Nature of Protein:
Proteins are polymers of amino acids. Few proteins exist as single polypeptide chains, whereas others take place as two or more polypeptides related noncovalently. Proteins belonging to later group are known as multisubunit proteins. Proteins may happen as simple proteins, containing only amino acid residues or as conjugated proteins, having permanent related chemical components additionally to amino acids. Non-amino acid part of the conjugated protein is generally referred to as its prosthetic group. Conjugated proteins could be categorized on basis of chemical nature of their prosthetic group. For instance, in lipoprotein, prosthetic group is lipid while in metalloprotein, it is a metal.
There is lot of work gone in proteins to appreciate nature of proteins. Hydrophobic interaction is dominant force which drives proteins to perform biochemical reactions in all living being. Carbon is only element which contributes towards the hydrophobic interaction. Carbon distribution along protein sequence has been calculated by representing protein sequence as the series of atoms instead of amino acids. Provided any length, there is maximum frequency happens at 31.44% of carbon. That is any globular proteins prefer to contain 31.44% of carbon. This is not just in global but also in local. This recently recognized carbon distribution profile is hoped to assist in recognition and growth of active sites, study of protein strength, evolutionary understating of proteins, gene classification and to solve long-standing problem of protein-protein and protein-DNA specific and non-specific interactions.
Properties of Proteins:
Protein is a highly complex substance which is present in all living organisms. Proteins are of huge nutritional value and are directly incorporated in chemical processes necessary for life. Significance of proteins was identified by chemists in early 19th century who coined name for these substances from Greek proteios, means holding first place. Proteins are species-specific; which is, proteins of one species vary from those of another species. They are also organ-specific; for example, within the single organism, muscle proteins vary from those of brain and liver.
Proteins are known to contain the given major properties:
i) Proteins contain varying molecular weights. Molecular weights of insulin, hemoglobin and immunoglobulins are 5700, 68000 and 150000 respectively.
ii. Absorption of ultraviolet light. They strongly absorb U.V. light in region between 200 to 240 nm. This is because of amount of aromatic amino acid residues in them.
iii. They are charged molecules. They have characteristic isoelectric points (pI). They could be influenced by change in pH. Buffers are thus, utilized to stabilize their tertiary structure.
iv. They differ in size and shape. Shape of some proteins is globular while shape of some others is fibrous. This is anticipated as their amino acid sequence and compositions are also fairly different. For instance, albumin is oval while insulin is globular.
v. They have varying solubility property. Different agents like salts, organic solvent like ethanol, heat, heavy metals and organic acids have been utilized to precipitate proteins. Protein stability, thus, could be influenced by any of these agents.
Biological Functions of Proteins:
Proteins founded all over cells of organisms and carry out different biological functions. Functions of few proteins are given below:
i) Melanin- this protein is found in skin and is known to be accountable for skin coloration;
ii) Keratin- is structural protein of nails, hair, horns etc of animals;
iii) Hemoglobin- this is protein pigment of blood which transports oxygen and carbon dioxide in body of animals;
iv) Enzymes- this class of proteins catalyze many bio-reaction happening in cells of living things;
v) Myosin and Actin- are muscle proteins accountable for muscular contraction and relaxation.
vi) Immunoglobulins-these are proteins that give body with defense against infections;
vii) Rhodopsin-this protein is located in retina of eye. It participates in transmission of vision.
viii) Collagen-gives tensile strength to tendons, bones and ligaments.
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