Proteins: Classifications Properties and Functions

Proteins are complex, organic compounds composed of many amino acids linked together through peptide bonds and cross-linked between chains by sulfhydryl bonds, hydrogen bonds and van der Waals forces.Protein, highly complex substance that is present in all living organisms. There is a greater diversity of chemical composition in proteins than in any other group of biologically active compounds. The proteins in the various animal and plant cells confer on these tissues their biological specificity.Proteins are of great nutritional value and are directly involved in the chemical processes essential for life. 

Classification of proteins

Proteins can be classified as:

• Simple proteins. On hydrolysis they yield only the amino acids and occasional small carbohydrate compounds. Examples are: albumins, globulins, glutelins, albuminoids, histones and protamines.

1. Conjugated proteins. These are simple proteins combined with some non-protein material in the body. Examples are: nucleoproteins, glycoproteins, phosphoproteins, haemoglobins and lecithoproteins. Glycoproteins

They are proteins that covalently bind one or more carbohydrate units to the polypeptide backbone.
Typically, the branches consist of not more than 15-20 carbohydrate units, where you can find arabinose, fucose (6-deoxygalactose), galactose, glucose, mannose, N-acetylglucosamine (GlcNAc, or NAG), and N-acetylneuraminic acid (Neu5Ac or NANA).
Examples of glycoproteins are:
glycophorin, the best known among erythrocyte membrane glycoproteins;

fibronectin
 They  anchors cells to the extracellular matrix through interactions on one side with collagen or other fibrous proteins, while on the other side with cell membranes;
all blood plasma proteins, except albumin;
immunoglobulins or antibodies.

Chromoproteins

They are proteins that contain colored  prosthetic groups. Typical examples are:
hemoglobin and myoglobin, which bind, respectively, one and four heme groups;
chlorophylls, which bind a porphyrin ring with a magnesium atom at its centre;
rhodopsins, which bind retinal.



Phosphoproteins
They are proteins that bind phosphoric acid to serine and threonine residues.
Generally, they have a structural function, such as tooth dentin, or reserve function, such as milk caseins (alpha, beta, gamma and delta), and egg yolk phosvitin.

• Derived proteins. These are proteins derived from simple or conjugated proteins by physical or chemical means. Examples are: denatured proteins and peptides.

Fibrous protein(Scleroprotein): We can find these proteins in animals and are insoluble in water.  Fibrous proteins are found in hair, muscle and connective tissue. Fibrous proteins are resistant to proteolytic enzymes and are coiled and exist in threadlike structures to form fibres. e.g. collagen, actin, and myosin, keratin in hair, claws, feathers, etc.

Globular proteins: Globular proteins are found in blood and tissue fluids in amorphous globular form with very thin or non-existent membranes. Collagenous proteins are found in connective tissue such as skin or cell membranes. These proteins, unlike fibrous proteins are soluble in water. They are made up of polypeptides that are coiled about themselves to form oval or spherical molecules e.g. albumin, insulin, and hormones like oxytocin, etc.

Crystalline proteins: These proteins are exemplified by the lens of the eye and similar tissues. Enzymes are proteins with specific chemical functions and mediate most of the physiological processes of life. Several small polypeptides act as hormones in tissue systems controlling different chemical or physiological processes. Muscle protein is made of several forms of polypeptides that allow muscular contraction and relaxation for physical movement.

Properties of proteins

• Proteins can also be characterized by their chemical reactions. Most proteins are soluble in water, in alcohol, in dilute base or in various concentrations of salt solutions. 
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• Proteins have the characteristic coiled structure which is determined by the sequence of amino acids in the primary polypeptide chain and the stereo configuration of the radical groups attached to the alpha carbon of each amino acid. 
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• Proteins are heat labile exhibiting various degrees of lability depending upon type of protein, solution and temperature profile. 
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• Proteins can be reversible or irreversible, denatured by heating, by salt concentration, by freezing, by ultrasonic stress or by aging. 
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• Proteins undergo characteristic bonding with other proteins in the so-called plastein reaction and will combine with free aldyhyde and hydroxy groups of carbohydrates to form Maillard type compounds.

Functions of Proteins

• Structural functions: Proteins are called as the building blocks of the body. They are an essential component of various structures in the cell and tissues. We also find these proteins in the outer membrane of all cells in the human body. We can also find structural proteins in hair, skin, and muscles. Proteins often act to strengthen these structures. Proteins working together can allow movement within the body, such as contraction of muscles and movement of food through the digestive system etc. They are needed for the growth, development, healing, and repair of tissues.

• Protective: Proteins are the main constituent of antibodies that protect our body against antigens and pathogens thus preventing infections.

• Hormonal regulation: Hormones are majorly composed of proteins. Hormones play a vital role in regulating muscle mass, sex hormones, and growth and development.

• Enzymes: Proteins are called as biological buffers because they, as enzymes, regulate many different biochemical reactions that are occurring in the body.

• Protein classification based on chemical composition
• On the basis of their chemical composition, proteins may be divided into two classes: simple and complex.