If the word “protein” only makes you think of juicy steaks or protein shakes, it may be a surprise to learn that 20% of each cell in our body is made of proteins . So, what are proteins and why does the body need them?
The name protein originates from the Greek “protos” which means first and it has that name for a good reason . Proteins are molecules that perform a whole zoo of functions in the cell. They are essential to every living organism. DNA, the blueprint for all living things, contains the recipes necessary to make each protein, hence coding for the “workers” that make our cells function.
Here are just a few examples of proteins and their roles:
HAIR: Your hair, nails, and skin are made of proteins called keratins that serve a protective function from UV irradiation and dehydration, among others. Your DNA determines the type of keratin you have. People with curly hair, for instance, have more disulfide bonds in their keratin, which causes the protein chain to curve .
EYES: Your eye lens is also made up of proteins. These proteins are called crystallins. They pack tightly to increase the refractive index of the lens, yet are transparent. In fact, over 90% of your eye lens are made of crystallins . Another amazing property of crystallins is their longevity. Most proteins are replaced within two days; however, crystallins essentially last a lifetime [5,6]! Sadly, protein accumulation results in damage with age, making the lens less transparent which contributes to vision deterioration .
BLOOD: Red blood cells also use proteins. They have a special protein called hemoglobin, which carries oxygen to tissues and removes waste products, such as carbon dioxide (CO2). This protein teams up with iron to accomplish the task, which gives blood its red color .
Your immune system also uses proteins. The immune system is a fleet of specialized cells circulating in the body and produces proteins called antibodies that can recognize and bind foreign molecules and particles (proteins on viral surfaces), targeting them for destruction .
We need to make lots of different kinds of proteins every day to accomplish these tasks and many more, but we aren’t the only ones making proteins. All living organisms, including plants and animals, make proteins, and we get proteins as part of our diet.
So, what happens to proteins when you ingest them? Proteins are made up of building blocks called amino acids. Dietary proteins are first chopped up into pieces (by enzymes called proteases) and these building blocks are used to either make new proteins or are “burned” or energy.
You have probably heard of the term “essential amino acids” before. Essential amino acids cannot be produced by the body alone, so proteins that contain those essential amino acids are absolutely required in our diets .
Most proteins are short-lived – they only live for a few hours to days, after which they are destroyed and recycled into amino acids . But why go through all the trouble of making a protein today just to break it down tomorrow, over and over again? As an area of ongoing research, we know that proteins get damaged .
Just like tires are replaced throughout the lifetime of a car, proteins need to be replaced in our lifetimes. A rapid turnover can ensure that the proteins that stick around are of good quality and will not cause trouble. It is also one way our cells regulate the function and amount of certain proteins, so that we only have specific proteins functioning at the time needed. Proteins whose functions are no longer needed are rapidly degraded. Failure to get rid of some proteins when necessary can cause problems, as seen in diseases including cancer .
Just how much work is involved in making a protein? The largest protein known to date is Titin, which acts as a molecular spring in muscles. It consists of ~30,000 amino acids and it takes a lot of time to make. Protein synthesis occurs at a rate of ~200 amino acids per minute, so it takes 2-3 hours to make one molecule of Titin ! Most proteins are much shorter than Titin with a median of ~400 amino acids long. These are produced in a matter of minutes .
Considering that proteins are so essential to all functions in the human body, it is not surprising that many diseases are caused by mistakes in protein sequencing or abnormal protein behavior. A mutation in the DNA recipe encoding the protein produces a “half-baked” defective protein. For example, cystic fibrosis is caused by a dysfunctional copy of the CFTR protein which is involved in the production of sweat, digestive fluids, and mucous . Sometimes a protein with a correct sequence can misbehave. For example, in prion disease, the prion protein changes its shape and forms an aggregate which interferes with the proper function of nerve cells .
It is not surprising that scientists devote their whole lives to the study of protein structure and function since proteins are so complicated — there is an estimated number of ~20,000 proteins in humans alone ! Our journey to understanding the function of proteins in human health and disease has only just begun with an important journey into the cell, a world made of proteins.
Zhanna Hakhverdyan, Know Science contributor
- Freitas Jr., Robert A. (1999). Nanomedicine,. Landes Bioscience. Tables 3–1 & 3–2. ISBN 1-57059-680-8