Question

Propose a reason for why so many of the essential amino acids belong in the aliphatic and aromatic groups.

Answer 1

Classifications of Amino Acids

Experts classify amino acids based on a variety of features, including whether people can acquire them through diet. Accordingly, scientists recognize three amino acid types:
1. Nonessential
2. Essential
3. Conditionally essential

However, the classification as essential or nonessential does not actually reflect their importance as all 20 amino acids are necessary for human health.

Eight of these amino acids are essential (or indispensable) and cannot be produced by the body. They are:
• Leucine
• Isoleucine
• Lysine
• Threonine
• Methionine
• Phenylalanine
• Valine
• Tryptophan

Histidine is an amino acid that is categorized as semi-essential since the human body doesn't always need it to properly function therefore dietary sources of it are not always essential. Meanwhile, conditionally essential amino acids aren't usually required in the human diet, but do become essential under certain circumstances.

Finally, nonessential amino acids are produced by the human body either from essential amino acids or from normal protein breakdowns. Nonessential amino acids include:
• Asparagine
• Alanine
• Arginine
• Aspartic acid
• Cysteine
• Glutamic acid
• Glutamine
• Proline
• Glycine
• Tyrosine
• Serine

An additional amino acids' classification depends upon the side chain structure, and experts recognize these five as:
• Cysteine and Methionine (amino acids containing sulfur)
• Asparagine, Serine, Threonine, and Glutamine (neutral amino acids)
• Glutamic acid and Aspartic acid (acidic); and Arginine and Lysine (basic)
• Leucine, Isoleucine, Glycine, Valine, and Alanine (aliphatic amino acids)
• Phenylalanine, Tryptophan, Tyrosine and Histidine (aromatic amino acids)

One final amino acid classification is categorized by the side chain structure that divides the list of 20 amino acids into four groups - two of which are the main groups and two that are subgroups. They are:
1. Non-polar
2. Polar
3. Acidic and polar
4. Basic and polar

For example, side chains having pure hydrocarbon alkyl or aromatic groups are considered non-polar, and these amino acids are comprised of Phenylalanine, Glycine, Valine, Leucine, Alanine, Isoleucine, Proline, Methionine and Tryptophan. Meanwhile, if the side chain contains different polar groups like amides, acids and alcohols, they are classified as polar. It includes Tyrosine, Serine, Asparagine, Threonine, Glutamine, and Cysteine. If the side chain contains carboxylic acid, the amino acids in the acidic-polar classification are Aspartic Acid and Glutamic Acid. Furthermore, if the side chain consists of a carboxylic acid and basic-polar, these amino acids are Lysine, Arginine, and Histidine.

Properties of Amino Acids

The properties of α-amino acids are complex, yet simplistic in that every molecule of an amino acid involves two functional groups: carboxyl (-COOH) and amino (-NH2).

Each molecule can contain a side chain or R group, e.g. Alanine is an example of standard amino acid containing methyl side chain group. The R groups have a variety of shapes, sizes, charges, and reactivities. This allows amino acids to be grouped according to the chemical properties of their side chains.

Table of common amino acid abbreviations and properties

Name	Three letter code	One letter code	Molecular Weight	Molecular Formula	Residue Formula	Residue Weight (-H2O)	pKa	pKb	pKx	pl
Alanine	Ala	A	89.10	C3H7NO2	C3H5NO	71.08	2.34	9.69	–	6.00
Arginine	Arg	R	174.20	C6H14N4O2	C6H12N4O	156.19	2.17	9.04	12.48	10.76
Asparagine	Asn	N	132.12	C4H8N2O3	C4H6N2O2	114.11	2.02	8.80	–	5.41
Aspartic acid	Asp	D	133.11	C4H7NO4	C4H5NO3	115.09	1.88	9.60	3.65	2.77
Cysteine	Cys	C	121.16	C3H7NO2S	C3H5NOS	103.15	1.96	10.28	8.18	5.07
Glutamic acid	Glu	E	147.13	C5H9NO4	C5H7NO3	129.12	2.19	9.67	4.25	3.22
Glutamine	Gln	Q	146.15	C5H10N2O3	C5H8N2O2	128.13	2.17	9.13	–	5.65
Glycine	Gly	G	75.07	C2H5NO2	C2H3NO	57.05	2.34	9.60	–	5.97
Histidine	His	H	155.16	C6H9N3O2	C6H7N3O	137.14	1.82	9.17	6.00	7.59
Hydroxyproline	Hyp	O	131.13	C5H9NO3	C5H7NO2	113.11	1.82	9.65	–	–
Isoleucine	Ile	I	131.18	C6H13NO2	C6H11NO	113.16	2.36	9.60	–	6.02
Leucine	Leu	L	131.18	C6H13NO2	C6H11NO	113.16	2.36	9.60	–	5.98
Lysine	Lys	K	146.19	C6H14N2O2	C6H12N2O	128.18	2.18	8.95	10.53	9.74
Methionine	Met	M	149.21	C5H11NO2S	C5H9NOS	131.20	2.28	9.21	–	5.74
Phenylalanine	Phe	F	165.19	C9H11NO2	C9H9NO	147.18	1.83	9.13	–	5.48
Proline	Pro	P	115.13	C5H9NO2	C5H7NO	97.12	1.99	10.60	–	6.30
Pyroglutamatic	Glp	U	139.11	C5H7NO3	C5H5NO2	121.09	–	–	–	5.68
Serine	Ser	S	105.09	C3H7NO3	C3H5NO2	87.08	2.21	9.15	–	5.68
Threonine	Thr	T	119.12	C4H9NO3	C4H7NO2	101.11	2.09	9.10	–	5.60
Tryptophan	Trp	W	204.23	C11H12N2O2	C11H10N2O	186.22	2.83	9.39	–	5.89
Tyrosine	Tyr	Y	181.19	C9H11NO3	C9H9NO2	163.18	2.20	9.11	10.07	5.66
Valine	Val	V	117.15	C5H11NO2	C5H9NO	99.13	2.32	9.62	–	5.96

