3. ATP Synthesis

ATP Synthase

source: http://www.rcsb.org/pdb/101/motm.do?momID=72
source: http://www.rcsb.org/pdb/101/motm.do?momID=72

Nature’s miracle enzyme! It even has its own fan site: http://www.atpsynthase.info/index.html. Sadly because of its awesomeness, creationists love to use to it as proof of intelligent design. what-ever. It is a remarkable machine though.

For starters, check out the following chapter from this great online Biochemistry book: http://www.ncbi.nlm.nih.gov/books/NBK22388/

ATP Synthase: Function

ATP synthase catalyzes the formation of ATP from ADP and orthophosphate:

Image ch18e26.jpg

ATP Synthase: Structure

It’s a massive molecule at over 500K Dalton consisting of two subunits, often called “motors”: one driven by proton motive force (PMF), the other by ATP hydrolysis.

Copyright © 2002, W. H. Freeman and Company.
Copyright © 2002, W. H. Freeman and Company.

The hydrophobic F0 subunit is embedded in the mitochondrial membrane and serves as the proton channel for the complex. Its “motor” is driven by the PMF.

Within F0 is a ring called the c-ring that contains 10-15 subunits:

c-chain: two alpha helices with a loop spanning the membrane
c-ring: A ring of phydrophic polypeptides made up of 10-15 chains.

At one side of the the c-ring is a single chain called the a-chain with the protein channels that isolate a couple of c-chains from surrounding lipids. A critical role in rotation is the c-ring and the flow of protons back to the matrix. That is, it rotates as protons flow through it.

The F1 subunit is a giant ball that protrudes into the mitochondrial matrix. Its “motor” is driven by ATP.

F1 contains five polypeptide chains including the γ and ϵ chains, which form the axle (also called the stalk). The stalk iterates with the c-ring of F0 subunit and will rotate through a ring of 6 gains: 3 alpha, 3 beta, that form the rest of F1.

The bulk of F1 are the α and β subunits, which are arranged in a hexameric ring. These form the “enzyme core” of ATP Synthase. Both can bind ATP. The β only has a catalytic role. α has a a regulator role. The rotation of c-ring leads to the rotations of the stalk– transmits rotator motion from F0 to F1.

Unlike an axle, the stalk isn’t symmetrically contracting each beta chain in a different manner, leading to three distant conformations. As the stalk turns, each beta chain alternates through each of the three conformations. This affects the ATP binding to each beta chain, important for catalysis.

Changes of conformation (alpha, beta) ring during rotation is possible because of the stator, which holds the alpha/beta ring still– resist rotations.

The structure of stator is not fully known!

ATP Synthase forms an angle dimer. The angle might be responsible for the instinctive “crinkled” nature of the cristae in mitochondria. Inhibition of dimer => alter shape of cistae, probably because of stator.

Here’s a nice animation and summary.

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