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Biochemistry

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Moderate

Determining the Charge on a Polypeptide

BIOCHM-KLEYKC

Consider the following polypeptide and the table of pK values to determine which of the following choices correctly states the net charge on the polypeptide at the given pH.

Glu – Gly – Arg – Leu – Gln – Lys

Amino Acid ${α-COOH}$ $α-NH_{3}^{+}$ R-Group
$pK_{1}$ $pK_{2}$ $pK_{R}$
Gly 2.34 9.60 -
Arg 2.17 9.04 12.48
Lys 2.18 8.95 10.53
Tyr 2.20 9.10 10.10
Cys 1.80 10.80 8.30
His 1.82 9.17 6.00
Glu 2.19 9.67 4.25
Asp 1.88 9.60 3.65
A

At pH = 1, the net charge on this polypeptide equals +5.

B

At pH = 6, the net charge on this polypeptide equals +3.

C

C. At pH = 10.5, the net charge on this polypeptide equals -4.

D

If His is joined by​ a peptide bond to the C-terminus, the net charge of the new polypeptide at pH = 8 equals +1.