The name protein (proteios Greek  = pre-eminent or first) was first suggested, in 1838, by a Swedish chemist Berzelius to a Dutch chemist Mulder, who referred it to the complex organic nitrogenous substances found in the cells of the living beings. They are naturally occurring nitrogenous polymers of different alpha-amino acids linked by peptide (—CONH) linkage. On hydrolysis, they give a mixture of alpha-amino acids. Thus proteins are biopolymers of alpha-amino acids. They contain carbon, hydrogen, nitrogen, sulphur, and oxygen as constituent elements. They may contain cobalt, manganese, zinc, iron, copper, etc.

They are present in animals as well as plants. In the animal kingdom, it occurs in forms such as silk, wool, hair, nail, skin, haemoglobin of blood, and blood plasma. In the plant kingdom, it occurs in high concentration in seeds. Their sources are pulses, milk, eggs, fish, meat, etc. They are important because they regulate metabolic processes. They are essential constituents of all living matter.

Classification of Proteins:

Classification on the Basis of Composition:

Simple proteins:

They on acid hydrolysis give only alpha-amino acids. e.g.

• Albumins: egg albumin, Serum albumin
• Globulins: Tissue, vegetable,  and Serum globulin
• Glutenins: Glutenin in wheat.
• Protamines: Occur in the nucleic acid.

Conjugated proteins:

They on hydrolysis (acids or alkalies or enzymes) give alpha-amino acids and non-protein group. e.g.

• Glycoproteins: Contain carbohydrate as a prosthetic group. e.g. egg white, mucin of saliva
• Nucleoproteins: Contain nucleic acid as a prosthetic group. e.g. components of viruses, chromosomes, and ribosome structures.
• Chromoproteins: Contain chlorophyll as a prosthetic group. e.g. Haemoglobin
• Phosphoproteins: Contain Phosphoric acid as a prosthetic group. e.g. Casein of milk and vielline of egg yolk.
• Lipoproteins: Contain fats as a prosthetic group. e.g. membrane structure, lipids transported in the blood.
• Flavoprotein: Contain flavine Adenine Dinucleotide (FAD) as a prosthetic group. This protein is important in the electron transport chain in respiration.
• Metal-protein: Contain metal as a prosthetic group. e.g. the protein in plants nitrate reductase, which converts nitrates into nitrites.

Derived proteins:

Natural proteins undergo structural change because of heat, chemical reagents, enzymes, acids, or alkalies and form degraded products of proteins (derived).

The flow is Proteins → Proteoses → Peptones → Polypeptides → Simple peptides → amino acids

Classification Based on Structure:

Globular proteins:

• They have a globular or elliptical shape. They are also called as spheroproteins.
• They are soluble in water, acids, and bases. They form a colloidal solution with water.
• They are made up of not only primary, secondary but also tertiary and occasionally quaternary structures.
• They form enzymes, antibodies, and some hormones (insulin), etc. They are needed for the formation of chemical messengers like hormones in the body. They are essential for the formation of transporters of other particles through the membrane.
• e.g. egg albumin (egg white), Casein in milk

Fibrous proteins:

• They have a fiber-like structure. They are also called as scleroproteins.
• They are elongated strand-like structures and are usually present in the form of rods or wires.
• They are insoluble in water, weak acids, and weak bases but soluble in strong acids and alkalis.
• They have primary and secondary structures. They are made up of a single unit or structure which is repeated multiple times.
• They perform a structural function in the cell. They are needed for the formation of tough structures like connective tissue, tendons, and fibers of the muscle.
• e.g. collagen, elastin, keratin of hair, Nails, horn, feathers. fibroin of silk.

Classification Based on Functions:

Enzymic Proteins.

• They are the most varied and most highly specialized proteins and shows catalytic activity. Almost all enzymes are globular proteins.
• Enzymes catalyze a variety of reactions.
• Examples: Urease, amylase, catalase, cytochrome C, alcohol dehydrogenase

Structural Proteins.

