Fall 2015: Discussion 10: Mitochondrial Genomes & Transport and Signaling
MCB 150
Part 1 Key:
The figure below is a simplified picture of mitochondrial membranes.
Answer the questions that follow.
ADP+Pi
ATP
Chaperonin
ATP
ADP+Pi
ADP+Pi
pH8
ATP
Matrix
TIM
Inner
membrane
Intermembrane
Space
+
H
Outer
membrane
pH7
Cytosol
Porin
TOM
ADP+Pi
+
H
ATP
MitochondrialTransitsignal(orpresequence)
Transitpeptidase(orMPP)
Chaperones
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Fall 2015: Discussion 10: Mitochondrial Genomes & Transport and Signaling
1. a. Label the cytosol and the matrix.
b. Describe how you knew this.
The matrix has a pH that is more basic than the cytosol. Protons are pumped
from the matrix into the intermembrane space during the ETC. There is no
way, other than ATP synthase, for protons to cross the inner membrane back
into the matrix.
2. a. What is the pH of the intermembrane space?
~7
b. Explain your above answer and add a pictorial representation of
this on the figure.
Porins are located in the outer membrane and allow protons to flow freely from
the intermembrane space to the cytosol. Therefore the pH of the
intermembrane space is the same as the cytosol.
3. Label the following components of the figure:
a.
b.
c.
d.
Outer mitochondrial membrane
Inner michochondrial membrane
TOM
TIM
4. Draw and explain the process of how a mitochondrial protein gets
into the matrix of the mitochondria and properly folded. Assume
two scenarios: when the protein needs help to properly fold and
when the protein needs extra help to properly fold.
Part II of the book pages 703-5 has a great explanation and pictures.
Translation of mt proteins occurs in the cytoplasm.
1. A mt transit sequence (or presequence) is presented. Cytosolic chaperone
proteins take the protein to the mt outer membrane and TOM. The
chaperone proteins keep the protein from folding.
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MCB 150
Fall 2015: Discussion 10: Mitochondrial Genomes & Transport and Signaling
2. In order for the protein to pass though TOM it needs to be unfolded (hence
the chaperones) and the chaperones need to be peeled off, an ATP
dependent process.
3. The mt presequence (or transit sequence) helps the amino terminus into the
matrix.
4. Once there, the transit sequence is cleaved by transit peptidase (aka
mitochondrial presequence peptidase, MPP)
5. Chaperones help the protein to enter the matrix. Translocation, which
requires ATP hydrolysis, is thought to driven by the chaperons (Hsp70s).
6. Chaperones either help the protein to fold, or transport them to the
chaperonin complex. ATP hydrolysis is required to help the protein fold
properly.
5. Indicate the energy requiring steps in this process. What form of
energy is needed? What does this energy do?
1. ATP hydrolysis: peels cytosolic chaperones off of the protein so the protein
can get through TOM
2. The Electrochemical Gradient: Helps the protein, or at least the N terminus,
which is positively charged, get into the matrix, which is more negatively
charged.
3. ATP hydrolysis: to help the protein fully enter the matrix via the action of
chaperones.
4. ATP hydrolysis is required to help the protein fold properly. The amount
depends on how much help the protein needs. For instance does it need help
via a chaperonin?
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MCB 150