question_answer

  Explain the terms primary and secondary structure of proteins. What is the difference between a-helix and p-pleated sheet of proteins?                                                                                                                                                          [3]

Answer:

  Structure of proteins The structure of proteins is much more complex than that of any of the families of organic compounds studied. This structural complexity arises from the fact that proteins are such large   molecules,   often   referred   to   as macromolecules. A protein may consist of just a single strand, or it may include several strands. A protein may have just a few a-amino acids or it may have thousands.

S. No.	Protein	Structure-type	Number of amino acids
1.	Oxytocin	Single-stranded protein	9amino acids
2.	Insulin	Two-chain protein Single-chain protein	51amino acids 153
3.	Myoglobin	Single-chain protein	153amino acids
4.	Gamma globulin	Four-chain protein	1320amino acids

There  are four levels of protein structure that are important in understanding how proteins function. The  simplest two-dimensional  description  of connectivity is called the primary structure. The three-dimensional gross structure-specifically, the secondary, tertiary and quaternary structure-describes how protein molecules are aggregated in order to carry out their specific functions. Primary structure The most basic aspect of protein structure is the primary structure-a specific sequence in which the a-amino acids are joined together by peptide bonds. Primary structure is represented using the three-letter abbreviations for the a-amino acids. Primary structure of insulin has two chains of insulin held together by two disulphide (?S?S?) linkages.                                                   [1] Secondary structure How a protein chain is arranged or oriented in three-dimensional space is described by secondary structure. Resonance delocalisation within the amide (peptide) group  causes  restricted  rotation  about the nitrogen-carbon bond because it acquires partial double bond character. The NH ? C = O grouping lies in a plane with bond angles of about 120°. The carbon atoms to which the nitrogen and the carbonyl carbon are attached (a-carbon of both amino acids) must also lie in the same plane, Therefore, the four atoms of the peptide backbone (?C?N?C?C?) lie in a plane, but there are two variants. There is free rotation about the single bonds in a peptide. Therefore, following are the rotational options for the peptide. The secondary structure of a protein is generally held in place by hydrogen bonding between the ?NH group of one amide and the C = O group of another amide. Because a protein chain contains a large number peptide bonds, there is extensive hydrogen bonding between peptide linkages of two separate chains or between peptide linkages within a single chain. hydrogen bonding between ?NH group of one amide and C = O group of another amide Secondary structure can be in the form of   [1] 1.Helix In which a protein chain is coiled about a central axis. It is held in place by hydrogen bonding that occurs between the hydrogens of ?NH groups and oxygens of C = O groups that are oriented toward each other on the inside of the helix. It takes 3.6 amino acids to complete one turn of the helix to enable such hydrogen bonding and a 13-member ring is formed by H-bonding. a-helix is also known as 3.6,3 helix. 2. Pleated sheet In which two protein chains are lined up parallel to each other but running in opposite directions ?that is, the N-terminus of one chain is near the C- terminus of the other. This puts the ?NH group of one chain opposite the C = O group of the other chain, permitting hydrogen bonding that holds the sheet together. Examples are given below

Helix	Pleated sheet
keratin	Fibroin
Myosin	Protein of silk
Actin	Dragline of silk (spider wave)