protein post-translational modification

Question 1

protein modification

Answer 1

takes place in the golgi apparatus
proteins are packaged into vesicles and tagged
can be modified during or after mRNA translation
proteins must carry a signal sequence/peptide to be modified

Question 2

protein modification - proteolysis

Answer 2

cleaving the polypeptide allows the fragments to fold into different shapes

Question 3

protein modification - covalent addition of molecules

Answer 3

acetyl group
methyl group
phosphate group
sugar moietes
lipid moietes
small peptides

Question 4

proteolysis function

Answer 4

to make protein isoforms from a single mRNA
to activate a protein (zymogen > active form)
enhances proper protein folding
enhances insertion of proteins to membranes or lumen of organelles
protein localisation

Question 5

insulin maturation

proteolysis example

Answer 5

preproinsulin > proinsulin > insulin
preproinsulin > proinsulin due to proteolysis of the signal sequence
proinsulin > insulin due to proteolysis of chain C

Question 6

covalent modification examples

Answer 6

histone proteins
phosphorylation
ubiquitination
glycosylation

Question 7

histone proteins

covalent modification example

Answer 7

acetylation > activation of a gene
methylation > inactivation of a gene

Question 8

phosphorylation

covalent modification example

Answer 8

added phosphate groups alter the shape of the protein
addition of a phosphate group to serine, threonine and tyrosine residues of a protein by kinase enzymes to activate the protein
reversible process, removal of a phosphate group by phosphatase enzymes
e.g. CDK/cyclin complex

Question 9

ubiquitination

covalent modification example

Answer 9

addition of ubiquitin to a protein to remove defective, non-functional proteins in the cell
signals proteins to the proteasome for degradation

Question 10

glycosylation

covalent modification example

Answer 10

adding sugars is important for targetting and recognition
the addition of a sugar molecule to the side chains of amino acids to change the proteins conformation
N-linked > to the nitrogen atom of asparagine
O-linked > to the oxygen atom of serine and threonine