Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

26 Chapter 2
O-linked glycosylation
(via serine/threonine)
N-linked glycosylation
(via asparagine)
R 6 O
HO
R 3 O
O
O
α NH
NH C O
O
CH
H/CH 3 NH
(Man) n
GlcNAc
O
HO
OH
O
NH
O
H
N
β
O
NH
C O
CH
NH
R 3 = H or Gal or GlcNAc
R 6 = H or NeuAc or GlcNAc
Figure 2.24 O- and N-linked glycosylation. Gal, galactose; GlcNAc, N-acetylglucosamine; Man,
mannose; NeuAc, N-acetylneuraminic acid.
Problems
(1) Which of the amino acid side chains found in proteins would be
(a) positively charged or (b) negatively charged at pH 4, 7 and 10, respectively
(2) The amide bonds found in polypeptides all adopt a trans-conformation in
which the N2H bond is transcoplanar with the C5O. Why Certain
peptides containing proline have been found to contain cis-amide bonds
involving the amine group of proline. Explain.
(3) The following segment of RNA sequence is found in the middle of a gene,
but the correct reading frame is not known. What amino acid sequences
would be encoded from each of the three reading frames Comment on
which is the most likely encoded amino acid sequence.
5 0 -ACGGCUGAAAACUUCGCACCAAGUCGAUAG-3 0
(4) You have just succeeded in purifying a new enzyme, and you have obtained
an N-terminal sequence for the protein, which reads Met–Ala–Leu–Ser–
His–Asp–Trp–Phe–Arg–Val. How many possible nucleotide sequences
might encode this amino acid sequence If you want to design a 12-base
oligonucleotide ‘primer’ with a high chance of matching the nucleotide
sequence of the gene as well as possible, what primer sequence would you
suggest
(5) a-Helices in proteins have a ‘pitch’ of approximately 3.6 amino acid residues.
In order to visualise the side chain–side chain interactions in a-helices,
the structure of the helix is often represented as a ‘helical wheel’. This
representation is constructed by viewing along the length of the helix
from the N-terminal end, with the amino acid side chains protruding
from the central barrel of the helix, as shown below.