Protein function in the body

 

6.5: Protein’s Functions in the Body

1. 
2. Buy Print CopyDonate

• 
• Anonymous
• LibreTexts

Learning Objectives

• List protein’s functions in the body.

Proteins

 are the “workhorses” of the body and participate in many bodily functions. As you may recall, 

proteins

 come in all sizes and shapes and each is specifically structured for its particular function.

Proteins

 come in all sizes and shapes.

Structure and Motion

More than one hundred different structural 

proteins

 have been discovered in the human body, but the most abundant by far is 

collagen

, which makes up about 6 percent of total body weight. 

Collagen

 makes up 30 percent of bone tissue and comprises large amounts of tendons, ligaments, cartilage, skin, and muscle. 

Collagen

 is a strong, fibrous protein made up of mostly glycine and proline 

amino acids

. Within its quaternary structure three protein strands twist around each other like a rope and then these 

collagen

 ropes overlap with others. This highly ordered structure is even stronger than steel fibers of the same size. 

Collagen

 makes bones strong, but flexible. 

Collagen

 fibers in the skin’s dermis provide it with structure, and the accompanying elastin protein fibrils make it flexible. Pinch the skin on your hand and then let go; the 

collagen

 and elastin 

proteins

 in skin allow it to go back to its original shape. Smooth-muscle cells that secrete 

collagen

 and elastin 

proteins

 surround blood vessels, providing the vessels with structure and the ability to stretch back after blood is pumped through them. Another strong, fibrous protein is 

keratin

, which is what skin, hair, and nails are made of.

Collagen

: A strong protein made up of three intertwined peptides. Image used wtih permission (CC-SA-By; Nevit Dilmen).

The closely packed 

collagen

 fibrils in tendons and ligaments allow for synchronous mechanical movements of bones and muscle and the ability of these 

tissues

 to spring back after a movement is complete. Move your fingers and watch the synchrony of your knuckle movements. In order to move, muscles must contract. The contractile parts of muscles are the 

proteins

 actin and 

myosin

. When these 

proteins

 are stimulated by a nerve impulse they slide across each other, causing a shortening of the muscle cell. Upon stimulation, multiple muscle cells shorten at the same time, resulting in muscle contraction.

Enzymes

Although 

proteins

 are found in the greatest amounts in connective 

tissues

 such as bone, their most extraordinary function is as 

enzymes

. 

Enzymes

 are 

proteins

 that conduct specific chemical reactions. An enzyme’s job is to provide a site for a chemical reaction and to lower the amount of 

energy

 and time it takes for that chemical reaction to happen (this is known as “catalysis”). On average, more than one hundred chemical reactions occur in cells every single second and most of them require 

enzymes

. The liver alone contains over one thousand enzyme systems. 

Enzymes

 are specific and will use only particular substrates that fit into their active site, similar to the way a lock can be opened only with a specific key. Nearly every chemical reaction requires a specific enzyme. Fortunately, an enzyme can fulfill its role as a catalyst over and over again, although eventually it is destroyed and rebuilt. All bodily functions, including the breakdown of 

nutrients

 in the stomach and small intestine, the transformation of 

nutrients

 into molecules a cell can use, and building all macromolecules, including protein itself, involve 

enzymes

.

Enzymes

 are 

proteins

. An enzyme’s job is to provide a site for substances to chemically react and form a product, and decrease the amount of 

energy

 and time it takes for this to happen.

Video 6.5.2$\mathrm{6.5.}2$: Enzyme

Watch this animation of an enzyme at work. (click to see video)

Hormones

Proteins

 are responsible for hormone synthesis. Recall that 

hormones

 are the chemical messages produced by the endocrine glands. When an endocrine gland is stimulated, it releases a hormone. The hormone is then transported in the blood to its target cell, where it communicates a message to initiate a specific reaction or cellular process. For instance, after you eat a meal, your blood glucose levels rise. In response to the increased blood glucose, the pancreas releases the hormone insulin. Insulin tells the cells of the body that glucose is available and to take it up from the blood and store it or use it for making 

energy

 or building macromolecules. A major function of 

hormones

 is to turn 

enzymes

 on and off, so some 

proteins

 can even regulate the actions of other 

proteins

. While not all 

hormones

 are made from 

proteins

, many of them are.

