Lecture 26 Flashcards
Which blood group is the universal acceptor
AB
Give an example of where different cells can process the same peptide into different hormones
Cleavage of opiomelanocortin can give rise to a whole range of different proteins which varies from cell to cell. Opiomelanocortin can be cleaved in neurons to give, B-MSH, B-endorphin, ?-MSH and ?-lipoprotein. In contrast, opiomelanocortin can be cleaved in the pituitary to give corticotropin (ACTH) and B-lipotropin
What are the three reasons why glycosylation is important
Increase protein stability in the harsh extracellular environment, enables cell-cell recognition as well as cross species separation
What is characteristic of the signal sequence that directs a protein to the mitochondria
Proteins destined for the mitochondria alternate between positive and hydrophobic amino acid residues
Trimming and growth of carbohydrate chains proceeds step-by-step in individual Golgi cisternae, T or F
T
Where does initial carbohydrate addition to proteins start
ER
Explain what is meant by SRPs and their receptors acting as molecular matchmakers
SRPs and SRP receptors function as molecular matchmakers as they connect the ribosomes synthesising proteins containing the endoplasmic reticulum signal sequence to available endoplasmic reticulum translocation channels
Describe the differences in the terminal oligosaccharide in people with the A, B and O antigens
People with the A blood type antigen contain an acetylated terminal galactose (N-acetylgalactosamine. People with the B blood type antigen possess a non-acetylated terminal galactose. AB patients will possess red blood cells with both the acetylated and the non-acetylated terminal galactose whereas O blood types will have neither.
Explain how proteins insert into the ER membrane once the SRP has bound to its receptor
The signal sequence in the protein opens the translocation channel. The signal sequence remains bound to the channel while the rest of the protein chain is threaded through the membrane as a large loop before it is then cleaved off and degraded. A protein plug then closes the translocation channel in the endoplasmic reticulum membrane
Give an example of protein trimming in the process of maturation
Protein trimming occurs prior to the secretion of inulin. The signal peptide is removed from preproinsulin which produces the still inactive proinsulin molecule. Proinsulin is then moved to the Golgi and passes through it. Then, in the final stage, several amino acids in the middle of the of protein (known as chain C) are removed. The cleavage of proinsulin produces mature insulin and the C-terminal peptide. Chain A and chain B are held together by disulphide bridges
What are signal patches
Signal patches are specific 3D arrangements of amino acids that are used to target proteins
Whereas membrane-bound and secreted proteins are made in the RER, where are nuclear and cytosolic proteins made
Nuclear proteins are made on the outer nuclear membrane whereas cytosolic proteins are made in the cytosol
What can be said about the affinity of start and stop transfer sequences in ER membrane proteins
They are hydrophobic
What is meant by the process of conformational maturation that occurs after signal sequence removal by signal peptidases
Conformational maturation is the process whereby the maturing proteins adopts a thermodynamically stable shape
Describe how transmembrane proteins insert into the ER membrane and how this differs from ER luminal proteins
Proteins that are meant to insert into the ER membrane contain stop transfer sequences that halt the transfer process into the endoplasmic reticulum lumen. Stop transfer sequences are released laterally into the lipid bilayer and form a membrane spanning segment anchoring the protein to the membrane
What is the initial purpose of carbohydrate chains added to proteins
Quality control tags
How does insertion into the ER membrane differ for multipass transmembrane proteins
Mulitpass membrane proteins have internal signal sequences consisting of several stop and start pairs which help to stitch the protein into the membrane. Combinations of Start-transfer and Stop-transfer signals determine the orientation of multipass membrane proteins in the membrane
Explain how proteins are transported and insert into the ER membrane
Signal sequences direct the protein to the endoplasmic reticulum through the action of signal recognition particles (SRPs). SRPs bind to the signal sequence in a newly synthesised protein as it emerges from the ribosome. Protein synthesis then slows down until the SRP-ribosome complex binds to the SRP receptor on the endoplasmic reticulum membrane. The SRP is then released passing the ribosome to the protein translocation tunnel in the endoplasmic reticulum membrane.
Give an example of where defective protein glycosylation causes disease
Myelination of neurons requires complex gangliosides enriched in sialic acid. Deficiency in sialic acid causes severe psychomotor delay evident by the end of the first year of life
Which blood group is the universal donor
O
What attribute of protein glycosylation is responsible for the limitations in xenotransplantation and anima donors
Humans contain B-galactose attached to proteins whereas animals use ?-galactose
Ribosomes associate with the membrane of the target compartment via a direct interaction, T or F
F - Electron microscopy has shown that ribosomes associate with the membrane via protein translocators present in the membrane
How does the structure of the various blood type antigens account for the antibodies that those patients will possess
A antigen patients will possess antibodies for the B antigen and vice versa. O group blood people will have antibodies for both A and B antigens whereas if you are AB you would have neither.
What is the role of signal recognition particles in membrane and secreted protein synthesis
SRPs direct the delivery of the ribosome and protein to the membrane
