Have you ever thought of your body as a collection of microscopic chains? Perhaps not. But in reality, “at the level of its smallest relevant components,” says the book The Way Life Works, life employs “THE CHAIN AS ITS ORGANIZING PRINCIPLE.” For that reason, just a small defect in some of these chains can have a MAJOR impact on our health.
What are these chains? How do they function? And how do they relate to our health and well-being?
Basically, they are chainlike molecules that fall into TWO main categories. The molecules we will consider in this article are the PROTEINS. The others are the molecules that STORE and TRANSMIT genetic information –DNA and RNA. Of course, the two groups are INTIMATELY related. In fact, one key function of DNA and RNA is to produce life’s vast array of PROTEINS.
HOW ARE PROTEINS MADE?
1. Inside a cell’s nucleus, DNA contains instructions for each protein
2. A section of DNA is unzipped, and the genetic information is made into a messenger RNA
3. Ribosomes –“message-reading, protein assemblers” –bind to the RNA
4. Transfer RNAs carry amino acids to the ribosome
5. As the ribosome “reads” the RNA, it links together single amino acids in a specific order to form a chain –the protein
6. The chainlike protein must fold up precisely to perform its function. Imagine, a typical protein is over 300 “links” long!
HOW DOES DNA “SPELL” EACH PROTEIN?
[a] DNA uses just four “letters”: A,T,C,G
[b] The DNA “spelling” is transcribed to an RNA form. RNA uses U [uracil] instead of T
[c] Each three-letter sequence “spells” a specific “word” or amino acid
[d] In this way, each of the 20 common amino acids can be “spelled.” “Words” are linked together to form a chain, or “sentence” –the protein
HOW DOES A PROTEIN “FOLD”
1. Single amino acids are linked together to form a chain, then…
2. They form patterns, such as coils or pleats, then...
3. Fold into a complex three-dimensional structure, which may be…
4. Just one subunit of a complex protein
CATALYSTS, GUARDS, AND POSTS
Proteins are by far the most diverse of life’s larger molecules. The protein family includes ANTIBODIES, ENZYMES, MESSAGERS, STRUCTURAL PROTEINS, and TRANSPORTERS. The vast array of ANTIBODIES, or IMMUNOGLOBULINS, defend against foreign invaders such as bacteria and viruses. Other globulins help to seal off blood vessels damaged by trauma.
Enzymes serve as catalysts, speeding up chemical reactions, such as those involved with digestion. In fact, “without enzymes you would soon starve, because it would take 50 years to digest a typical meal,” explains the book The Thread of Life.
Enzymes go about their work in assembly-line fashion, each protein performing a specific task. For instance, the enzyme maltase breaks down maltose, a sugar, into two glucose molecules. Lactase breaks down lactose, or milk sugar.
Other enzymes combine atoms and molecules to form new products. And they perform their work with blinding speed. A single molecule of enzyme can catalyze thousands of chemical reactions per second!
Some proteins are classified as hormones and act as messengers. Released into the bloodstream, they stimulate or decrease the activity of other body parts. Insulin, for example, stimulates cells to absorb glucose, their energy source.
Structural proteins such as collagen and keratin are the main components of cartilage, hair, nails and skin. All these proteins are “the cell’s equivalent of posts, beams, plywood, cement, and nails. Let us now see what proteins are made of and how their chainlike structure relates to their function.
COMPLEXITY BASED ON SIMPLICITY
An alphabet is a basic element of many languages. From that list of letters, words are built. In turn, words form sentences. At the molecular level, life employs a similar principle. A master “alphabet” is provided by DNA.
Amazingly, this “alphabet” consists of just four letters –A, C, G, and T, which are symbols for the chemical bases ADENINE, CYTOSINE, GUANINE, and THYMINE. From these four bases, DNA via an RNA intermediate gives rise to AMINO ACIDS, which could be compared to words.
Unlike normal words, however, amino acids all have the same number of letters, namely, three. “Protein-assembling machines,” called RIBOSOMES, link the amino acids together. The resulting chains, or proteins, could be likened to sentences. With more elements than a spoken or written sentence, a typical protein may contain about 300 to 400 amino acids.
According to one reference work, there are hundreds of amino acids that occur in nature, but only about 20 kinds are found in most proteins. These amino acids can be arranged in an almost endless number of combinations. Consider: If just 20 amino acids form a chain 100 amino acids long, that chain can be arranged in over 10¹ᴼᴼ different ways –that is, 1 followed by 100 zeros!
PROTEIN SHAPE AND FUNCTION
A protein’s shape is critical to its role in the cell. How does a chain of amino acids influence the shape of a protein? Unlike the loose links in a metal or plastic chain, amino acids join together at certain angles, forming regular patterns.
Some of these patterns resemble coils like those of a telephone cord or folds like those of a pleated cloth. These patterns are then “folded,” or shaped, to form a more complex three-dimensional structure. The shape of a protein is anything but haphazard. Indeed, a protein’s form is critical to its function, which becomes all too evident when a flaw occurs in the amino acid chain.
WHEN THE CHAIN HAS A DEFECT
When proteins have defects in the amino acid chain or are incorrectly folded, they can cause a number of diseases, including sickle-cell anemia and cystic fibrosis. Sickle-cell anemia is a genetic disease in which the hemoglobin molecules in red blood cells are abnormal.
A molecule of hemoglobin consists of 574 amino acids arranged in four chains. A switch of just one amino acid in two of the four chains turns normal hemoglobin into its sickle-cell variant. Most cases of cystic fibrosis are due to a protein that lacks the amino acid phenylalanine at a key position in the amino acid chain.
Among other things, this defect interferes with the balance of salt and water needed in the membranes that line the gut and lungs, causing the mucus that coats these surfaces to become abnormally thick and sticky.
A severe lack of or absence of certain proteins lead to disorders such as albinism, a deficiency in pigmentation, occurs when a key protein called TYROSINASE is either defective or absent. This affects the production of melanin, a brown pigment normally present in human eyes, hair, and skin. Hemophilia is caused by very low levels of or lack of protein factors that help blood to coagulate. Other disorders attributed to defective proteins include lactose intolerance and muscular dystrophy, to name a few.
ONE THEORY ON THE MECHANISM OF DISEASE
In recent years scientists have focused their attention on a disease that some attribute to an abnormal form of a protein called a PRION. The theory is that disease results when defective prions bind to normal prion proteins, causing the normal protein to misfold. The result is “a chain reaction that propagates the disease and generates new infectious material” says the journal Scientific American.
What may have been an instance of prion-based disease first came to public attention in the 1950’s in Papua New Guinea. Certain isolated tribes engaged in a form of cannibalism for religious reasons, and this led to a disease called KURU, with symptoms similar to those of Creutzfeldt-Jakob disease. Once the afflicted tribes gave up this religious ritual, the incidence of KURU rapidly declined, and it is now virtually unknown.
Happily, though, proteins are usually folded correctly and go about their tasks with amazing cooperation, efficiency, and fidelity. This is remarkable considering that there are over 100,000 different kinds of proteins in the human body, all complex chains arranged into thousands of types of folds.
The world of proteins is still largely uncharted. To learn more, researchers are now devising sophisticated computer programs that may predict the shape of proteins from their amino acid sequence. Still, even the little we know about proteins establishes clearly that these “chain of life” not only possess a high level of organization but also reflect a profound intelligence.