Amino acid

• 1. Introduction

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  Amino acids are organic compounds carrying two reactive parts: an amino group and a carboxylic acid group. More than 500 of them exist in nature. Yet biology bets almost everything on a tiny subset. Just 22 alpha-amino acids get woven into proteins, and only those 22 appear in the genetic code of life. In the form of proteins, amino-acid residues make up the second-largest component of human muscle and tissue, with only water ahead of them. Why did life converge on so few molecules out of hundreds? Why do all the chiral ones share the same handedness? And how does a single building block stretch from the inside of a stomach enzyme to a sweetener in a soda? The answers begin with a French chemist, an asparagus stalk, and the year 1806.

• 2. Naming The Invisible

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  The systematic name of alanine is 2-aminopropanoic acid, a label that points to a structure no chemist expects to find floating in water. The IUPAC-IUBMB Joint Commission on Biochemical Nomenclature names amino acids using a fictitious neutral form, where the amino group is unprotonated and the carboxyl group is undissociated. The Commission warned that these names should not imply such structures represent any appreciable fraction of the real molecules. Classification offers more honest handles. Amino acids can be sorted by where their core groups sit, giving alpha-, beta-, and gamma-amino acids. Others group by polarity, by ionization, or by side-chain type, whether aliphatic, acyclic, aromatic, or polar. The carbon next to the carboxyl group, called the alpha-carbon, carries the amine, a hydrogen, and a side chain unique to each amino acid. That side chain, written as R, is where the real diversity lives, and it decides almost everything that follows.

• 3. The Left-Handed Rule

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  With the single exception of glycine, the alpha-carbon of every proteinogenic amino acid is stereogenic, meaning it can exist in mirror-image forms. Glycine escapes this because its side chain is just another hydrogen atom. All chiral proteinogenic amino acids share the L configuration. They are the left-handed enantiomers, a striking uniformity given that mirror images are chemically possible. Right-handed D-amino acids do appear in nature, though sparingly. They turn up in bacterial envelopes, in the neuromodulator D-serine, and in some antibiotics. On rare occasions a D-amino acid residue sits inside a protein, converted from its L form by post-translational modification. Threonine raises the stakes further. It carries two chiral centers rather than one, the L center at the alpha-carbon plus a second at the beta-carbon, yielding the full specification (2S,3R)-L-threonine. This single-handed chemistry is one of the deepest puzzles biology hands to anyone studying the origin of life.

• 4. What Side Chains Decide

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  Five amino acids carry a charge at neutral pH, and those charges do real structural work. Side chains with opposite charges form salt bridges, electrostatic contacts that hold a protein together or bind two proteins at an interface. Aspartate, glutamate, and histidine often mediate the binding of metal ions into protein structures. Aspartate and glutamate are the two negatively charged residues at neutral pH, and their carboxylate groups act as Brønsted bases in most settings. Inside the stomach, the aspartic protease pepsin flips this script, using catalytic aspartate or glutamate residues as Brønsted acids. Histidine plays a subtler game. Its imidazole group has a pKa of 6.0, leaving it only about 10 percent protonated at neutral pH, so it slips easily between its acid and base forms and shuttles protons during enzyme reactions. Cysteine breaks from the simple categories entirely. It forms covalent disulphide bonds with other cysteines, links that shape protein folding and stability and prove essential to the formation of antibodies. Glycine and proline each warp protein structure in their own way, glycine through the flexibility of having no side chain, proline through a ring that joins back onto the alpha amino group and locks it in place.

• 5. The Zwitterion Truth

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  In water near neutral pH, an amino acid is not the neutral structure its name suggests. It favors a zwitterionic form, with a deprotonated carboxyl group and a protonated amino group at once. Water makes this possible. Its high dielectric constant and hydrogen-bonding network stabilize the separated charges, so the neutral form is not present to any measurable degree at physiological pH. A zwitterion has a net charge of zero, but because it carries both a positive and a negative site, calling it uncharged is misleading. Move the molecule out of water and the picture changes. In low-dielectric environments like organic solvents or the interior of a cell membrane, charge separation is poorly stabilized, and proton transfer yields a neutral form. In the gas phase, with no solvation at all, the lowest-energy structure of most amino acids is likewise neutral. Acidity drives its own transformation. Below pH 3 the carboxylate becomes protonated, producing an ammonio carboxylic acid, the state relevant to acid-loving enzymes like pepsin in the stomach and in lysosomes. Each amino acid has an isoelectric point, the pH at which traces of positive and negative forms balance to an average net charge of zero. At that point the molecule has zero mobility in electrophoresis and minimal solubility, a property that lets some amino acids be isolated from solution simply by tuning the pH.