They are usually inert to biochemical reactions. They maintain the native form and position of the organs. The cell wall and primary fibrous constituents of the cell have structural proteins.

• Collagen: It is found in connective tissue such as tendons, cartilage, a matrix of bones, and the cornea of the eye. Leather is almost pure collagen.
• Elastin: It is found in ligaments. It is capable of stretching in two dimensions.
• Keratin: It constitutes almost the entire dry weight of hair, wool, feathers, nails, claws, quills, scales, horns, hooves, tortoiseshell, and much of the outer layer of skin.
• Fibroin: It is the major component of silk fibres and spider webs.
• Resilin: The wing hinges of some insects are made of resilin, which has nearly perfect elastic properties.

Transport or Carrier Proteins:

• Certain proteins, especially in animals, are involved in the transport of many essential biological factors to various parts of the organisms.
• Hemoglobin of erythrocytes carries oxygen to tissues. The blood plasma contains lipoproteins, which carry lipids from the liver to other organs. Ceruloplasmin transports copper in the blood.

Nutrient and Storage Proteins:

Ovalbumin is the major protein of egg white. The milk protein, casein stores amino acids. The seeds of many plants store nutrient proteins, required for the growth of the germinating seedlings. Ferritin, found in some bacteria and in plant and animal tissues, stores iron.

Contractile or Motile Proteins:

• They give an ability to contract, move about, and change shape to cells. Actin and myosin function in the contractile system of skeletal muscle and also in many nonmuscle cells.
• Microtubules are built up of Tubulin.

Defense Proteins:

• They defend organism against invasion by other species or protect them from injury.
• Immunity: The antibodies (or immunoglobulins), the specialized proteins made by the lymphocytes of vertebrates, can precipitate or neutralize invading bacteria, viruses or foreign proteins from another species.
• Blood Clotting: Fibrinogen and thrombin, although enzymic, are blood-clotting proteins that prevent loss of blood when the vascular system is injured.

Regulatory Proteins:

• They regulate the cellular or physiological activity. They are called hormones.
• For example, insulin regulates sugar metabolism and growth hormone which is required for bone growth.
• The cellular response to many hormonal signals is often mediated by a class of GTP-binding proteins called G-proteins.
• Some of them bind to DNA and regulate the biosynthesis of enzymes and RNA molecules involved in cell division.

Toxic proteins.

Snake venom, bacterial toxins, and toxic plant proteins are toxic. They have defensive functions.

Science > Chemistry > Biomolecules > Proteins and Their Classification

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When a human blood sample is prevented from clotting and spun in a test tube (centrifuged), in a machine called a centrifuge, the blood separates into a   straw coloured liquid called plasma and a dark brown mass of blood cells. The lower layer consists of white blood cells, blood platelets, and red blood cells. Collectively, these are the formed elements, which make up about 45% of the total volume of whole blood; the percentage of blood attributed to red blood cells is called the hematocrit. The hematocrit is defined as the percentage of blood volume that is occupied by erythrocytes. The normal hematocrit is approximately 45 percent in men and 42 percent in women.

The upper layer is plasma, which contains a variety of inorganic and organic molecules dissolved or suspended in water. Plasma accounts for about 55% of the total volume of whole blood.

Composition of Plasma:

Plasma is the straw-coloured non-living, liquid part of blood. It makes up about 55 – 60% of blood volume and 5.5 % of body weight. Blood corpuscles and platelets are suspended in it. Blood without clotting factor is called serum. The characteristic straw color of plasma is due largely to a waste product of hemoglobin breakdown called bilirubin.

It is the liquid portion of blood, and about 92% of plasma is water. The remaining 8% of plasma is composed of various salts (ions) and organic molecules. The salts, which are dissolved in plasma, help maintain the pH of the blood. Small organic molecules such as glucose, amino acids, and urea are also dissolved in plasma. The large organic molecules in plasma include hormones and the plasma proteins.

