© J. Dirk Nies, Ph.D.
Acids and bases are important and versatile compounds, vital for our lives and to our economy. They occur naturally in our bodies, they are found everywhere in the environment, and they appear throughout our economy. They are important players in the manufacture of fertilizers, explosives, dyestuffs, pigments, rubber, plastics and paper. They serve as catalysts to speed up synthesis of pharmaceuticals. Acids, in particular, are used in purification of petroleum, leaching and refining of ores, cleaning and pickling of metals. They are leather tanning agents and textile scouring agents. In ultrapure form, they are essential to the electronics industry. They are employed in the preparation and preservation of food products. And acids and bases appear in many common household items as well, such as vinegar and ammonia. From car batteries to hair dyes, acids and bases are a ubiquitous part of our lives.
When mixed together, acids and bases often yield more benign, water-soluble salts. For example, in equal proportions, corrosive hydrochloric acid (muriatic acid) combined with caustic sodium hydroxide (lye) gives common table salt, sodium chloride. Salts of acids and bases are found the world over. This is why the ocean is so salty. When naturally occurring acids and bases come in contact, they neutralize each other and the resultant salts are carried by rain and snow into the sea.
Acids and bases play vital roles in biology. Living organisms actively strive to maintain them in balance. In humans, for example, our blood, like ocean water, is slightly alkaline. Its pH is regulated with acids and bases to stay within the narrow range of 7.35 to 7.45. In the plant kingdom, different species have adapted to the acidity or alkalinity of soils in which they thrive. Rhododendron and azaleas love acidic soils (pH between 4.5 and 6), and acid-forming soil amendments such as aluminum sulfate, iron sulfate and sulfur can be added to adjust soil pH into this optimal range for these plants. Conversely, many grasses do not do well under overly acidic conditions. Basic soil amendments such as lime (calcium carbonate) and dolomite (calcium/magnesium carbonate) that raise soil pH are helpful in this regard.
As important as acids and bases are for life, they are eclipsed in value and versatility by a special class of compounds that have an acid and a base built into their structure. Specifically, their acidity is imparted by the presence of a carboxylic acid group (-COOH) and their basicity is imparted by an amino group (-NH2). These substances are the building blocks of life. They are the primary organic stuff of which we are made. They are the amino acids.
The simplest amino acid is glycine (NH2CH2COOH ). Glycine can be thought of as a derivative of vinegar (acetic acid, CH3COOH) in which one of hydrogens of the CH3 group has been replaced with an amino group. Or you can picture them as a derivative of ammonia (NH3) in which one of the hydrogens has been substituted with acetic acid. As shown in its chemical formula, the amino group on the left and the carboxylic acid group on the right are attached to the same carbon atom in the middle of glycine. As a rule, biologically important amino acids contain these acid and base groups attached to the same carbon atom.
Amino acids can and do interact with each other as acids and bases. But what makes them so special is that they also can form sturdy links between each other. When the acid group (-COOH) of one amino acid is chemically bound to the basic group (-NH2) of another amino acid by the removal of water (H2O), this linkage is called a peptide bond (-CONH-). When dozens or more amino acids are linked together through peptide bonds, the resultant molecules are called proteins. The first protein to be sequenced was the hormone insulin (in the late 1940s). By figuring out the sequence of amino acids in insulin, scientists demonstrated conclusively that proteins were polymers of amino acids. Hemoglobin, which makes up about 97 percent of our red blood cells’ dry content, was the first 3-dimensional protein structure to be solved (in the late 1950s).
Other types of smaller compounds can be made from amino acids as well. For example, NutraSweet, which is 200 times sweeter than sugar, is made from two amino acids held together by a single peptide bond. And even individual amino acids such as glutamate and gamma-aminobutyric acid (GABA) have important biological functions, serving as neurotransmitters that convey information across the synapses of our nervous system.
Twenty-two different amino acids comprise the protein-making “toolkit” of life. These nearly two dozen amino acids differ from one another by the presence of additional groups of atoms attached to the structural template of glycine. These groups impart properties that influence reactivity and behavior, for example, by adding additional positive or negative charge, or by making the amino acid more or less soluble in water.
Proteins are molecular workhorses. They perform a multitude of biologically important functions. No other class of compounds is more prevalent in the human body. Thousands of different proteins exist within each of us, and their properties and functions are determined primarily by the sequence of amino acids within their structure.
As enzymes, proteins help replicate DNA and they catalyze metabolic reactions. The rate acceleration they achieve often is astounding. In the case of the enzyme orotidine 5’-phosphate decarboxylase, the time to complete the reaction it catalyzes decreases from 78 million years without the enzyme to 18 milliseconds in its presence!
As structural materials, proteins build muscle, connective tissue, hair and nails. We clothe our bodies and clad our feet with proteins; silk is a natural protein fiber, leather is principally protein. Everyday products ranging from glue to gelatin are made of proteins.
Most microorganisms and plants can biosynthesize all the amino acids they need. In contrast, humans ,along with many other animals, lack the enzymes necessary to synthesize many of them. From an evolutionary point of view, it can make sense not to expend energy and resources making these essential amino acids when we can obtain them from the foods we eat. The downside of this out-sourcing strategy is that our health and even our survival become absolutely dependent upon the presence of these essential amino acids in our diet.
Scientists have identified nine amino acids that we do not make in our bodies and must therefore obtain through our diet. These essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Other amino acids such as arginine, cysteine and tyrosine are required in the diets of infants and growing children because the metabolic pathways that synthesize these amino acids are not yet fully developed.
We obtain amino acids by eating foods high in protein content. Meat, poultry, fish, beans, nuts, seeds, grains, many dairy products, and specialty items such as spirulina made from blue green algae are all excellent sources of protein. As a general rule, individual plant species tend not to have all of the amino acids essential for good nutrition in ideal proportions. Eating plant foods in combination, such as corn and beans, or rice and beans, provides the essential amino acids in better balance. Proteins ingested during meals are disassembled into their constituent amino acids by our digestive system. Using stomach acids and enzymes called proteases, the body breaks apart the peptide bonds linking the amino acids together. We then use these freed amino acids for protein synthesis or break them down further for their energy content.
As vital as proteins are to human health, most of our allergies result from our immune system improperly identifying benign proteins as harmful. (Snake venom, for example, is made of proteins, so we need the ability to assess proteinaceous friend from foe.) Proteins found in cat dander, latex, and tree and grass pollen are common allergens. Foods proteins that trigger allergic reactions are found most commonly in milk, eggs, peanuts, tree nuts, seafood, shellfish, soy and wheat. There are no cures for protein allergies, the best treatment is avoidance, where possible.
In summary, amino acids are extraordinary compounds possessing both acidic and basic groups within their structures. Because we lack the ability to synthesize some amino acids that are necessary for life, their presence in our diet is essential. When linked together in the proper sequence through peptide bonds, they form proteins. These substances constitute the bulk of all the organic materials we are made of, they speed up and direct the synthesis of virtually all important biochemical reactions, and they participate in nearly all other processes that occur within our bodies.