Chemiosmosis
Sections
INNER MITOCHONDRIAL MEMBRANE
- Contains the ETC, ATP synthase, ADP-ATP transporter, phosphate translocase and more
- Impermeable to small molecules (H+, ATP, ADP & Pi)
- ETC pumps protons across impermeable inner membrane: generates chemiosmotic gradient (proton-motive force)
ATP SYNTHASE STRUCTURE
F0 (c, gamma, and epsilon subunits)
- Cylindrical structure embedded in membrane
- Channel through which H+ flows down gradient
F1 (alpha and beta subunits)
- Sits on top of F0 on matrix side
Stator (a, b, and delta subunits)
- Prevents F1 from rotating as F0 does
ATP SYNTHESIS
- H+ from intermembrane space enters F0
- H+ protonates asparagine residue within channel
- Induces rotation of c-ring
- Electrochemical energy (H+ gradient) converted to mechanical energy (rotation)
- On the matrix side, ADP & Pi bind F1 beta subunit
- Beta subunit interacts with rotating F0: activates and catalyzes formation of ATP
- ATP released into matrix along with H+ that passes through channel
ADP-ATP TRANSPORTER
- Antiporter
- Driven by the electrical potential across membrane
- Intermembrane space more positive than matrix
- ATP has -4 charge while ADP has -3 charge
- Charge difference favors movement of ATP OUT of negatively charged matrix
PHOSPHATE TRANSLOCASE
- Symporter: pumps H+ & Pi from intermembrane space into matrix
- Driven by pH gradient across inner membrane
- pH greater in matrix (more basic, less H+) and lower in intermembrane space (more acidic, more H)
- Protons & Pi move from intermembrane space into matrix
- For every 4 H+ pumped into matrix: 3 drive ATP synthase & 1 drives Pi transport
CLINICAL CORRELATIONS
Uncouplers
- Proteins that make inner mitochondrial membrane permeable to H+
- Example: 2,4 dinitrophenol (DNP)
Brown adipose tissue (BAT)
- Specialized adipose tissue that facilitates non-shivering thermogenesis
- Contains many mitochondria & uncouplers
Full-Length Text
- Here we will learn chemiosmosis, the ATP generating portion of oxidative phosphorylation.
- To begin, draw two curved lines to represent the inner mitochondrial membrane.
- Label the matrix above it and the intermembrane space below it.
- Indicate that the inner mitochondrial membrane is:
- Rich in proteins, like: the electron transport chain complexes!
- For this reason, it is impermeable to small molecules, including H+, ATP, ADP and phosphate.
- Start a table.
- Remind ourselves that that the electron transport chain actively pumps protons across the impermeable inner membrane, which generates the chemiosmotic gradient (the proton-motive force).
- Now, denote that the key player in chemiosmosis, ATP synthase comprises:
- Two rotors (rotary motors): F0 and F1 held together by a stator (which is the stationary part of a motor).
Now, let's draw ATP Synthase.
- Embed F0, a cylindrical structure, in the membrane; it is the proton pore that allows protons to flow down their concentration gradient – from the intermembrane space into the matrix.
- Show that it comprises multiple C-subunits, which is why it's often called the C-ring.
- Show that gamma and epsilon subunits attach to the top of the cylinder and extend into the matrix.
- They form a central shaft that extends into the second rotor, F1, which we draw, now, as a mushroom top.
- Show that F1 comprises six subunits that alternate alpha and beta.
- Thus, indicate that this structure is often referred to as the alpha-beta hexamer.
Finally, let's draw the stator, which is stationary.
- Adjacent to the c-ring, embed another shape (a) in the inner membrane.
- Then, attach to it a shaft that comprises two long subunits (b).
- Finally, use a small delta subunit to connect B to F1.
- Indicate that the stator comprises a, b, and delta; it prevents F1 from rotating as the c-ring rotates beneath it.
Now, let's learn what ATP synthase does.
- Step 1: Show that H+ from the intermembrane space enters F0.
- Step 2: H+ protonates an asparagine residue within the channel.
- Step 3: This reaction catalyzes the rotation of the c-ring.
- Specifically, the electrochemical energy of the proton-gradient is converted to the mechanical energy of rotation.
- Step 4: On the matrix side, show that ADP and phosphate bind the beta subunit of F1.
- Step 5: The Beta subunit interacts with rotating F0, which activates and catalyzes the formation of ATP, which is released into the matrix along with the H+ that passes through the channel.
What happens when ATP synthase runs out of ADP and phosphate?
- There are two proteins in the inner mitochondrial membrane that make sure it never does.
- Draw the ADP-ATP transporter, a transmembrane protein.
- Show that it exchanges ADP in the intermembrane space for ATP in the matrix.
- What drives this exchange?
- Use an arrow to illustrate the electrical potential across the membrane: the intermembrane space is more positive than the matrix.
- Now, indicate that ATP has a -4 charge while ADP has a -3 charge; this charge difference favors the movement of ATP OUT of the negatively charged matrix.
- Finally, write that the electrical potential across the membrane drives the ADP-ATP translocator.
What about phosphate?
- Draw phosphate translocase, another transmembrane protein.
- Show that it is a symporter: it pumps H+ and phosphate from the intermembrane space into the matrix.
- What drives this activity?
- Use an arrow to illustrate the pH gradient across the membrane: the pH is greater in the matrix (more basic) and lower in the intermembrane space (more acidic).
- Write that the pH gradient drives the symporter.
- For every 4 protons pumped into the matrix: 3 drive ATP synthase and 1 drives the transport of phosphate.
- Finally, let's learn how ATP synthase activity changes with the rate of respiration.
- Indicate that the rate of respiration depends on the following: the concentration of ADP and phosphate over the concentration of ATP.
- This ratio increases during exercise, for example, as ATP stores are depleted.
- Thus the rate of respiration increases during exercise.
- As a clinical correlation, write that 2,4 dinitrophenol (DNP) is an uncoupler that was once sold as a weight loss pill. It is a protein that makes the inner mitochondrial membrane permeable to protons, which results in "proton leak." This disrupts the proton-gradient and as a result "uncouples" the phosphorylation of ADP from electron transport.
- The result? The respiration rate rapidly increases because it is no longer dependent on ADP, phosphate and ATP. Instead of being used to pump protons and fuel ATP synthase, all the energy released by the electron transport chain is dissipated as heat. ,
- This mechanism is used by newborn infants to stay warm.
- Write that brown adipose tissue (BAT) is specialized adipose tissue that facilitates non-shivering thermogenesis.
- It has high concentrations of mitochondria, which render it brown in color.
- It also contains uncoupling proteins that transport protons back across the inner membrane into the matrix in a controlled manner.
- Newborns cannot shiver and rely on this mechanism to generate heat; they have BAT along their neck, breast, back and kidneys.