You grope around to turn off the loud alarm, and drag your feet to the bathroom. You brush your teeth, wash up with soap contained in various containers, and dry your hair with a hair dryer. After a quick breakfast, you get dressed, and leave the house, carrying your purse and cell phone.
When we take a look at the items that are used while getting ready to leave for work, most of them are made of plastic. Starting with watches, toothbrushes, detergent bottles, hair dryers, utensils, cooking utensils, functional fabrics, bags, and cell phone display stands—all these would not have been around us if we did not have plastic. The 21st century is worth being defined as the “age of plastic.”
Many high-tech products are made of synthetic polymers1 such as plastic (synthetic resin), synthetic fibers, and synthetic rubber. Then, what material are living beings, including human beings, made of?
1. Polymer: A large molecule with a molecular weight of more than 10,000, usually consisting of 100 or more atoms. Dozens to hundreds of monomers in a row are synthesized to form long polymer chains.
Life consists of protein
Humans and other living things are composed of natural polymers called protein. When you look at the human body, you can see that most of the body parts—various substances in the cells, blood components, organs, muscles surrounding the organs, skin, hair, nails, etc.—are made up of various types of proteins. It is no exaggeration to say that half of the human body, except water, is made of protein2.
2. Hard bones are completed after the bone matrix containing collagen protein as a main component is hardened and calcium phosphate crystals are added to it.
Proteins fold and bend according to the arrangement of amino acids to form a three-dimensional form. In case of hemoglobin, ring-like four chains are gathered into one.
Plastics are polymers that form long and large molecular chains through the process of polymerization of low molecular weight monomers. Similarly, proteins, which are natural polymers, are made of monomers called amino acids consisting of carbon, hydrogen, oxygen, and nitrogen. Animo acids which are separated are connected by a peptide bond as if two objects are glued together and form protein. Insulin, whose sequence of amino acids was identified for the first time after a decade-long study, consists of 51 amino acids.
Proteins are not in the form of a single string, but folded and bent according to the arrangement of amino acids, taking the form of three-dimensional shapes such as round balls, plates, long fibers, and coils. Only then can they properly demonstrate the unique functions and properties of a particular protein. The hemoglobin in the blood is composed of four polypeptide chains; each chain, which is like a ring, come together to form one like a transforming robot.
The structure of a protein is very sensitive to the environment. Protein-rich egg whites are originally transparent, but they turn white when stirred or heated. Such changes in the protein’s molecular structure due to changes in the external environment are called denaturation. This is why meat turns brown when it is cooked.
Protein with a thousand different faces
There are a total of 20 amino acids that are the source of protein. Since there are only 20 amino acids corresponding to the monomers of protein, you may think that the type of protein synthesized is probably not as diverse as the synthetic polymer. However, suppose that only 10 of the 20 amino acids are freely linked. There are 2010 different combinations. Proteins are made up of hundreds to tens of thousands of amino acids, so there are countless combinations of them. This allows the synthesis of a variety of proteins to meet the functions required in the body.
In our body, genetic information determines the order of amino acids that make up proteins. The amino acids designated by the DNA code are passed in sequence, like beads in a string, and the synthesis proceeds in a tightly controlled state. As a result, since the sequence and number of amino acids are always regulated, the lengths and types of the chains are always the same, which is why the proteins that are made have consistent properties.
The amino acids, which compose the chains, either pull or resist each other, depending on their properties. Therefore, protein is spontaneously folded into a stable state. Sometimes, with the help of another protein called chaperone, it forms three-dimensional forms.
It is known that there are 100,000 kinds of proteins in our body. There are only about 30,000 genes that can be called blueprints for proteins, but as a result of the editing process, several different proteins are produced from one gene, or sugar chains are attached to other proteins.
Protein, a special capability holder
The abilities of proteins vary just like the various types of proteins. The stratum corneum on the skin’s surface, hair, and nails are all insoluble in water and are made of hard, keratin proteins. Bone is made from a protein called collagen, and both transparent and tough lenses and elastic muscles are made of protein.
Proteins not only shape the body, but also play a role in supporting vital activities such as catalysts, information transfer and transport, like enzyme proteins that speed up chemical reactions in the body. In the brain where neurons are gathered, memories and emotions are created by exchanging neurotransmitters between cells. A wide variety of proteins is involved in this complex process. There are many different tasks, such as making antibodies against pathogens invading the body, involving in the senses, and binding to DNA to regulate gene expression.
When it comes to a good elastic material, the first thing that comes to mind is rubber. But the uterus, which is a body organ, has a more surprising function. When the fetus is fully grown, the uterus grows up to 500 times larger than its original size, but it returns to its original size after the baby is born. Most organs that have repetitive movements such as blood vessels, vocal organs, or the heart, are made of elastin protein with good elasticity. Blood vessels do not lose elasticity or break even over decades of heartbeats unless there is a pathological problem in them.
Some have good adhesion. It is a protein called dopa, found in marine organisms that usually stick to stones such as barnacles and mussels. Adhesives tend to lose their adhesion when there is moisture, but they don’t lose their adhesion even in water when using this protein. By studying these properties, a biocompatible protein glue is being invented to replace surgical sutures.
Fish in the Antarctic Ocean do not freeze even at the cryogenic temperature. This is because they have antifreeze proteins in their body that restrain the growth of ice crystals even at water temperatures below freezing. This antifreeze protein, which is found in some insects and plants that live in extreme conditions, is already being used in a variety of industries, including blood conservation and frozen food quality control.
There are also proteins that can make a bulletproof jacket. The silk protein from spiders is more than five times stronger than steel of the same thickness; it is so strong that a spider web of 1 cm [0.4 inch] in diameter can pull a jet. The spider silk protein which has high strength and elasticity will be most likely to be used in various fields such as parachutes, artificial ligaments, and suspension bridge cables.
When it comes to protein, people usually think of protein-rich foods such as meat, beans, and eggs. However, the true nature of protein is that it is a natural polymer that plays a wide range of roles in the body. Various proteins govern the life activities of all living things, from birth to death and during growth. Although the number of monomers is much smaller than that of synthetic polymers, the diversity and excellent properties are beyond those of synthetic polymers, considering that they are materials that constitute the human body.
Proteomics which is the study of the functions and changes of proteins which are made inside the body is in the spotlight no less than Genomics, as a high-growth industry that will lead scientific civilization in the 21st century. This is because proteins made from genes contain secrets that cannot be studied by studying only genes. There are a myriad of unknown proteins in living things that we do not even know. What humans have discovered is actually just the tip of the iceberg.