Lecithin is a major constituent of cell membrane, while sphingomyelin forms myelin sheath of nerves

ethanolamine), and (iii) sphingomyeli. Choline, which contains -CH3 groups is an important constituent of lecithin and sphingomyelin. Lecithin is a major constituent of cell membrane, while sphingomyelin forms myelin sheath of nerves. Lecithin is also a major component of plasma lipoproteins. Fatty liver. Excess fat in the liver. It develops, (i) if fatty acid input in the liver is very high (eg, starvation in man or excess dietary fat in the animal), or (ii) when the liver cannot dispose fatty acids because of lack of hpotropins, which in turn, can result from lack of CH3- groups, that is, lack of choline. CH3- groups cannot be synthesized in our body, so, they have to be supplied through foods. Once CHS- is available, choline can be synthesized. Methionine as well as choline are called methyl donors. Fatty acids can be synthesized in the adipose tissue (mainly) and the liver. For their synthesis, H (from NADH) are required and this supply increases with insulin. 1. Introduction. Structure of proteins : Amino acid structure, D and L form, classification, peptide bond, peptides. Primary, secondary, tertiary structure of protein, Classification of proteins Denaturation of protein Introduction All organic compounds contain carbon and hydrogen; proteins in addition contain oxygen and nitrogen. In addition, some proteins contain sulphur. Of the three proximate constituents of our body (carbohydrate, fat and protein), the protein is the most important one. Following are some of the chief functions of the proteins in our body: all enzymes are proteins. protein is a part ofthe cell membrane structure. structural proteins form the basis of the different organs of our body. contractile proteins, like actin, myosin and others are the basis of muscular contraction. Therefore, they are vita) to cardiac, smooth and skeletal muscular contraction many hormones (eg. insulin, parathormone) are protein in nature. as conjugated protein, they are present in hemoglobin, nucleoprotein and others, where the protein portion is vital for the actions ofthe entire life span. After the expiry of the span, the old molecule of protein is replaced by a new molecule. Structure of proteins A protein is made up of several amino acids. The different amino acids ofthe protein are )inked up to form a poly peptide which is the fundamental basis of protein struc-ture. An amino acid, however, is not a protein, although, a polypeptide is. Amino acid The general formula of the amino acids is, RCHNH2 CO OH, were R is a variable side chain. for example, in glycine, R = H, whereas in alanine, R = CHS and so on. Optical isomerism and stereoisomerism Assuming, the side chain R is other than H (only in glycine R = H; in all others it is other than H) the central carbon atom C is asymmetric carbon atom, as shown below: Therefore, all amino acids (excepting glycine) are optically active and belong to either (+) or (-) series. Amino acids also show stereo isomensm and hence an amino acid can be either D or L ammo acid. All naturally occurring amino acids in our body which are used for protein synthesis belong to L series but some naturally occurring amino acids are levorotatory (-) while others are dextrorotatory (+). Further, most amino acids (see table 7.11.2) have NH2 group at a position. Thus, as an example, aianine is a amino propionic acid; again, alanine is L (+) ammo acid; it therefore, may be expressed as L (+) alanine . Classification of amino acids Usually, amino acids are classified into two major groups, viz. (i) aliphatic amino acids, (ii) aromatic and heterocyclic amino acids. Aliphatic amino acids are divided into four subgroups, viz , (i) monoamino monocarboxylic amino acids, (ii) monoamino dicarboxylic amino acids, (iii) diamino monocarboxylic ammo acids and (iv) sulphur containing amino acids. Table 7.11.2 shows the trivial (popular) names, chemical names and structural formule of the amino acids found in the human body. The common abbreviated forms (3 letter code) are also shown. There are 20 amino acids in the human proteins. There are some amino acids (for example, ornithine, citrulline, homocysteine) which perform important functions and yet are not part of the proteins in our body In the plants, many other amino acids also occur Peptide bond Consider the two amino acids, alanine (CH3 CHNH2 COOH) and glycine (CH2NH2COOH). These two amino acids can combine to form 'alanylAlycine', as follows : Or in other words, amino acids can conjugate with other amino acids and the junctional bridge between two such conjugated amino acids, viz, CO:NH, is called a peptide bond. Peptides Thus, alanine and glycine conjugate to form alanyl glycine. Alanylglycine is a peptide. As it contains two amino acids, it is a dipeptide. Similarly, a tripeptide contains three amino acids. A polypeptide contains several aminoacids. Usually, the term polypeptide is used where the number of amino acids in the peptide chain is within 50, whereas if this exceeds 50, it is called protein. Primary and secondary structures of protein A protein molecule may consist of one or several polypeptide chains. Sequence of amino acids in a polypeptide chain is called primary structure of protein, whereas the spatial configuration of the protein molecules is called the secondary structure. Thus, a polypeptide chain (primary structure) may have a helical (secondary structure) pattern. The term, tertiary structure refers to the ultimate shape of the molecule in a three dimensional space. Thus the tertiary structure of the protein of Hb, is orange like (globular), some other protein has a cigar like appearance, and so on. Classification of
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