Functions of proteins

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Proteins are very important macromolecules with essential functions in the body.

Proteins make up to 50% of the dry body weight and have specific functions. All the activities like structural support, growth, respiration, digestion, energy production, osmoregulation are brought about by proteins. The chemical structures and chemical compositions regulate the roles of proteins.

Proteins in structural support

The structural proteins form the basic matrix of structural supports and protective covers.
Keratin, elastin and collagen are a few examples these types, carrying out the supportive and protective role.
Keratins (α and β) are the primary constituent of growths and structures of skin like hair (also wool), nails (also claws and hoofs), horns and beaks.
Elastin is very important for pressure wave propagation functions in arterial walls, especially in large elastic blood vessels like aorta.
Elastin contributes to the elasticity of skin, lung, bladder and elastic cartilage.
Collagen is fibrous, mostly found outside the cells and forms the extracellular matrix giving structure to the animal body.
It is the main component of the matrix of bone, cartilage, tendons, ligaments, fascia and skin.

Functions of proteins in movement and contraction

All our body movements are brought about by movement of muscles.
Muscles are categorized as skeletal muscle (bringing about skeletal movement, locomotion and maintenance of posture), smooth muscle (bring about involuntary movements like, movement of intestine, urinary bladder, stomach, esophagus, intestines, blood vessels) and cardiac muscle (for beating of the heart).
All these muscles are mainly made of muscle cells.
Muscle cells contain myofibrils which are organised into sub-units called sarcomeres.
The filaments of myofibrils are constructed from contractile proteins.
Thin type of myofibrils consists mainly of actin and the thick type of myofibril consists mainly myosin.

Proteins in control of growth differentiation

There are proteins having a regulatory role like growth differentiation.
Growth differentiation factors (GDF) are named GDF1 to GDF15.
These factors regulate complex processes in ovulation, embryonic development, muscle development (GDF8-myostatin) and inflammatory and apoptotic pathways (GDF15) during diseases and injuries.

Functions of proteins in gene expression

The messages and codes in the genes are not totally expressed at the same time.
They are switched on and off in respect to the phase of growth, organ under concern and biological timetable.
In expression of DNA and cell differentiation repressorproteins bind to specific DNA segments and prevent the formation of the particular product of that segment.
These DNA-binding proteins are coded by regulator genes.
The segment bound by repressor is called operator.
By binding to operator, the repressor prevents the creation of messenger RNA by the RNA polymerase.

Functions of proteins in tissue repair and maintenance

Continuous supply of amino acids and building of proteins is necessary for the tissue growth, repair and maintenance.
Constantly new tissues of hair, skin, nails and teeth are added to our body to replace the dead cells which have sloughed off.
Epidermis and blood cells have a life of about one month and are to be constantly replaced.
The cell in the gastro-intestinal system lining last only for 2 weeks.
There is an elaborate process of tissue regeneration all over the body with their use.
In special circumstances like injury and damage, increased activity of cell formation is induced and new tissue is formed.

Functions of proteins as hormones

These are messengers helping in regulating certain body chemical activities.
Many categories of hormones are secreted by endocrine glands like thyroid gland, pancreas, adrenal gland, ovary, testis and thymus gland for carying out many vital biological actities.
Complex hormones like melatonin and seratonin affect the sleep and mood whereas thyroid hormones affect the basal metabolic rate and synthesis of proteins.
Insulin regulates glucose utilisation whereas a myriad hormones regulate reproduction.

Functions of proteins as enzymes and catalysts

Proteins as enzymes and catalysts facilitate chemical reactions in the body.
Enzymes are large structures having essential kinetic functions in bringing about all biochemical reactions.
The functions of the gastrointestinal system are facilitated by different enzymes secreted for digestion of proteins, fats and carbohydrates.

Functions of proteins as antibodies and immune protection

Antibodies are proteins and are responsible for binding to specific foreign bodies and make them harmless, ineffective and eliminate them.
There are two types of main functions for immunoproteins made and used by the body, namely innate immune system and adaptive immune system.
In innate immune system the innate response is triggered by pattern recognition receptors (proteins) which identify pathogen-associated molecular patterns (PAMPs), which are associated with cellular stress or microbial pathogens. Cytokines (like interleukins, chemokines, interferons) which are glycoproteins are released to combat the antigen in various ways.

