What distinguishes you chemically from any other human being are minute differences in your particular body proteins (enzymes, antibodies, and others). These differences are determined by your proteins’ amino acid sequences, which are written into the genes you inherited from your parents and ancestors. The genes direct the making of all the body’s proteins.
The human body has more than 20,000 genes that code for hundreds of thousands of proteins. Relatively few proteins have been studied in detail, although this number is growing rapidly with the surge in knowledge gained from sequencing the human genome. Only a few of the many roles proteins play are described here, but these should serve to illustrate proteins’ versatility, uniqueness, and importance.
Below are some of the important functions of protein in your body:
1. As Structural Components
A great deal of the body’s protein is found in muscle tissue, which allows the body to move. The amino acids of muscle protein can also be released when the need is dire, as in starvation. These amino acids are integral parts of the muscle structure, and their loss exacts a cost in functional proteins. Other structural proteins confer shape and strength on bones, teeth, tendons, cartilage, blood vessels, and other tissues. These proteins exist in a stable form and are more resistant to breakdown than are the proteins of muscles.
2. As Enzymes
Enzymes are catalysts that are essential to all life processes. Enzymes in the cells of plants or animals put together the pairs of sugars that make disaccharides and the long strands of sugars that make starch, cellulose, and glycogen. Enzymes also dismantle these compounds to free their constituent parts and release energy. Enzymes also assemble and disassemble lipids, assemble all other compounds that the body makes, and disassemble all compounds that the body can use for building tissue and other metabolic work. Enzymes put amino acids together to make needed proteins, too. In other words, these proteins can even make other proteins.
The protein story moves in a circle. To follow the circle in nutrition, start with a person eating food proteins. The food proteins are broken down by digestive enzymes, proteins themselves, into amino acids. The amino acids enter the cells of the body, where other proteins (enzymes) put the amino acids together in long chains whose sequences are specified by the genes.
The chains fold and twist back on themselves to form proteins, and some of these proteins become enzymes themselves. Some of these enzymes break apart compounds; others put compounds together. Day by day, in billions of reactions, these processes repeat themselves, and life goes on. Only living systems can achieve such self-renewal. A toaster cannot produce another toaster; a car cannot fix a broken-down car. Only living creatures and the parts they are composed of—the cells—can duplicate and repair themselves.
3. As Transporters
A large group of proteins specializes in transporting other substances, such as lipids, vitamins, and minerals, around the body. To do their jobs, those sub-stances must move from place to place within the blood, into and out of cells, or around the cellular interiors. Two familiar examples: the protein hemoglobin transports oxygen from the lungs to the cells and the lipoproteins transport lipids in the watery blood.
4. As Regulators of Fluid and Electrolyte Balance
Proteins help maintain the body’s fluid and electrolyte balance. The body’s fluids are contained in three major body compartments: (1) the spaces inside the blood vessels, (2) the spaces within the cells, and (3) the spaces between the cells (the interstitial spaces outside the blood vessels). Fluids flow back and forth between these compartments, and proteins in the fluids, together with minerals, help to maintain the needed distribution of these fluids.
Proteins are able to help determine the distribution of fluids in living systems for two reasons: first, proteins cannot pass freely across the membranes that separate the body compartments, and second, they are attracted to water. A cell that “wants” a certain amount of water in its interior space cannot move the water around directly, but it can manufacture proteins, and these proteins will hold water.
Thus, the cell can use proteins to help regulate the distribution of water indirectly. Similarly, the body makes proteins for the blood and the interstitial (intercellular) spaces. These proteins help maintain the fluid volume in those spaces. Excess fluid accumulation in the interstitial spaces is called edema.
Not only is the quantity of the body fluids vital to life, but so is their composition. Special transport proteins in the membranes of cells continuously transfer substances into and out of cells to maintain balance. For example, sodium is concentrated outside the cells, and potassium is concentrated inside. The balance of these two minerals is critical to nerve transmission and muscle contraction. Any disturbance in this balance triggers a major medical emergency. Such imbalances can cause irregular heartbeats, kidney failure, muscular weakness, and even death.
