Amino Acids: Function, Structure, Types

Amino acids are one of the four main macromolecules of life, the others being carbohydrates, lipids and nucleic acids. They serve primarily as the monomeric units of proteins. The 20 naturally occurring amino acids are found in all living things, from bacteria to humans.

Since amino acids make proteins and proteins account for most of your body mass, these acids are literally the stuff from which people (and other animals) are made.

Deficits in one or more amino acids can lead to incomplete or poorly constructed tissues and are also believed to play a role in the genesis of some cancers.

The human body is capable of synthesizing 10 of these acids, but the other 10 must be obtained from dietary sources and are therefore called essential amino acids. These are sometimes offered as essential amino acid supplements.

Amino acids that the body can manufacture are called nonessential amino acids, a somewhat misleading term since the body does in fact require them.

Each amino acid has both a capital one-letter abbreviation and a three-letter abbreviation (e.g., tyrosine goes by both "tyr" and "Y"). Sometimes, amino acids are modified after they have already become incorporated into proteins (an example is hydroxylation of proline).

Amino acids have become popular in dietary supplements among people interested in overall health and those hoping to build muscle mass through a combination of weight training and nutritional interventions.

• The first amino acid to be identified was asparagine, isolated from asparagus juice in 1806.   

The universal structure of all amino acids is a central carbon atom that has a carboxyl group, an amino group, a hydrogen atom and an "R" side chain that varies from amino acid to amino acid bound to it.

The carboxyl group consists of a carbon atom double-bonded to an oxygen atom and also bonded to a hydroxyl (-OH) group. It can be represented as -CO(OH) and it's this that earns these compounds the designation "acid," as the hydrogen atom in the hydroxyl component is readily donated, leaving behind a -CO(O-) group.

The 20 amino acids found in nature are called alpha-amino acids because the amino (-NH2) group is attached to the alpha carbon of the carboxylic acid, which is the carbon next to the -CO(OH) group. This carbon is also the "central" carbon described above.

Amino acids vary in mass from 75 grams per mole (glycine) to 204 grams per mole (tryptophan), and on average are smaller than the sugar glucose (180 grams per mole).

If every amino acid were observed with equal frequency in nature, each one would account for about 5 percent of the amino acids in protein structures (100 percent divided by 20 amino acids = 5 percent per amino acid).

In reality, these frequencies of occurrence vary from a little over 1.2 percent (tryptophan and cysteine) to just under 10 percent (leucine).

The "R" side chains, or simply R-chains, fall into various subcategories that both describe and determine the biochemical behavior of the amino acid as a whole. One common scheme categorizes amino acids as hydrophobic, hydrophilic (or polar), charged or amphipathic.

Hydrophobic comes from the Greek for "water-fearing," and these eight amino acids are so labeled because their side chains are nonpolar, meaning that they do not carry either a net electrostatic charge or an asymmetrically distributed charge. As a result of this property, hydrophobic amino acids are typically found on the interior of proteins, "safe" from water.

Similarly, these acids' hydrophilic peers tend to assemble on the exterior surfaces of proteins. Charged and amphipathic molecules, meanwhile, display their own charms and peculiarities.

Following is a list of individual amino acids along with some of their distinguishing features. They are presented in order of their one-letter abbreviations for ease of reference, but if you elect to try to memorize the names of the amino acids, you should use whatever grouping scheme or other trickery that makes this task as easy as possible.

These eight amino acids are generally grouped together and are sometimes called "nonpolar" instead of hydrophobic although they essentially mean the same thing. They participate on the interior of proteins in _van der Waals interactions_, which are like covalent or ionic bonds but much weaker and more transient.

• Alanine (ala or A): The second-lightest as well as the second-most-abundant amino acid.
• Glycine (gly or G): Does not actually have a full side chain (the side chain for glycine is a single hydrogen) and is therefore placed with other nonpolar compounds by default, but is often found near the surface of proteins and may plausibly be labeled "hydrophilic" for this reason.
• Phenylalanine (phe or F): Like tyrosine and tryptophan, this is an aromatic amino acid, which has nothing to do with its smell (amino acids are odorless) and instead indicates the presence of a phenyl group (a six-carbon ring containing three double bonds).
• Isoleucine (ile or I): An isomer of leucine with a single methyl (-CH3) group attached to a different carbon on the R-chain. (Isomers have the same number and type of atoms, but different spatial arrangements.)
• Leucine (leu or L): As with its isomer, leucine is a branched-chain amino acid (BCAA), a reference to the construction of the R-chain. Because most animals cannot synthesize BCAAs, these are two of the essential amino acids.
• Methionine (met or M): One of two sulfur-containing amino acids, the other being cysteine. Sometimes classified as amphipathic or even polar depending on its surroundings.
• Proline (pro or P): The amino group of proline exists in a five-atom ring instead of as a terminal -NH2 group.
• Valine (val or V): Another BCAA; equivalent to a leucine molecule with a backbone methyl group excised. 

Tryptophan is sometimes included with this group, but it is actually amphipathic.

These amino acids are often called "polar, but uncharged." They pepper the exterior surfaces of proteins and do not recoil in the presence of water.

• Cysteine (cys or C): Contains a sulfur atom; accounts for only 1.2 percent of amino acids in nature.
• Histidine (his or H): Histidine contains not one but two -NH2 groups, making it a very versatile amino acid thanks to its ability to take on- or offload protons (i.e., hydrogen atoms) at multiple locations. In some sources, histidine is listed primarily as amphipathic. 
• Asparagine (asn or N): Chemically, this is aspartic acid with an amino group replacing the acidic hydrogen of the carboxyl group.
• Glutamine (gln or Q): Identical to glutamtic acid with an amino group replacing the acidic hydrogen of the carboxyl group.  
• Serine (ser or S): The hydrophilic properties of serine are owed to the fact that it contains a hydroxyl group.
• Threonine (thr or T): Similar in structure to a sugar called threose, and named after it.

These compounds behave much like hydrophilic (polar) amino acids in that they readily interact with water, but they carry a net charge of +1 or -1. This makes them acids (proton donors) or bases (proton acceptors) at the pH, or acidity, of the human body.

• Aspartic acid (asp or D): Deprotonated at physiological (body) pH, conferring a negative charge on the molecule. Also called aspartate.
• Glutamic acid (glu or E): Deprotonated at physiological pH. Also called glutamate.
• Lysine (lys or K): A base, and protonated at physiological pH.   
• Arginine (arg or R): Also a base and protonated at physiological pH.   

"Amphipathic" translates roughly to "feeling both" in Greek, and these amino acids can function as both non-polar (hydrophobic) and polar (hydrophilic), almost like a softball player who is not a superstar pitcher or batter but can function capably in both roles in a sport in which most players' capacities are highly specialized.

They do not carry a net charge, but the distribution of electrical charge along the R-chains of these amino acids is markedly asymmetrical.

• Tyrosine (tyr or T): Its hydroxyl group can both donate and accept a hydrogen bond, so tyrosine sometimes "acts" hydrophilic.
• Tryptophan (try or W): The largest amino acid; a precursor to the neurotransmitter serotonin (5-hydroxytryptamine).