Question

1- All of the following are types of modifications that can result in an active, functional protein EXCEPT:

a. Covalent disulfide bonds between cysteine amino acids

b. Cleavage of proteins

c. Glycosylation

d. Addition of lipids to the N- or C-terminus

e. Amyloidosis

2- All of the following are reversible protein modifications EXCEPT:

a. Proteolysis

b. Phosphorylation

c. Acetylation

d. Methylation

e. Ubiquitination

3- How can protein-protein interactions regulate enzymes?

a. These interactions change the pH of the local environment, causing enzymes to be less efficient

b. These interactions change the conformation of the enzyme, which may inactivate the enzyme

c. These interactions change the enzyme’s amino acid sequence, which may enhance enzyme activity

d. These interactions prevent translation of the enzyme’s mRNA, leading to lower levels of the enzyme

e. These interactions send enzymes out of the cell, so that they can no longer catalyze cellular processes

Answer 1

1. Answer. e. Amyloidosis is a rare systemic disease that occur due to the deposition of amyloid. Amyloid is an abnormal deposition of proteins, which is insoluble and gradually deposits in the organ causing its progressive malfunction. All other options are a type of post-translational modification which helps the protein to achieve a specific 3D conformation essential for its function. The specific structure of the protein determines its function because if the protein structure gets altered the interaction between amino acids as well as their interaction with other molecules get altered, which can disrupt the protein function.

• Covalent disulfide bonds between cysteine amino acids help in protein folding and formation of specific 3D structure.
• Cleavage of proteins- a lot of proteins are synthesized as a precursor which get modified by proteolytic cleavage which results an active form of protein. Eg, Insulin is synthesized as preproinsulin.
• Glycosylation Carbohydrate side chains are added to form glycoproteins.
• Addition of lipids to the N- or C-terminus- Addition of lipids at the ends directs the protein to its functional location. Eg. Lipid acylation directs the protein to the plasma membrane.

2. Answer. a.

• Proteolysis is an irreversible post-translational modification which involves the specific hydrolytic cleavage of peptide bond producing smaller peptides and /or amino acids. Eg. Cleavage of N- terminal and C-terminal amino acids to modify the proteins, cleavage of preproinsulin at specific sites to secrete active insulin, etc.
• Phosphorylation, acetylation, methylation and ubiquitination are reversible covalent modification by addition phosphoryl, acetyl, methyl, and ubiquitin groups to the proteins. The groups play an essential role in determining the specific function of the proteins. Eg. Ubiquitin plays an essential role in protein breakdown (proteolysis).

3. Answer. b. Protein-protein interactions change the conformation of the enzyme altering its function. Proteins mostly interact via hydrophobic bonds, van-der waals forces and salt bridges, these interactions modify the ligand/substrate binding sites. Hence, altering the activity of enzymes.