Amino acids are crystalline solids which usually are water soluble and only sparingly dissoluble in organic solvents. Their solubility depends on the size and nature of the side chain. Amino acids have very high melting points, up to 200-300°C. Their other properties varying for each particular amino acid.

20 Amino Acids and their Functions

Only 20 amino acids are found in the human peptides and proteins. These naturally occurring amino acids are used by cells to synthesize peptides and proteins. They are typically identified by generic formula: H₂NCHRCOOH.

The primary difference between the 20 amino acids is a different structure of R group. Below the essential amino acids and their respective functions are shown.

Non-polar, aliphatic residues

	Glycine (G/Gly). Slices DNA and produces different amino acids. One of the three most important glycogenic amino acids. Read more about Glycine.
	Alanine (A/Ala). Important source of energy for muscle. One of the three most important glycogenic amino acids. The primary amino acid in sugar metabolism. Boosts immune system by producing antibodies. Read more about Alanine.
	Valine (V/Val). Essential for muscle development. Read more about Valine.
	Leucine (L/Leu). Beneficial for skin, bone and tissue wound healing. Read more about Leucine.
	Isoleucine (I/Ile). Necessary for the synthesis of hemoglobin. Read more about Isoleucine.
	Proline (P/Pro). Critical component of cartilage, aids in joint health, tendons and ligaments. Keeps heart muscle strong. Read more about Proline.

Aromatic residues

	Phenylalanine (F/Phe). Beneficial for healthy nervous system. It boosts memory and learning. Read more about Phenylalanine.
	Tyrosine (Y/Tyr). Precursor of dopamine, norepinephrine and adrenaline. Increases energy, improves mental clarity and concentration, can treat some depressions. Read more about Tyrosine.
	Tryptophan (W/Trp).Necessary for a synthesis of neurotransmitter serotonin. Effective sleep aid, due to conversion to serotonin. Reduces anxiety and some forms of depression. Treats migraine and headaches. Stimulates growth hormone Read more about Tryptophan.

Polar, non-charged residues

	Serine (S/Ser). One of the three most important glycogenic amino acids, the others being alanine and glycine. Maintains blood sugar levels, and boosts immune system. Myelin sheaths contain serine. Read more about Serine.
	Threonine (T/Thr). Required for formation of collagen. Helps prevent fatty deposits in liver. Aids in antibodies' production. Read more about Threonine.
	Cysteine (C/Cys). Protective against radiation, pollution and ultra-violet light. Detoxifier, necessary for growth and repair of skin. Read more about Cysteine.
	Methionine (M/Met). An antioxidant. Helps in breakdown of fats and aids in reducing muscle degeneration. Read more about Methionine.
	Asparagine (N/Asn).One of the two main excitatory neurotransmitters. Read more about Asparagine.
	Glutamine (Q/Gln). Essential for helping to maintain normal and steady blood sugar levels. Helps muscle strength and endurance. Gastrointestinal function, provides energy to small intestines. Read more about Glutamine.

Positively charged residues

	Lysine (K/Lys). Component of muscle protein, needed in the synthesis of enzymes and hormones. It is also a precursor for L-carathine, which is essential for healthy nervous system function. Read more about Lysine.
	Arginine (R/Arg). One of the two main excitatory neurotransmitters. May increase endurance and decrease fatigue. Detoxifies harmful chemicals. Involved in DNA synthesis. Read more about Arginine.
	Histidine (H/His). Found in high concentrations in hemoglobin. Treats anemia, has been used to treat rheumatoid arthritis. Read more about Histidine.

Negatively charged residues

	Aspartate (D/Asp). Increases stamina and helps protect the liver; DNA and RNA metabolism, immune system function. Read more about Aspartate.
	Glutamate (E/Glu). Neurotransmitter that is involved in DNA synthesis. Read more about Glutamate.