Elementary plant Biochemistry and Biotechnology (2+1)

b) Reactions due to carboxyl group

Decarboxylation
 
• The carboxyl group of amino acids is decarboxylated to yield the corresponding amines. Thus, the vasoconstrictor agent, histamine is produced from histidine.
• Histamine stimulates the flow of gastric juice into the stomach and the dilation and constriction of specific blood vessels.
• Excess reaction to histamine causes the symptoms of asthma and various allergic reactions.
  

Essential amino acids

• Most of the prokaryotic and many eukaryotic organisms (plants) are capable of synthesizing all the amino acids present in the protein. But higher animals including man possess this ability only for certain amino acids.
• The amino acids, which are needed for normal functioning of the body but cannot be synthesized from metabolic intermediates, are called essential amino acids.
• These must be obtained from the diet and a deficiency in any one of the amino acids prevents growth and may even cause death.
• Methionine, Arginine, Threonine, Tryptophan, Valine, Isoleucine, Leucine, Phenylalanine, Histidine, and Lysine are the essential amino acids (Remember MATTVILPHLY)

Peptide

• Amino acids are linked together by formation of covalent bonds. The covalent bond is formed between the α-carboxyl group of one amino acid and the α-amino group of the next amino acid.
• The bond so formed between the carboxyl and the amino groups, after elimination of a water molecule is called as a peptide bond and the compound formed is a peptide.
  
• The peptide formed between two amino acids is a dipeptide; three amino acids is a tripeptide. Few amino acids are an oligopeptide and many amino acids is a polypeptide.
• In writing the peptide structure, the amino terminal (N-terminal) amino acid is written first and carboxyl terminal (C-terminal) amino acid written last.



Peptides of physiological interest

• Glutathione is a commonly occurring tripeptide (ɣ-glutamyl cysteinyl glycine) in many living organisms.
  
• It has a role in detoxification of toxic compounds in physiological system.
• The nanapeptides (nine amino acids), oxytocin and vasopressin are important animal peptide hormones. Oxytocin induces labor in pregnant women and controls contraction of uterine muscle.
  
• Vasopressin plays a role in control of blood pressure by regulating the contraction of smooth muscles. A dipeptide L-aspartyl-L-phenylalanine, is of commercial importance.
  
• This dipeptide is about 200 times sweeter than cane sugar. The methyl ester of this dipeptide is called as aspartame and marketed as an artificial sweetener for diabetics