Functions of Proteins

Proteins are the most abundant organic molecules present on earth. They are present abundantly in every living cell. Proteins are the polymers made up of thousands of amino acids linked via peptide bonds. Long chains of amino acids known as polypeptides fold around themselves in several ways to form complex structures called proteins. 

Functions performed by proteins can be divided into different categories. Some functions are essential at the cellular level while others are required for the better performance of the body as a whole. Here, we will try to understand different functions performed by proteins in our body through various examples. 

All enzymes are proteins

Enzymes are the proteins that are required for any chemical reaction to take place in our body. They catalyze the biochemical reaction so that life can proceed. 

An example of enzymatic reaction in our body is glycolysis. This is the process by which energy is released from a glucose molecule. This energy is required to carry out several processes taking place within a cell. The process of glycolysis involves around 10 steps each requiring a particular enzyme. The absence of a single enzyme stops the process and energy from glucose cannot be obtained. 

Synthesis of proteins also requires specific enzymes. Protein synthesis involves transcription of DNA into mRNA and then translation of mRNA by ribosomes. Both these steps require enzymes that are proteins. For example;

• RNA polymerase is an enzyme required to join RNA nucleotides in the process of transcription. 
• Aminoacyl tRNA synthetase is an enzyme that attaches specific amino acids to tRNA so that it can be used in protein synthesis. 

Thus, from obtaining energy to making proteins, all chemical processes in living organisms need enzymes, and all enzymes are proteins. The role of proteins as enzymes is the most important and crucial function performed by proteins. 

Proteins act as receptors on cell membranes

Proteins are essential components of all the cell membranes and membranes of the organelles. One of the functions of these membrane proteins is that they act as receptors. Hormones, neurotransmitters, and other signalling molecules bind to these receptors and convey signals to cells. In this way, proteins play a role in cell signalling that is essential for the coordinated function of all the cells present in our body.  Take the following example to understand the role of proteins as receptors. 

• Insulin is a hormone that controls the glucose levels in our blood. It performs its function by binding to its receptor that is a protein. Insulin binds to its receptor that sends signals for the opening of glucose channels so that glucose can be taken up from the blood into the liver and muscle cells. If the insulin receptors are not present, the blood glucose levels cannot be regulated. 

This and various other examples in our body prove why proteins are necessary for cell signalling and coordination of cellular functions. 

Some hormones are also proteins

Proteins not only act as cellular receptors but also hormones. Insulin and Glucagon are the two hormones that are protein in nature. Both these hormones are required for the regulation of blood glucose levels. They control the uptake and release of glucose by the cells, glycolysis and gluconeogenesis, as well as the synthesis and degradation of glycogen. The roles of these hormones in our body are listed below;

• Insulin is released by the pancreas when blood glucose levels are high. It promotes glucose uptake by the cells, its breakdown as well as its storage in the form of glycogen. It also inhibits the synthesis of new glucose molecules from non-carbohydrate sources (gluconeogenesis). 
• Glucagon is released by the pancreas when the blood glucose levels are low. It promotes the breakdown of glycogen to release glucose. It also promotes gluconeogenesis. 

Proteins act as transport channels in cell membranes

Proteins present in cell membranes also act as transport channels. Substances that are not permeable through membranes due to their size or charge can enter the cell through these protein channels. One protein channel is specific for one or more substances. Examples of protein channels are given below;

• Aquaporins are the protein channels that allow the passage of water molecules through cells
• GLUT (glucose transporter) are the transporters for glucose molecules
• Sodium channels allow the passage of sodium ions within the cell
• Potassium channels allow only potassium ions to pass through them
• Calcium channels are specific for calcium ions only

These are the few examples of protein channels present in membranes.

Proteins maintain the shape and structure of a cell

This is another important cellular function performed by proteins. Cytoskeleton is made up of several interlinked protein filaments. The proteins in the cytoskeleton are organized in the form of microtubules, microfilaments, and intermediate filaments. All these components of the cytoskeleton are arranged in a particular fashion that maintains the shape of a cell. Important proteins that make cytoskeleton include actin and tubulin. In the absence of these proteins, it would not be possible for a cell to maintain its structure. 

Proteins are involved in cell division

Cell division is the process by which a mature adult parent cell divides into daughter cells. Proteins are required for this process also. 

During cell division, the chromosomes of a cell are divided into two halves by pulling apart. This separation of chromosomes is done by proteins known as spindle fibers. 

Proteins are also required for the division of cytoplasm that occurs after the chromosomes have been divided. 

Proteins are required for transport within a cell

Specific transport proteins are needed for intracellular transport of different substances. The different proteins that are involved in intracellular proteins are known as motor proteins. These proteins use energy in the form of ATP and travel along the microtubules to transport various substances within the cytoplasm of a cell. An example of motor proteins is kinesin protein. It is involved in the transport of various substances in axons of neurons. 

Proteins are needed for oxygen transport

This function of proteins is essential for the survival of the body as a whole. Two proteins are involved in this process, hemoglobin and myoglobin. 