Water:

Plasma is composed of about 90 to 92% water. Acts as a solvent and suspending medium for blood components.  

Plasma Proteins:

Plasma proteins or serum proteins constitute 6 to 8% of plasma. Important plasma-proteins are. The plasma proteins constitute, by weight, most of the plasma solutes. They can be classified, according to certain physical and chemical reactions, into three broad groups: the albumins and globulins, and fibrinogen, which function in blood clotting. Most plasma proteins are made in the liver. An exception is the antibodies produced by B lymphocytes, which function in immunity. Certain hormones are plasma proteins made by various glands.  It must be emphasized that the plasma proteins normally are not taken up by cells; cells use plasma amino acids, not plasma proteins, to make their own proteins. Plasma proteins must be viewed quite differently from most of the other organic constituents of plasma, which use the plasma as a medium for transport to and from cells. In contrast, most plasma proteins perform their functions in the plasma itself or in the interstitial fluid.

• Fibrinogen and prothrombin: It constitutes 4% of the plasma proteins and required for blood clotting.
• Serum albumin: The albumins are the most abundant of the three plasma protein groups and are synthesized by the liver. It makes up 58% of the plasma proteins. They are partly responsible for blood viscosity, the regulation of water movement between tissues and blood and osmotic pressure; acts as a buffer; transports fatty acids,
  free bilirubin, and thyroid hormones.
• Globulins or Gamma globulins or Immunoglobins (Ig): It accounts for 38% of the plasma proteins. They act as antibodies and are associated with the defence mechanism of the body. They Transport lipids, carbohydrates, hormones, and ions like iron and copper; antibodies complement are involved in immunity

Inorganic Salts and Ions (Minerals):

They form 1-2 % of the plasma and includes electrolytes like Sodium, potassium, calcium, magnesium, chloride, iron, phosphate, hydrogen, hydroxide, bicarbonate. They are involved in osmosis, membrane potentials, and acid-base balance.

Dissolved Nutrients:

Glucose, lipids, vitamins, fatty acids, amino acids, and cholesterol. They act as sources of energy and basic “building blocks” of more complex molecules. Vitamins Promote enzyme activity.

Hormones and Enzymes:

Blood acts as the transport system for the transportation of regulatory substances called hormones secreted by different glands.  Thus plasma contains hormones and enzymes. Enzymes catalyze chemical reactions; hormones stimulate or inhibit many body functions.

Dissolved Gases:

Oxygen, It is necessary for aerobic respiration; terminal electron acceptor in an electron-transport chain, Carbon dioxide, a Waste product of aerobic respiration; as bicarbonate, helps buffer blood and nitrogen.

Excretory Substances:

Ammonia, urea, uric acid, creatine, and creatinine. Urea, uric acid, creatinine , and ammonia salts  are the breakdown products of protein metabolism; excreted by the kidneys. Bilirubin is the breakdown product of red blood cells; excreted as part of the bile from the liver into the intestine. Lactic acid is the end product of anaerobic respiration; converted to glucose by the liver.

Functions of Plasma:

Transport Nutrients:

Delivering nutrients to the body is a critical function of the circulatory system. Plasma of the blood is the carrier of all nutrients. After food is digested and assimilated, its component nutrients like carbohydrates, proteins, minerals, fats, and vitamins are absorbed into the bloodstream. Each of these nutrients is vital for healthy body function.

Transport of Waste Products:

The plasma collects metabolic waste products like urea, creatinine, and other chemical wastes and toxins and transports them to the liver, kidneys, skin, and lungs (excretory organs) for elimination from the body.

Transport of Hormones:

Hormones are chemical messengers produced by endocrine glands that affect distant organs. Hormones are released into the bloodstream through which they travel to target sites. The plasma collects the hormones from the endocrine glands and serves as the transportation connection between the glands and the organs or tissues.

Transport of Other Products:

Albumin transports the molecule bilirubin, a breakdown product of hemoglobin. Lipoproteins, whose protein portion is a globulin, transport cholesterol.

