Amino acids are the fundamental units of oligopeptide and polypeptide drugs. There are 20 well – known natural amino acids that form the basic structure of proteins. They can be classified into four major groups: non – polar (hydrophobic) amino acids, polar uncharged (hydrophilic) amino acids, basic (polar positively charged) amino acids, and acidic (polar negatively charged) amino acids.
Classification of Single Amino Acids
Let’s summarize the structure, classification, and major physiological roles in the body of common natural amino acids that make up natural proteins. Here, the polarity, hydrophobicity, or hydrophilicity refers to the side – chain R group, not the entire amino – acid molecule. If the R group consists only of H or the elements C and H, the amino acid is non – polar and hydrophobic. If it contains polar side – chains such as – OH, – SH, – COOH, or – NH₂, the amino acid is polar and hydrophilic.
- Non – polar amino acids (9): Glycine, Alanine, Phenylalanine, Leucine, Isoleucine, Proline, Valine, Tryptophan, Methionine.
- Polar uncharged amino acids (6): Serine, Tyrosine, Threonine, Cysteine, Glutamine, Asparagine.
- Basic amino acids (3): Histidine, Lysine, Arginine.
- Acidic amino acids (2): Aspartic Acid, Glutamic Acid.
In recent years, the 21st and 22nd basic amino acids, selenocysteine and pyrrolysine, have been discovered in some proteins. The 22nd amino acid is only found in archaea and bacteria. In total, there are 21 basic natural amino acids that make up human proteins. Except for glycine, these basic amino acids are all L – configuration α – amino acids, which play crucial roles in maintaining life activities and regulating biological functions, such as participating in protein formation, cell signal transduction, membrane transport, enzyme catalysis, DNA synthesis or repair, and the removal of toxins in the body.
There are 8 amino acids that the human body cannot produce on its own, known as essential amino acids, which need to be obtained from food: Valine, Isoleucine, Leucine, Phenylalanine, Methionine, Tryptophan, Threonine, and Lysine. There are also 2 amino acids, Arginine and Histidine, that the human body cannot synthesize in sufficient amounts, especially for infants, so they are called semi – essential amino acids and need to be partly obtained from food.
Now, let’s take a closer look at these 22 natural amino acids and their main functions in organisms.
- Glycine (Glycine, G or Gly), α – aminoacetic acid
It is one of the non – polar α – amino acids (some also consider it a polar uncharged amino acid, but from a biological R – group – based definition, it is non – polar). It is a non – essential amino acid. It is highly hydrophilic, easily soluble in water, and hardly soluble in ethanol and ether. It is a component of the endogenous antioxidant glutathione. During severe stress, it is often supplemented externally, and is sometimes referred to as a semi – essential amino acid. Besides participating in protein formation, it also plays a role in nerve signal transmission and toxin removal in the body. - Alanine (Alanine, A or Ala), α – aminopropionic acid
It is a non – polar α – amino acid and a non – essential amino acid. It is soluble in water but insoluble in ethanol, ether, and acetone. It is a hydrophobic amino acid with optical activity. In the organism, it mainly participates in protein formation and cellular energy metabolism (metabolism of sugars and acids). - Valine (Valine, V or Val), β – methyl – α – aminobutyric acid
It is a non – polar α – amino acid, one of the 8 essential amino acids, and one of the three branched – chain amino acids (valine, leucine, isoleucine). It is soluble in water but insoluble in ethanol. It is a hydrophobic amino acid and a glucogenic amino acid. Together with leucine (a ketogenic amino acid) and isoleucine (a glucogenic and ketogenic amino acid), it is related to the tricarboxylic acid cycle, enabling the conversion of the three major nutrients (proteins, sugars, and fats) in the body, promoting body growth, tissue repair, blood – sugar regulation, and energy provision. It can also remove nitrogen and toxins from the body. - Leucine (Leucine, L or Leu), γ – methyl – α – aminovaleric acid
It is a non – polar α – amino acid, one of the 8 essential amino acids, and one of the three branched – chain amino acids. It is easily soluble in formic acid, slightly soluble in water, and insoluble in ethanol. It is a hydrophobic amino acid. It participates in protein synthesis in the body, regulates blood sugar, promotes insulin secretion, and maintains blood – sugar and nitrogen balance. - Isoleucine (Isoleucine, I or Ile), β – methyl – α – aminovaleric acid
It is a non – polar α – amino acid, one of the 8 essential amino acids, and one of the three branched – chain amino acids. It is slightly soluble in water and insoluble in ethanol. It is a hydrophobic amino acid. It participates in protein synthesis, regulates blood sugar, provides energy, maintains nitrogen balance, and promotes tissue growth. - Phenylalanine (Phenylalanine, F or Phe), β – phenyl – α – aminopropionic acid
It is a non – polar α – amino acid, one of the 8 essential amino acids, and an aromatic amino acid. It is soluble in water but insoluble in ethanol and ether. It is a hydrophobic amino acid. In the body, it is converted into tyrosine by phenylalanine hydroxylase and participates in the synthesis of neurotransmitters and hormones, as well as the body’s sugar and fat metabolism. - Proline (Proline, P or Pro), α – carboxytetrahydropyrrole
It is a non – polar α – cyclic imino acid and a non – essential amino acid. It is soluble in hot water and ethanol. It is a hydrophobic amino acid. In the body, it participates in collagen synthesis, tissue repair, and enzyme catalysis. - Tryptophan (Tryptophan, W or Trp), α – amino – β – (3 – indolyl) propionic acid
