Question

In what form do proteins cross the mitochondrial membranes?

A) As free amino acids
B) As folded tertiary structures
C) As unfolded polypeptides
D) As quaternary structures

Answer 1

Proteins cross the mitochondrial membranes as unfolded polypeptides, despite being initially folded into tertiary structures in the cytoplasm. They unfold to pass through the membrane and refold once inside the mitochondrion.

Step-by-step explanation:

Proteins cross the mitochondrial membranes as unfolded polypeptides. Although mitochondrial proteins are synthesized in the cytoplasm and initially fold into their tertiary structures, they must be unfolded to pass through the mitochondrial membranes. Upon reaching the mitochondrial membrane, these proteins expose an N-terminal signal peptide that targets them to the mitochondria. They bind to receptor proteins on the outer membrane and are subsequently translocated into the mitochondrion in an unfolded state. The protein enters the mitochondrion and refolds into its functional tertiary structure with the help of mitochondrial chaperones.