Body Temperature Regulation:

Plasma Picks up excess body heat from the deep-seated heat-producing organs and brings it to the skin to be excreted. If body temperature drops, surface blood vessels constrict(decrease in size) to conserve body heat. Thus it  helps in regulating the body temperature

Disease Protection and Healing:

There are three types of globulins, designated alpha, beta, and gamma globulins. The alpha and beta globulins, produced by the liver, bind to metal ions, to fat-soluble vitamins, and to lipids, forming the lipoproteins. Antibodies, which help fight infections by combining with antigens, are gamma globulins. The immunoglobins of plasma act as antibodies and attack the foreign intruder in the body. They neutralize these harmful foreign agents. Thus plasma is responsible for the immunity of the body.

Fibrinogen present in the plasma is responsible for clotting of blood which is important for stopping the blood flow from the wounds.

Maintain Haemostasis and Osmoregulation:

Plasma supplies water to different tissues at the same time and removes excess of water produced during metabolic activities. Thus it maintains water balance in the body. Osmotic pressure is a force caused by a difference in solute concentration on either side of a membrane. The plasma proteins, particularly the albumins, contribute to the osmotic pressure, which pulls water into the blood and helps keep it there.

Acid-Base Buffer:

Plasma proteins act as acid-base buffers and maintain blood pH within a range. Plasmaproteins are able to take up and release hydrogen ions; therefore, the plasma proteins help buffer the blood and keep its pH around 7.40.

Disorders Related with Blood Plasma:

Oedema or Edema:

Oedema is swelling that occurs when too much fluid becomes trapped in the tissues of the body, particularly the skin. It most often occurs in the skin, especially on the hands, arms, ankles, legs, and feet. However, it can also affect the muscles, bowels, lungs, eyes, and brain. It usually starts slowly, but the onset can be sudden.

In case of a person suffering from protein deficiency, a fall in plasma protein leads to escape of excess volume of water from the blood to tissues. Due to excess of fluid of fluid in tissues causes swelling of feet. The state is called oedema.  The condition mainly occurs in older adults and women who are pregnant. Symptoms include skin that retains a dimple after being pressed for a few seconds,  puffiness of the ankles, face, or eyes, higher pulse rate and high blood pressure.

Diuretics are a type of medication. They help get rid of excess fluid by increasing the rate of urine production by the kidneys.

Cholesterol:

Cholesterol is present in plasma. Cholesterol has a tendency to deposit on the walls of blood vessels leading to a condition called atherosclerosis. The liver is responsible for producing and clearing cholesterol in the body.

Dietary cholesterol increases plasma total cholesterol concentrations in humans. There is a relationship between increased plasma cholesterol concentrations and cardiovascular disease risk. Dietary guidelines have consistently recommended to such person limiting food sources of cholesterol. Potential sources of dietary cholesterol are limited to animal foods; eggs, dairy products, and meat.

LDL (low-density lipoprotein) cholesterol is also called “bad” cholesterol. LDL can build up on the walls of arteries and increase the chances of getting heart disease. HDL (high-density lipoprotein) cholesterol is also called “good” cholesterol. HDL protects against heart disease by taking the bad cholesterol out of the blood and keeping it from building up in arteries. Along with cholesterol, triglycerides form plasma lipids. Excess triglycerides in plasma have been linked to the occurrence of coronary artery disease in some people.

Everyone over the age of 20 should get their cholesterol levels checked at least once every 5 years by a test called “Lipid profile”. Everyone over the age of 40 should get their cholesterol levels checked at least once a year.

Lifestyle changes such as exercising and eating a healthy diet are the first line of defence against high cholesterol. The choice of medication for high cholesterol depends on individual risk factors, age,  current health and possible side effects. Common choices include Statins, Bile-acid-binding resins, Cholesterol absorption inhibitors, Injectable medications. The choice of medication for high triglycerides is Fibrates, Niacin, Omega-3 fatty acid supplements.

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