Hemoglobin

It is a protein present in the red blood cells. Hemoglobin is made up of four polypeptide chains, two alpha chains and two beta chains,  that are coiled around each other. Each of these polypeptide chains carries one heme group (containing an iron atom). 

This protein is responsible for the transport of oxygen from the lungs to the tissue fluid. One molecule of oxygen can bind to four molecules of oxygen. It binds to oxygen molecules present in the air while passing through the lungs. These oxygen molecules are released when the blood passes through the tissues. 

Any deficiency or abnormality of hemoglobin impairs the oxygen transport by the blood. Our cells cannot survive in the absence of oxygen. Any interruption in oxygen supply will result in cell death in the affected tissues. 

Myoglobin

Myoglobin is another protein involved in the transport of oxygen. It is made up of a single polypeptide chain having a heme group. It is a cytoplasmic protein having a higher affinity for oxygen molecules meaning that it can bind to oxygen even when the concentration of oxygen is high. Its function is to transport oxygen from tissue fluid to the cells.

Because of its high affinity for oxygen, myoglobin releases oxygen at very low concentrations. This feature of myoglobin is responsible for storing oxygen in tissues. 

Proteins are necessary for the transport of various substances in the blood

Although blood acts as a transport medium, proteins are necessary to hold and transport some substances that cannot dissolve in blood. This function of proteins is also essential for the proper functioning of the body. Some examples of transport proteins present in blood are as follows.

• Albumin is the major transport protein in blood. It acts as a carrier for fatty acids, steroids, thyroid hormones, lipophilic drugs, heavy metals, calcium ions, and bilirubin
• Prealbumin is another transport protein in blood that carries steroid hormones, thyroxine, and vitamin A
• Haptoglobin is a transport protein that carries any free hemoglobin that is present in plasma
• Thyroxine binding protein is specific for thyroid hormone
• HDL is a lipoprotein that transports cholesterol from tissues to the liver
• LDL is another lipoprotein that transports cholesterol from the liver to the tissues

Proteins are involved in Muscle contraction

Muscle contraction is the process that enables us to perform our daily life tasks such as walking, running, sitting, standing, writing and even speaking. This process of muscle contraction is also because of proteins. Contractile proteins are present in muscle fibers. These proteins interact in a particular fashion that enables contraction and relaxation of muscles. The most important contractile present are;

• Actin
• Myosin
• Troponin

Proteins prevent edema

Edema is a condition in which excess fluid leaks from the blood vessels and collects in the tissue spaces. The loss of fluid from blood results in decreased blood pressure. It is a potentially lethal condition that can compromise the effective delivery of blood to body tissues. 

Proteins present in blood known as plasma proteins prevent the leakage of fluid through capillaries due to their osmotic effects. The oncotic pressure due to plasma proteins keeps water inside the blood vessels preventing its leakage into the tissue fluids, thus preventing edema. If these proteins are absent, edema develops in different parts of the body. 

Proteins protect our body against diseases

This function is performed by antibodies. Antibodies or immunoglobulins are the plasma proteins that are produced in response to various disease-causing agents entering our body. They fight against these pathogens and prevent our bodies from their harmful effects. If antibodies are already present in our body against a pathogen, they destroy the pathogen before it causes any disease. This process is known as immunity. 

Proteins are needed for digestion

The process of digestion involves breaking down the complex substances present in our diet into simpler ones so that they can be absorbed into the blood. The breakdown of various dietary substances into simpler molecules takes place in our digestive system by enzymes that are proteins in nature. 

Proteins also act as storage substances

Proteins are the polymers of amino acids. They act as storage substances that store thousands of amino acids. These amino acids are released from proteins when needed in the body. Examples of storage proteins are;

• Casein present in milk
• Albumin present in egg

These proteins provide the essential amino acids needed in the body to make several proteins. Moreover, in time of starvation, the proteins present in the body can also be used as an energy source to provide the calories needed for carrying out various body functions. 

Proteins control the expression of Genes

Gene expression is a process by which the information in a particular gene is copied in the form of mRNA and later, this mRNA is used by ribosomes to make the protein coded by that gene. 

This process of gene expression is controlled by transcription factors. These transcription factors allow the transcription of genes of only those proteins that are currently required in the body. 

The transcription factors are also proteins in nature. Thus, proteins regulate their own synthesis by regulating gene expression. 

Summary

Proteins are the polymers made up of amino acids. They are involved in almost all the processes taking place in our body. A summary of the functions performed by proteins is as follows;

• As enzymes, proteins are required for all chemical processes in living organisms
• As hormones and cellular receptors, they are needed for cellular signalling and co-ordination
• As transport channels, protein are needed for the entry of ions and larger-sized particles into the cells
• Being components of cytoskeleton, they maintain the shape of cells
• Spindle fibers are protein fibers that are needed for cell division
• Hemoglobin and myoglobin are the proteins required for oxygen transport
• Albumin and other plasma proteins are needed for the transport of lipids, drugs and other substances in the blood
• Contractile proteins are needed for muscle contraction
• Antibodies are the proteins that protect our bodies from harmful disease
• Plasma proteins maintain fluid balance in our body 
• They regulate gene expression
• Proteins also provide energy to the body in times of starvation

References

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