Fluid and Acid-Base Balance

Proper protein intake enables the basic biological processes of the body to maintain the status quo in a changing environment. Fluid balance refers to maintaining the distribution of water in the body. If too much water in the blood suddenly moves into a tissue, the results are swelling and, potentially, cell death. Water always flows from an area of high 

concentration

 to one of a low 

concentration

. As a result, water moves toward areas that have higher concentrations of other 

solutes

, such as 

proteins

 and glucose. To keep the water evenly distributed between blood and cells, 

proteins

 continuously circulate at high concentrations in the blood. The most abundant protein in blood is the butterfly-shaped protein known as 

albumin

. 

Albumin

’s presence in the blood makes the protein 

concentration

 in the blood similar to that in cells. Therefore, fluid exchange between the blood and cells is not in the extreme, but rather is minimized to preserve the status quo.

The butterfly-shaped protein, 

albumin

, has many functions in the body including maintaining fluid and acid-base balance and transporting molecules. Cartoon representation of the molecular structure of protein registered with 1ao6 code. (Public Domain; Jawahar Swaminathan and MSD staff at the European Bioinformatics Institute).

Protein is also essential in maintaining proper pH balance (the measure of how acidic or basic a substance is) in the blood. Blood pH is maintained between 7.35 and 7.45, which is slightly basic. Even a slight change in blood pH can affect body functions. Recall that acidic conditions can cause protein 

denaturation

, which stops 

proteins

 from functioning. The body has several systems that hold the blood pH within the normal range to prevent this from happening. One of these is the circulating 

albumin

. 

Albumin

 is slightly acidic, and because it is negatively charged it balances the many positively charged molecules, such as hydrogen protons (H+), 

calcium

, potassium, and 

magnesium

 which are also circulating in the blood. 

Albumin

 acts as a buffer against abrupt changes in the concentrations of these molecules, thereby balancing blood pH and maintaining the status quo. The protein hemoglobin also participates in acid-base balance by binding hydrogen protons.

Transport

Albumin

 and hemoglobin also play a role in molecular transport. 

Albumin

 chemically binds to 

hormones

, 

fatty acids

, some vitamins, essential minerals, and drugs, and transports them throughout the 

circulatory system

. Each red blood cell contains millions of hemoglobin molecules that bind oxygen in the lungs and transport it to all the 

tissues

 in the body. A cell’s plasma membrane is usually not permeable to large polar molecules, so to get the required 

nutrients

 and molecules into the cell many transport 

proteins

 exist in the cell membrane. Some of these 

proteins

 are channels that allow particular molecules to move in and out of cells. Others act as one-way taxis and require 

energy

 to function (Figure 6.5.1$\mathrm{6.5.}1$).

Figure 6.5.1$\mathrm{6.5.}1$: Molecules move in and out of cells through transport 

proteins

, which are either channels or carriers. Facilitated diffusion in cell membrane, showing ion channels (left) and carrier 

proteins

 (three on the right). (Public Domain; LadyofHats).

Protection

Earlier we discussed that the strong 

collagen

 fibers in skin provide it with structure and support. The skin’s dense network of 

collagen

 fibers also serves as a barricade against harmful substances. The 

immune system

’s attack and destroy functions are dependent on 

enzymes

 and 

antibodies

, which are also 

proteins

. An enzyme called lysozyme is secreted in the saliva and attacks the walls of bacteria, causing them to rupture. Certain 

proteins

 circulating in the blood can be directed to build a molecular knife that stabs the cellular membranes of foreign invaders. The 

antibodies

 secreted by the white blood cells survey the entire 

circulatory system

 looking for harmful bacteria and viruses to surround and destroy. 

Antibodies

 also trigger other factors in the 

immune system

 to seek and destroy unwanted intruders.

Proteins

 play a role in protecting the body against unwanted intruders. 

Antibodies

 surround and attack an influenza virus via "antigens" on thier surface. Each antibody binds to a specific antigen; an interaction similar to a lock and key. (Public Domain; Fvasconcellos and USA government).

Wound Healing and Tissue Regeneration

Proteins

 are involved in all aspects of wound healing, a process that takes place in three phases: inflammatory, proliferative, and remodeling. For example, if you were sewing and pricked your finger with a needle, your flesh would turn red and become inflamed. Within a few seconds bleeding would stop. The healing process begins with 

proteins

 such as bradykinin, which dilate blood vessels at the site of injury. An additional protein called fibrin helps to secure 

platelets

 that form a clot to stop the bleeding. Next, in the proliferative phase, cells move in and mend the injured tissue by installing newly made 

collagen

 fibers. The 

collagen

 fibers help pull the wound edges together. In the remodeling phase, more 

collagen

 is deposited, forming a scar. Scar tissue is only about 80 percent as functional as normal uninjured tissue. If a diet is insufficient in protein, the process of wound healing is markedly slowed.

