Which one of the following is an essential amino acid?

Which of the following structures of proteins represents its constitution?

In an amino acid,the carboxylic group ionises at $pH = 2.56$ $(pK_{a_1})$ and the ammonium ion ionises at $pH = 9.38$ $(pK_{a_2})$. The isoelectric point of the amino acid is at:

In a linear tetrapeptide (constituted with different amino acids),the value of (number of amino acids) - (number of peptide bonds) is:

The major product formed in the following reaction is:

The prosthetic group of haemoglobin is