question 16. Which of the proteins below are synthesized on the membrane bound ribosome of the...

question 16. Which of the proteins below are synthesized on the membrane bound ribosome of the RER?

A. Peripheral membrane proteins of plasma membrane

B. The enzymes required for glycolic

C. The pex5 receptor that binds preoxisomal proteins and transports them to the peroxisomal matrix

D. Enzymes of the inner mitochondrial membrane

E. None of the above

Expert Solution

The correct option is E

None of the above.

The proteins which are destined for exocytosis, various membranes (ER and golgi body) or lysosome as well as plasma membrane integrated proteins are synthesized on rough endoplasmic reticulum.

Whereas, cytosolic proteins or those destined for nucleus, mitochondria, peroxisome as well as peripheral membrane proteins are sythesized by free ribosomes (polysomes).

Whether a protein will be synthesized on rough endoplasmic protein (RER) or on free ribosome is decided by a N terminal signal sequence. If N terminal signal sequence is present then protein synthesis happens in membrane bound form. If N terminal signal sequence is lacking then it is synthesized on free ribosomes. N terminal signal is usually 12 -35 amino acid long. It contains central cluster of hydrophobic amino acids. Overall, the N terminal region carries a net positive charge.

The A option is incorrect- Peripheral proteins unlike integral membrane proteins are synthesized by free cytosolic ribosomes hence, this option is incorrect

Option B is incorrect. All the glycolytic enzymes are cytosolic. They all lack N terminal signal sequence. Therefore, all are synthesized by free ribosomes.

Option C is incorrect- Pex 5 receptor protein is also cytosolic. Pex 5 helps in transporting cytosolic peroxisomal proteins to peroxisome. Pex 5 binds to peroxisomal target sequence (PTS) and carries the protein inside the peroxisome by binding with membrane bound receptor (made up of Pex 2-10-12). The most notable point here is, that Pex 5 is although a receptor but is a cytosolic protein. Hence it is also synthesized on free cytosolic ribosomes. This option os also incorrect.

Option D is incorrect- Mitochondrial proteins are also synthesized on free cytosolic ribosomes. The inner membrane proteins are transported by TIM complex (translocase of inner membrane). Mitochondrial proteins are transported in unfolded state with the help of ATP and HSPs. Hence, this option is also incorrect.

Synthesis of inner mitochondrial membrane bound proteins happens on free ribosomes or polysomes. Later on they

gladiator answered 2 years ago

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