intracellular proteolysis

Question 1

What is proteolysis?

Answer 1

process of the cleaving of proteins with proteases by cleaving the peptide bond

Question 2

What is the difference between specific and non specific proteolysis?

Answer 2

specific proteolysis only cleaves substrate with a specific sequence whereas non specific is usually protein degradation

Question 3

what are examples of protein activation?

Answer 3

digestive enzymes

clotting factors

Question 4

protein degradation

Answer 4

Ubiquitylation - the attachment of ubiquitin to the protein forming a proteasome(large protease)

Ubiquitin activating enzymes - E1

Ubiquitin conjugating enzymes - E2

Ubiquitin ligase - E3

Non specific as it can occur in any proteins

Question 5

what are metalloprotease?

Answer 5

any protease enzyme whose catalytic mechanism involves a metal
for example meltrin has a significant role in myogenesis which is the fusion of muscle cells during embryo development

Question 6

what are endopeptidases?

Answer 6

break peptide bond in middle of polypeptide

Question 7

what are exopeptidases?

Answer 7

break of polypeptide from end of chain

these include aminopeptidases and carboxypeptidases

Question 8

protein activation

Answer 8

Specific proteolysis: protein activation of digestive enzymes

An inactive precursor referred to as pro-proteins are cleaved to form the active protein 

Specific cleavage causes conformational changes which expose the active site  

Inactive chymotrypsinogen forms active π-chymotrypsin by a trypsin cleavage  

Certain residues are removed at different positions resulting in the formation of active α-chymoytrypsin and the fragments are held together with disulfide bonds  

Activation of clotting factors

Serine proteases and its glycoprotein co-factor are activated to become active components that then catalyse the next reaction 

This ultimately results in a cross-linked fibrin

Question 9

protein degradation

Answer 9

Necessary to prevent the build up of unwanted protein and permittingthe breakdown of amino acids

Phase 1: protein adapted by ubiquitylation 

Phase 2: degradation 

Lysosomal degradation is usually unspecific

Question 10

Phase 1

Answer 10

E1: ubiquitin-activating enzyme

E2: ubiquitin-conjugating enzyme 

E3: ubiquitin ligase 

Forms thioester bond between carboxyl end of ubiquitin and a cysteine E1. (Using ATP) 

Transfer of ubiquitin to a cysteine onto E2 

E3 transfer ubiquitin from E2 to target protein 

Many different E3 enzymes exist for specific target proteins

Question 11

Phase 2

Answer 11

A large complex known as the 26 proteasome degrades Ubiquitinated proteins

Question 12

What is the meaning of half life of proteins?

Answer 12

time taken for protein to degrade the protein in half and measures how stable the protein is

Question 13

what is the N-end rule?

Answer 13

The N-terminal amino acid of a protein determines its half life

Question 14

the SREBP cycle

Answer 14

For cholesterol regulation

At low cholesterol levels: 

Ubiquitin targets Insig for degradation  

Secretory proteins escort SCAP/SREBP to golgi complex  

In golgi, two proteases release regulatory domain of SREBP 

The regulatory domain enters the nucleus and the transcription of lipid synthesizing enzymes are stimulated  

This increases cholesterol synthesis