While wound healing takes place only after an injury is sustained, a different process called tissue regeneration is ongoing in the body. The main difference between wound healing and tissue regeneration is in the process of regenerating an exact structural and functional copy of the lost tissue. Thus, old, dying tissue is not replaced with scar tissue but with brand new, fully functional tissue. Some cells (such as skin, hair, nails, and intestinal cells) have a very high rate of regeneration, while others, (such as heart-muscle cells and nerve cells) do not regenerate at any appreciable levels. Tissue regeneration is the creation of new cells (cell division), which requires many different 

proteins

 including 

enzymes

 that synthesize RNA and 

proteins

, transport 

proteins

, 

hormones

, and 

collagen

. In a hair follicle, cells divide and a hair grows in length. Hair growth averages 1 centimeter per month and fingernails about 1 centimeter every one hundred days. The cells lining the intestine regenerate every three to five days. Protein-inadequate diets impair tissue regeneration, causing many health problems including impairment of nutrient digestion and absorption and, most visibly, hair and nail growth.

Energy

 Production

Some of the 

amino acids

 in 

proteins

 can be disassembled and used to make 

energy

. Only about 10 percent of dietary 

proteins

 are catabolized each day to make cellular 

energy

. The liver is able to break down 

amino acids

 to the carbon skeleton, which can then be fed into the citric acid cycle. This is similar to the way that glucose is used to make ATP. If a person’s diet does not contain enough 

carbohydrates

 and fats their body will use more 

amino acids

 to make 

energy

, which compromises the synthesis of new 

proteins

 and destroys muscle 

proteins

. Alternatively, if a person’s diet contains more protein than the body needs, the extra 

amino acids

 will be broken down and transformed into fat.

Key Takeaways

• The many shapes and sizes of 
  proteins
   allow them to perform a vast array of functions, including: acting as 
  enzymes
   and 
  hormones
  , and providing for fluid and acid-base balance, transport, protection, wound healing and tissue regeneration, and 
  energy
   production.
• Without 
  adequate
   intake of protein containing all the essential 
  amino acids
  , all protein functions will be impaired.

Discussion Starter

1. Given protein’s critical roles in the 
   immune system
   , what do you think might occur more frequently in a person whose diet is protein deficient?

reflection 

Proteins are crucial for numerous functions within the body, each shaped specifically to perform a distinct role effectively. This reflection will explore the multifaceted roles of proteins and emphasize their importance in health and disease prevention.

Structural and Functional Roles of Proteins

Proteins like collagen and keratin provide structural support to various body parts. Collagen, which makes up a significant portion of bone, skin, and muscle tissue, is incredibly strong and flexible. This protein is essential for maintaining the resilience and elasticity of these tissues, helping them to withstand stress without breaking. Similarly, keratin strengthens hair and nails, providing durability and protection against physical damage.

Enzymatic and Hormonal Functions

Proteins also function as enzymes, facilitating countless chemical reactions in the body that are vital for survival. These reactions occur every second and are crucial for everything from digestion to DNA replication. Enzymes reduce the energy required for reactions, ensuring efficiency and speed in metabolic processes.

Hormones, many of which are proteins, serve as chemical messengers that regulate bodily functions. For example, insulin, a protein hormone, helps manage blood glucose levels by signaling cells to absorb glucose from the bloodstream. This regulatory role is critical for energy balance and overall metabolic health.

Transport and Maintenance of Homeostasis

Proteins like albumin play a key role in transporting substances such as fatty acids, hormones, vitamins, and minerals throughout the body. Albumin also helps maintain fluid balance and pH levels in the blood, preventing sudden shifts that could disrupt cellular function and overall health.

Immune Protection and Tissue Repair

In the immune system, proteins act as antibodies that identify and neutralize foreign invaders such as viruses and bacteria. These proteins are fundamental in defending the body against infections and diseases. Additionally, proteins are involved in wound healing and tissue regeneration, processes that repair damaged tissues and replace old or dying cells with new ones.