5. As Regulators of Acid-Base Balance
Proteins also help maintain the balance between acids and bases within the body’s fluids. Normal body processes continually produce acids and bases, which must be carried by the blood to the kidneys and lungs for excretion. The blood must do this without upsetting its own acid-base balance. Blood pH is one of the most tightly controlled conditions in the body. If the blood becomes too acidic, vital proteins may undergo denaturation, losing their shape and ability to function. A similar situation arises when there is an excess of base. These imbalances are known as acidosis and alkalosis, respectively, and both can be fatal.
Proteins such as albumin in blood help to prevent acid-base imbalances. In a sense, the proteins protect one another by gathering up extra acid (hydrogen) ions when there are too many in the surrounding medium and by releasing them when there are too few. By accepting and releasing hydrogen ions, proteins act as buffers, maintaining the acid-base balance of the blood and body fluids.
6. As Antibodies
Proteins also defend the body against disease. A virus—whether it is one that causes flu, smallpox, measles, or the common cold—enters the cells and multiplies there. One virus may produce 100 replicas of itself within an hour or so. Each replica can then burst out and invade 100 different cells, soon yielding 10,000 viruses, which invade 10,000 cells. Left free to do their worst, they will soon overwhelm the body with disease.
Fortunately, when the body detects these invading antigens, it manufactures antibodies, giant protein molecules designed specifically to combat them. The antibodies work so swiftly and efficiently that in a healthy individual, most diseases never get started. Without sufficient protein, though, the body cannot maintain its army of antibodies to resist infectious diseases.
Each antibody is designed to destroy a specific antigen. Once the body has manufactured antibodies against a particular antigen (such as the measles virus), it “remembers” how to make them. Consequently, the next time the body encounters that same antigen, it produces antibodies even more quickly. In other words, the body develops a molecular memory, known as immunity.
7. As Hormones
The blood also carries messenger molecules known as hormones, and some hormones are proteins. (Some hormones are sterols, members of the lipid family.) Among the proteins that act as hormones are glucagon and insulin.
8. As a Source of Energy and Glucose
Without energy, cells die; without glucose, the brain and nervous system falter. Even though amino acids are needed to do the work that only they can perform—build vital proteins—they will be sacrificed to provide energy and glucose during times of starvation or insufficient carbohydrate intake. When glucose or fatty acids are limited, cells are forced to use amino acids for energy and glucose.
When amino acids are degraded for energy or converted into glucose, their nitrogen-containing amine groups are stripped off and used elsewhere or are incorporated by the liver into urea and sent to the kidneys for excretion in the urine. The fragments that remain are composed of carbon, hydrogen, and oxygen, as are carbohydrates and fat, and can be used to build glucose or fatty acids or can be metabolized like them.
The body does not make a specialized storage form of protein as it does for carbohydrates and fat. Glucose is stored as glycogen in the liver and muscles, and fat as triglycerides in the adipose tissue, but body protein is available only as the working and structural components of the tissues. When the need arises, the body dismantles its tissue proteins and uses them for energy. Thus, over time, energy deprivation (starvation) always incurs the wasting of lean body tissue as well as fat loss.
Summary of Protein Functions
- Structural components. Proteins form integral parts of most body tissues and confer shape and strength on bones, skin, tendons, and other tissues. Structural proteins of muscles allow movement.
- Enzymes. Proteins facilitate chemical reactions.
- Transporters. Proteins transport substances such as lipids, vitamins, minerals, and oxygen around the body.
- Fluid and electrolyte balance. Proteins help to maintain the distribution and composition of various body fluids.
- Acid-base balance. Proteins help maintain the acid-base balance of body fluids by acting as buffers.
- Antibodies. Proteins inactivate disease-causing agents, thus protecting the body.
- Hormones. Proteins regulate body processes. Some, but not all, hormones are proteins.
- Energy and glucose. Proteins provide some fuel, and glucose if needed, for the body’s energy needs.
- Other. The protein fibrin creates blood clots; the protein collagen forms scars; the protein opsin participates in vision.