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Introduction

Amino acids serve as the fundamental molecular units that construct all proteins in living organisms. These 20 standard amino acids combine in various sequences to form the diverse proteins essential for biological functions. This article examines all 20 proteinogenic amino acids, their classifications, biochemical properties, and roles in human physiology.

1. Classification of Amino Acids

(1) By Polarity

• Nonpolar (Hydrophobic): Alanine, Valine, Leucine
• Polar Uncharged: Serine, Threonine, Asparagine
• Positively Charged: Lysine, Arginine, Histidine
• Negatively Charged: Aspartate, Glutamate

(2) By Essentiality

Essential: Must be obtained from diet (9 amino acids)

Nonessential: Can be synthesized by the body (11 amino acids)

2. The Complete List of 20 Amino Acids

Table 1: Standard Amino Acids and Their Properties

Amino Acid	3-Letter Code	1-Letter Code	Classification
Alanine	Ala	A	Nonpolar
Arginine	Arg	R	Positive
Asparagine	Asn	N	Polar
Aspartic acid	Asp	D	Negative
Cysteine	Cys	C	Polar
Glutamine	Gln	Q	Polar
Glutamic acid	Glu	E	Negative
Glycine	Gly	G	Nonpolar
Histidine	His	H	Positive
Isoleucine	Ile	I	Essential
Leucine	Leu	L	Essential
Lysine	Lys	K	Essential
Methionine	Met	M	Essential
Phenylalanine	Phe	F	Essential
Proline	Pro	P	Nonpolar
Serine	Ser	S	Polar
Threonine	Thr	T	Essential
Tryptophan	Trp	W	Essential
Tyrosine	Tyr	Y	Polar
Valine	Val	V	Essential

3. Biochemical Characteristics

(1) Structural Features

All contain:

Amino group (-NH₂)

Carboxyl group (-COOH)

Side chain (R-group) determining properties

(2) Unique Amino Acids

Cysteine: Forms disulfide bonds

Proline: Creates kinks in protein chains

Glycine: Simplest structure

4. Essential vs. Nonessential Amino Acids

Table 2: Dietary Requirements of Amino Acids

Category	Amino Acids	Dietary Sources
Essential	Histidine, Isoleucine, Leucine	Meat, eggs, dairy
	Lysine, Methionine, Phenylalanine	Soy, quinoa, fish
	Threonine, Tryptophan, Valine	Nuts, seeds, beans
Conditionally Essential	Arginine, Cysteine, Glutamine	Produced during stress
Nonessential	Alanine, Asparagine, Aspartate	Synthesized internally

5. Biological Importance

(1) Protein Synthesis

Ribosomes link amino acids via peptide bonds

Sequence determines protein structure/function

(2) Specialized Functions

Neurotransmitters: Glutamate, Glycine

Precursors: Tyrosine (melanin/dopamine)

Detoxification: Cysteine (glutathione)

6. Health Considerations

(1) Deficiency Risks

Muscle wasting

Impaired immunity

Metabolic disorders

(2) Supplementation

BCAA (Branched-chain amino acids) for athletes

Complete proteins for vegetarians

Conclusion

The 20 standard amino acids represent nature’s universal toolkit for building proteins. Their chemical diversity enables the vast array of protein structures and functions necessary for life. Understanding these molecular building blocks provides insight into nutrition, biochemistry, and therapeutic development. A balanced diet containing all essential amino acids remains crucial for maintaining optimal health.