• 6. A Code With Exceptions

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  Twenty-two amino acids get incorporated into polypeptides, and of those only 20 are spelled out by the universal genetic code. The other two arrive through clever workarounds. Selenocysteine slips in when an mRNA contains a SECIS element, which reassigns the UGA codon away from its usual job as a stop signal. Just 25 human proteins carry selenocysteine in their primary structure, and the characterized selenoenzymes use it as the catalytic moiety in their active sites. Pyrrolysine is rarer still. Some methanogenic archaea use it in enzymes that produce methane, encoding it with UAG, a codon that normally tells the ribosome to stop. Even methionine has a variant. A modified form, N-formylmethionine, often serves as the initial amino acid of proteins in bacteria, mitochondria, and chloroplasts. Evolutionary studies hint at a sequence to all this. Glycine, alanine, aspartate, valine, serine, proline, glutamate, leucine, and threonine may belong to an early genetic code, while cysteine, methionine, tyrosine, tryptophan, histidine, and phenylalanine look like later additions. The single-letter codes reflect their own history, with W chosen for tryptophan because its double ring suggested the bulky letter, and K assigned to lysine as the alphabet's nearest neighbor to its initial L.

• 7. Beyond Proteins

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  Tryptophan is the precursor of the neurotransmitter serotonin, one of many jobs amino acids hold outside protein chains. Tyrosine, along with its precursor phenylalanine, feeds the catecholamine neurotransmitters dopamine, epinephrine, and norepinephrine. Glycine seeds the porphyrins, including heme, while arginine gives rise to nitric oxide, and aspartate, glycine, and glutamine supply pieces for nucleotides. Plants turn amino acids into weapons. Canavanine, an analogue of arginine found in many legumes, acts as an antifeedant that protects the plant from predators. Mimosine, an analogue of tyrosine in some legumes, can poison animals that graze on them. The nonstandard members carry their own duties. Carnitine ferries long-chain fatty acids into the mitochondrial matrix for beta-oxidation, gamma-aminobutyric acid serves as a neurotransmitter, and beta-alanine helps build pantothenic acid, vitamin B5, a component of coenzyme A. Several have moved into medicine. 5-HTP has seen experimental use against depression, L-DOPA treats Parkinson's disease, and eflornithine inhibits ornithine decarboxylase in the treatment of sleeping sickness. For humans the supply is not optional. Nine of the 20 standard amino acids, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine, are essential, because the body cannot make them fast enough and must take them from food.

• 8. From Spark To Industry

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  An electric arc passed through methane, hydrogen, and ammonia produced a large number of amino acids in the famous Urey-Miller experiment. The formation of amino acids and peptides is assumed to have preceded, and perhaps even induced, the emergence of life on Earth. Several hypotheses lean on the Strecker synthesis, in which hydrogen cyanide, simple aldehydes, ammonia, and water combine to make amino acids. One review notes that amino acids and even peptides turn up fairly regularly in experimental broths cooked from simple chemicals, because nucleotides are far more difficult to synthesize than amino acids. That chemistry now runs at commercial scale. Production usually relies on mutant bacteria that overproduce a single amino acid using glucose as a carbon source, while aspartic acid is made by adding ammonia to fumarate with a lyase. The food industry is a heavy consumer, using glutamic acid as a flavor enhancer and aspartame, aspartylphenylalanine 1-methyl ester, as an artificial sweetener. Amino acids also enter animal feed, since staples like soybeans run low on lysine, methionine, threonine, and tryptophan. Looking forward, their chelating ability is being put into fertilizers to correct deficiencies such as iron chlorosis, and polyaspartate, a water-soluble biodegradable polymer, is being explored for disposable diapers and as a corrosion inhibitor.