The Interleukins (secreted proteins /signaling molecules) create communication between white blood cells (leukocytes). Chemokines (chemotactic cytokines) are secreted by cells.
Their functions are to promote chemotaxis (chemically induced movement) of the nearby responsive cells towards the site of antigen.

Interferons (IFNS) have many antiviral activities like shutting down the synthesis of proteins in the host cells. In the functions of innate immune system, a complement system containing over 20 different proteins starts a biochemical cascade of attacking the surfaces of foreign cells.
This complements the functions of antibodies in killing the pathogens. In the innate immune system, white blood cells (leukocytes) like 'natural killer cells' which are cytotoxic lymphocytes kill the tumors and virus infected cells by releasing small cytoplasmic granules called perforin (cytolytic proteins) and granzyme (serine proteases) causing the target cell to die by apoptosis (programmed cell death).

Adaptive immune system has stronger immune response as well as immunological memory.
Each pathogen is remembered by signature antigen protein.
The responses are generated tailor made to specific antigen.

When a pathogen infects more than once the memory cells are quickly used to eliminate it.
Lymphocyte B cells and T cells carry receptor molecules which can recognize particular target antigen. Cytotoxins such as perforin are released into the target cell inducing it to undergo apoptosis.

Functions of proteins in Fluid and electrolyte balance and osmoregulation

Globulins and albumins are the important blood proteins molecules doing the functions of maintaining the fluid and electrolyte balance between the cells and the extracellular space.
The proteins present in the capillary beds (blood vessels of one-cell thickness) pull the blood fluid from the tissues into the capillary beds.
This osmotic pressure exerted by proteins is called colloid osmotic pressure or oncotic pressure.
These functions protects the body from fluid retention and edema.
If the blood plasma levels of proteins are reduced due to proteinuria (loss through urine) or as a result of malnutrition, oncotic pressure is reduced and fluid retention occurs leading to many health complications.
Proteins are abundant in biological membranes.
Cellular membranes form channels and pumps regulating fluids and ions inside and outside the cells.
Many cellular channels allow ions to move from higher concentration to lower concentration.
However for forcing ions from lower concentration to higher concentration (pumps-active transport) energy in the form of adenosine triphosphate (ATP) are required.
Proteins being amphoteric, perform the functions of buffering and maintaining the optimum blood pH.

Functions of proteins in transport and storage

One of the important functions of Proteins is the transport of essential molecules from one part of the body to other.

Functions of hemoglobin and myoglobin are primarily in transport of oxygen to the tissues.

Transferrin is transporter of iron and helps in the storage of iron in the liver.

Cytochrome C functions as electron transport and albumin transports fatty acids in the bloodstream.

Ovalbumin are storage proteins found in the egg white providing nutrition to the embryo.

Functions of proteins in metabolism and energy production

Though proteins are not primary source of energy, in the event of shortage of body fuels like carbohydrates and fats, they are utilized in energy production functions.
They provide the nitrogenous base adenine for ATP and also creatine phosphate in the cell metabolism functions.
Some amino acids by the process called gluconeogenesis are degraded to pyruvate and glucose for production of energy.
Some of the amino acids are reduced to ketone bodies (acetoacetate) which can be metabolised for energy by muscle and brain when blood glucose levels are low.

Functions of proteins as receptors and neurotransmitters

Many proteins do the functions of signal detection and its conversion into another type of signal.
The functions of proteins Rhodopsin in light detection and conversion of the photo-signals into neurosignals is well known.
Proteins play many important functions in generation as well as transmission of nerve impulses.
Many monoamines like dopamine, epinephrine, norepinephrine, serotonin, histamine, melatonin have neurotransmitters functions.
Many neuropeptides have been discovered and 'neuropeptide Y' is one of them having wide range of neurological functions in food intake, obesity and anorexia nervosa.
Acetylcholine receptor (AChR-an integral membrane) is one of the neuroreceptor proteins.
It is embedded in the 'postsynaptic density (PSD)' region at the membrane of postsynaptic neuron. These proteins function as neurotransmitter receptor.

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Functions of proteins