
| Standard AAs | 20 |
| Essential | 9 |
| Non-essential | 11 |
| General Formula | H₂N-CHR-COOH |
Amino acids are organic compounds containing both an amino group (-NH₂) and a carboxyl group (-COOH), along with a side chain (R group) specific to each amino acid. They serve as the building blocks of peptides and proteins and play numerous critical roles in metabolism, cell signaling, and biosynthesis.
Chemical Structure
General Structure
All standard amino acids share a common structural framework:
- Central carbon (α-carbon): The chiral center (except glycine)
- Amino group (-NH₂): Basic, accepts protons
- Carboxyl group (-COOH): Acidic, donates protons
- Hydrogen atom: Attached to α-carbon
- R group (side chain): Determines amino acid identity and properties
Stereochemistry
With the exception of glycine, all amino acids are chiral and exist as L- and D- enantiomers. Biological systems almost exclusively use L-amino acids for protein synthesis.
Classification
Essential Amino Acids
Nine amino acids cannot be synthesized by the human body and must be obtained through diet:
| Amino Acid | Abbreviation | Key Functions |
|---|---|---|
| Histidine | His (H) | Histamine precursor, pH buffering |
| Isoleucine | Ile (I) | BCAA, muscle metabolism |
| Leucine | Leu (L) | BCAA, mTOR activation, protein synthesis |
| Lysine | Lys (K) | Collagen synthesis, calcium absorption |
| Methionine | Met (M) | Methylation, cysteine precursor |
| Phenylalanine | Phe (F) | Tyrosine precursor, neurotransmitters |
| Threonine | Thr (T) | Collagen, elastin, immune function |
| Tryptophan | Trp (W) | Serotonin precursor, melatonin |
| Valine | Val (V) | BCAA, muscle metabolism |
Non-Essential Amino Acids
These can be synthesized by the body:
| Amino Acid | Abbreviation | Key Functions |
|---|---|---|
| Alanine | Ala (A) | Glucose-alanine cycle, energy |
| Arginine | Arg (R) | Nitric oxide precursor, urea cycle* |
| Asparagine | Asn (N) | Protein glycosylation |
| Aspartic Acid | Asp (D) | Urea cycle, neurotransmitter |
| Cysteine | Cys (C) | Disulfide bonds, glutathione* |
| Glutamic Acid | Glu (E) | Neurotransmitter, GABA precursor |
| Glutamine | Gln (Q) | Most abundant AA, gut/immune function* |
| Glycine | Gly (G) | Collagen, neurotransmitter, bile acids* |
| Proline | Pro (P) | Collagen structure |
| Serine | Ser (S) | Phosphorylation, cysteine precursor |
| Tyrosine | Tyr (Y) | Catecholamine precursor* |
* Conditionally essential under certain circumstances
Branched-Chain Amino Acids (BCAAs)
Three essential amino acids with branched aliphatic side chains:
- Leucine – Most potent for muscle protein synthesis
- Isoleucine – Glucose uptake into muscle
- Valine – Energy during exercise
BCAAs are unique because they are primarily metabolized in muscle tissue rather than the liver. They comprise approximately 35% of essential amino acids in muscle proteins.
Classification by Side Chain
Non-polar (Hydrophobic)
- Glycine, Alanine, Valine, Leucine, Isoleucine
- Proline, Methionine, Phenylalanine, Tryptophan
Polar Uncharged
- Serine, Threonine, Cysteine, Tyrosine
- Asparagine, Glutamine
Positively Charged (Basic)
- Lysine, Arginine, Histidine
Negatively Charged (Acidic)
- Aspartic acid, Glutamic acid
Biological Functions
Protein Synthesis
The primary role of amino acids is serving as building blocks for proteins:
- Linked by peptide bonds during translation
- Sequence determined by mRNA codons
- Fold into specific 3D structures
Metabolic Functions
- Energy production: Can be converted to glucose or ketone bodies
- Nitrogen balance: Source of nitrogen for biosynthesis
- Gluconeogenesis: Some AAs can form glucose
- Ketogenesis: Some can form ketone bodies
Biosynthetic Precursors
| Amino Acid | Derived Products |
|---|---|
| Tryptophan | Serotonin, melatonin, niacin |
| Tyrosine | Dopamine, norepinephrine, epinephrine, thyroid hormones |
| Histidine | Histamine |
| Glutamate | GABA |
| Arginine | Nitric oxide, creatine |
| Glycine | Heme, purines, creatine |
| Methionine | SAMe (methylation), cysteine |
Nutritional Aspects
Protein Quality
Protein quality is determined by amino acid composition:
- Complete proteins: Contain all essential AAs (animal sources, soy, quinoa)
- Incomplete proteins: Low in one or more essential AAs (most plant sources)
- Complementary proteins: Combining incomplete proteins to get all essentials (beans + rice)
Daily Requirements
WHO/FAO recommendations for essential amino acids (mg/kg body weight/day for adults):
| Amino Acid | Requirement |
|---|---|
| Leucine | 39 mg/kg |
| Isoleucine | 20 mg/kg |
| Valine | 26 mg/kg |
| Lysine | 30 mg/kg |
| Methionine + Cysteine | 15 mg/kg |
| Phenylalanine + Tyrosine | 25 mg/kg |
| Threonine | 15 mg/kg |
| Tryptophan | 4 mg/kg |
| Histidine | 10 mg/kg |
Amino Acid Supplements
Common Forms
- BCAA supplements: Leucine, isoleucine, valine (often 2:1:1 ratio)
- EAA supplements: All nine essential amino acids
- Glutamine: Gut health, immune support
- Arginine/Citrulline: Nitric oxide support
- Glycine: Sleep, collagen support
Amino Acid Disorders
Genetic conditions affecting amino acid metabolism:
- Phenylketonuria (PKU): Cannot metabolize phenylalanine
- Maple syrup urine disease: BCAA metabolism defect
- Homocystinuria: Methionine metabolism disorder
- Tyrosinemia: Tyrosine metabolism defect
Related Topics
- Peptides – Chains of amino acids
- Proteins – Large polypeptides
- Essential Amino Acids – Detailed coverage
- BCAAs – Branched-chain amino acids
References
- Nelson DL, Cox MM. "Lehninger Principles of Biochemistry." 8th ed. W.H. Freeman; 2021.
- WHO/FAO/UNU. "Protein and Amino Acid Requirements in Human Nutrition." WHO Technical Report Series 935. 2007.
- Wu G. "Amino acids: metabolism, functions, and nutrition." Amino Acids. 2009;37(1):1-17.