It is a non – polar α – amino acid, one of the 8 essential amino acids, and an aromatic amino acid. It is easily soluble in formic acid, slightly soluble in water, and insoluble in ethanol. It is a hydrophobic amino acid. In the body, it participates in plasma – protein renewal and the production of neurotransmitters. - Methionine (Methionine, M or Met), α – amino – γ – methylthiobutyric acid
It is a non – polar α – amino acid and one of the 8 essential amino acids. It is soluble in water but difficult to dissolve in ethanol and ether. It is a hydrophobic amino acid. In the body, it participates in protein synthesis, converts to cysteine, provides sulfur and methyl groups, and maintains the body’s growth, development, and nitrogen balance. - Serine (Serine, S or Ser), α – amino – β – hydroxypropionic acid
It is a polar α – amino acid (uncharged) and a non – essential amino acid. It is easily soluble in water but insoluble in ethanol, ether, and acetone. It is a hydrophilic amino acid. In the body, it participates in protein synthesis and nervous – system signal transmission. - Threonine (Threonine, T or Thr), α – amino – β – hydroxybutyric acid
It is a polar α – amino acid (uncharged) and one of the 8 essential amino acids. It is soluble in water but insoluble in ethanol. It is a hydrophilic amino acid and a glucogenic and ketogenic amino acid. In the body, it participates in protein phosphorylation, promotes growth and development, and has water – retention and moisturizing effects. - Tyrosine (Tyrosine, Y or Tyr), α – amino – β – (p – hydroxyphenyl) propionic acid
It is a polar α – amino acid (uncharged) and a non – essential amino acid (conditionally essential; non – essential when phenylalanine is sufficient). It is soluble in dilute hydrochloric acid, very slightly soluble in water, and insoluble in ethanol. It is a glucogenic and ketogenic amino acid. In the organism, it participates in protein formation and neuronal transmission. - Cysteine (Cysteine, C or Cys), α – amino – β – mercaptopropionic acid
It is a polar α – amino acid (uncharged) and a non – essential amino acid (conditionally essential; can be converted from the essential amino acid methionine in the body). It is soluble in water but insoluble in ether and acetone. It is a hydrophilic amino acid and a glucogenic amino acid. In the organism, it participates in protein formation, detoxifies a wide range of poisons (such as formaldehyde, acetaldehyde, and alcohol), and acts as an antioxidant to scavenge free radicals. - Asparagine (Asparagine, N or Asn), α – aminobutyramide acid
It is a polar α – amino acid (uncharged) and a non – essential amino acid. It is soluble in water but insoluble in ethanol and ether. It participates in the synthesis of glycoproteins. - Glutamine (Glutamine, Q or Gln), α – aminovaleramide acid
It is a polar α – amino acid (uncharged) and a non – essential amino acid. It is soluble in water but insoluble in ethanol and ether. It participates in protein synthesis and DNA/RNA synthesis. - Aspartic Acid (Aspartic Acid, D or Asp), α – aminobutanedioic acid
It is one of the 2 acidic amino acids and a non – essential amino acid. It is slightly soluble in water (soluble in boiling water) and difficult to dissolve in ethanol and ether. In the organism, it participates in protein formation, the ornithine cycle, and hormone regulation. - Glutamic Acid (Glutamic Acid, E or Glu), α – aminopentanedioic acid
It is one of the 2 acidic α – amino acids and a non – essential amino acid. It is easily soluble in dilute hydrochloric acid or 1 mol/L sodium hydroxide solution, soluble in hot water, slightly soluble in water, and insoluble in ethanol and ether. In the organism, it participates in protein metabolism and is an excitatory neurotransmitter in the brain. - Histidine (Histidine, H or His), α – amino – β – (4 – imidazolyl) propionic acid
It is a basic α – amino acid and one of the 2 semi – essential amino acids (especially for infants). It is soluble in water, slightly soluble in ethanol, and insoluble in ether. In the organism, it participates in the formation of metalloproteins and enzyme catalysis. - Lysine (Lysine, L or Lys), α, ε – diaminohexanoic acid
It is a basic α – amino acid and one of the 8 essential amino acids. It is easily soluble in water, slightly soluble in ethanol, and insoluble in ether. In the organism, it participates in protein formation, regulates metabolic balance, and enhances immune function. - Arginine (Arginine, R or Arg), α – amino – δ – guanidinovaleric acid
It is a basic α – amino acid and one of the 2 semi – essential amino acids. It is easily soluble in water, slightly soluble in ethanol, and insoluble in ether. It participates in protein formation and the ornithine cycle in the organism. - Selenocysteine (Selenocysteine, U or Sec), α – amino – β – selenohydroxypropionic acid
It is the last – discovered basic natural amino acid in the human body. It is the main form of selenium in proteins and the only amino acid containing a metalloid element. It participates in the formation of selenoproteins in the organism and is the active center of 25 known selenium – containing enzymes (such as glutathione peroxidase), with antioxidant activity. - Pyrrolysine (Pyrrolysine)
It is a basic α – amino acid and the last – discovered basic natural amino acid, found in some methanogenic archaea and bacteria. It participates in the catalysis of methyltransferases.
In conclusion, single amino acids are not only the building blocks of peptides and proteins but also play diverse and crucial roles in various physiological processes of organisms. Understanding their properties and functions is of great significance for fields such as biochemistry, nutrition, and medicine.