<prompt> Amino acid which acts as a buffer at the physiological pH of 7.4 is:GlycineHistidineLysineArginine  <response>: The pK value of imidazolium group of Histidine is 6.1 and therefore effective as a buffer at the physiological pH of 7.4. The buffering capacity of plasma proteins and hemoglobin is mainly due to histidine residue. Also Know: Paulys test is used to detect the amino acid group Imidazole Histidine. Ref: Textbook of Biochemistry By DM Vasudevan 5th Ed Page 20, 22 