Amino Acids

RUTH M. LEVERTON.

AMINO acids form the alphabet of the proteins. They have the same relation to proteins that letters have to words. At least 22 different letters make up the amino acid alphabet, and combinations of the same or different amino acids make a great variety of proteins.

Not all of the amino acids are present in each protein, but there are many, many more amino acids in a protein than there are letters in any word.

The number of amino acids contained in a single protein can be imagined by comparing the weight of a molecule of an amino acid with the weight of a molecule of protein.

The amino acids range in molecular weight from a low of 89 for glycine to a high of 777 for thyroxine. The wheat protein, gliadin, has a molecular weight of 27,500; zein in corn, 50,000. The molecular weight of hemoglobin in human blood is 63,000. One of the serum globulins in blood may have a molecular weight of more than a million.

The amino acids in a protein determine its chemical characteristics and its nutritive value and how it functions in the metabolism of the body.

All amino acids contain carbon, hydrogen, oxygen, and nitrogen. Three amino acids have sulfur, and two contain iodine.

The chemical structure of each amino acid includes an acid group and an amino group on adjoining carbon atoms.

The acid group looks like this:

It is attached to the carbon, which in turn is attached to the acid group.

SO THE CHEMICAL structure common to all amino acids is this arrangement:

One amino acid differs from another in the special group that is attached to the same carbon atom as the amino group, where the question mark appears. (Chemists use an R to represent this part of the molecule.)

The attachment may be as simple as a lone hydrogen atom to make the amino acid glycine. It may be a certain chain of carbon and hydrogen atoms to make leucine; a chain plus another amino group to make lysine; or a chain that includes an atom of sulfur to make methionine. Adding a more complicated arrangement, such as a ring of carbon and hydrogen atoms, would make phenylalanine; two rings, with some iodine attached, would make thyroxine. Regardless of what is added, however, the characteristic acid and amino groups are common to all amino acids.

A unique feature of the amino acids is that the arrangement of the carbon, hydrogen, oxygen, and nitrogen atoms (and others if they are present) can exist in two patterns.

One pattern is the mirror image of the other, like your left and your right hand. Nature makes only the left-hand pattern, called the L-form of amino acids, and this is the pattern found in all our foods. In general, the body can use only the L-form.

WHEN CHEMISTS make amino acids synthetically in the laboratory, they come out with a mixture of equal parts of the left-hand and the right-hand (D-form) patterns. The body cannot use the right-hand pattern of an amino acid except as a source of carbon and nitrogen, which it may build into the L-form of certain amino acids.

The amino group makes it possible for an amino acid to act like a base (also called alkali), while the acid group makes it possible for it to act like an acid. This dual action is one of the special characteristics of amino acids.

Whether they can act as an acid or a base depends on which is needed at the moment to keep the acid-base balance of the body, especially of the blood, within normal limits. Proteins are often referred to as buffers because of this ability, through their amino acids, to protect the body against sudden or great changes in its acid-base relationships.

It is through their amino and their acid groups that amino acids are joined together to make proteins. The acid group of one molecule of amino acid reacts with the amino group of another just as any acid and alkali react together. A molecule of water is formed and travels off, leaving the nitrogen of one amino acid joined to the carbon of the next amino acid. Such a joining is called a peptide linkage. A protein is a group of amino acids held together by peptide linkages.

Specific enzymes in the gastrointestinal tract attack the peptide linkages when proteins are digested. First, the protein is separated into many clumps of amino acids. And then the clumps are separated further into single amino acids, which are absorbed from the intestine and carried by the blood to the liver.

But the amino acids do not stay single for long. As soon as they leave the liver and are carried by the blood to different tissues, they are reassembled into the special combinations that make the proteins to replace cell material that has worn out, to add to tissue which needs to grow, or to make some enzyme or hormone or other active compound.

It is remarkable how the normal body has unerring accuracy in assembling amino acids into the vital substances needed in every location. If any amino acids are left over, they cannot be stored in the body for use at a later time. They are returned instead to the liver and stripped of their amino groups in a process called deaminization. The nitrogen leaves the body chiefly as urea through the urine, but the carbon, hydrogen, and oxygen fragments that are left can be used to provide energy. If the energy is not needed immediately, the fragments can be converted to fat and stored for use at a later time.

EIGHTEEN DIFFERENT amino acids commonly occur in our food supply.

Some are more important to us than others. The body can manufacture many of them from the materials supplied by the protein and other substances in our food.

There are eight amino acids that the body must have but cannot make from any materials. Our food must supply them completely formed and ready for use. They are valine, lysine, threonine, leucine, isoleucine, tryptophan, phenylalanine, and methionine. They are called the essential or indispensable amino acids because it is essential to have them supplied ready made.

Other amino acids are essential to life and health, too, but if our food does not provide any or enough of them, the body can make them from the raw materials supplied by the food. Therefore they are called nonessential or dispensable amino acids in reference to the fact that it is not essential for the food to furnish them ready made. They are glycine, tyrosine, cystine, cysteine, alanine, serine, glutamic acid, aspartic acid, arginine, histidine, proline, and thyroxine.

The presence in a protein of all of the essential amino acids in significant amounts and in proportions fairly similar to those found in body proteins classifies it as a complete protein meaning that it could supply completely the needs of the body for these amino acids.

The proportions in which the essential amino acids are required are as important as the amounts. Apparently the body wants these amino acids to be available from food in about the same proportions each time for use in maintenance, repair, and even growth.