Proteins are polymers of amino acid molecules which are bonded with each other through peptide linkage. We know that amino acids are organic molecules with two functional groups on the same carbon atom. These two functional groups are carboxyl group and an amino group. That carbon atom is called as an alpha carbon atom. This alpha carbon atom is also bonded with one hydrogen atom with one side chain that is represented as ‘R’.
The amino acids involve in the formation of a peptide linkage with similar or different molecules. The peptide linkage is an amide bond that is formed by the condensation polymerisation of amino group of one molecule and the carboxyl group of another molecule with the elimination of water molecules. This is an amide bond with structure –CO-NH-. Such type of amino acids is called as alpha amino acids. The presence of –NH2 and –COOH is common in all alpha amino acids, only the side chain vary from molecule to molecule. The side chain effect the polarity of a molecule because –NH2 group and –COOH on an alpha carbon atom can neutralise each other. On the basis of polarity, amino acids can be classified as polar and non-polar amino acids. Polar amino acids can again classify as positively charged and negatively charged amino acids. Let’s discuss different types of polar amino acids with examples.
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Non Polar Amino Acids have equal number of amino and carboxyl groups and are neutral. These amino acids are hydrophobic and have no charge on the 'R' group. The amino acids in this group are alanine, valine, leucine, isoleucine, phenyl alanine, glycine, tryptophan, methionine and proline.
These are some of the structures of non polar amino acids.
These amino acids do not have any charge on the 'R' group. These amino acids participate in hydrogen bonding of protein structure. The amino acids in this group are serine, threonine, tyrosine, cysteine, glutamine and aspargine.
Polar amino acids with positive charge have more amino groups as compared to carboxyl groups making it basic. The amino acids, which have positive charge on the 'R' group are placed in this category. They are lysine, arginine and histidine.
Polar amino acids with negative charge have more carboxyl groups than amino groups making them acidic. The amino acids, which have negative charge on the 'R' group are placed in this category. They are called as dicarboxylic mono-amino acids. They are aspartic acid and glutamic acid.
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