What is the molecular weight of a protein?

Consider the following sequence for aspartic acid:
$H_3N^+-CH(COOH)-CH_2COOH \leftrightarrow{pK_1=1.88} H_3N^+-CH(COO^-)-CH_2COOH \leftrightarrow{pK_R=3.65} H_3N^+-CH(COO^-)-CH_2COO^- \leftrightarrow{pK_2=9.60} H_2N-CH(COO^-)-CH_2COO^-$
The $pI$ (isoelectric point) of aspartic acid is:

For an amino acid,the ionization constant for the carboxyl group is $pK_{a_1} = 2.34$ and for the ammonium ion is $pK_{a_2} = 9.6$. At what $pH$ will the isoelectric point of the amino acid be?

Serine $(HOCH_2CH(NH_2)COOH)$ is an amino acid. The correct Fischer projection of $L$-serine is:

Identify the amino acids obtained by hydrolysis of the following compound: $NH_2-CH(CH_3)-CO-NH-CH_2-COOH$

The number of disulphide linkages present in insulin are