PSII is a Fixer-Upper


The final song from our Frozen parody reminds us that just because something is damaged or broken, doesn’t mean it can’t become whole again with a few repairs. This sentiment makes it the perfect theme song for my favorite enzyme, Photosystem II (PSII). Here’s the Disney version…



Is it the slower QB reduction?

Or lack of water oxidation?

No use trying state transitions,

Although we know it transfers well, PSII ends up photodamaged

Such photoinhibition is such an imposition

So it’s a bit of a fixer upper

So it falls behind

Its peculiar mechanism

Is a one-of-a-kind

So it’s a bit of a fixer upper, but we’re certain of this one

You can fix this fixer upper with a newly made D1

Is it the oxygen singlets?

In chlorophyll protein ringlets?

Is it the way the water splits?

Electrons zipping towards quinones

Causing D1’s aches and groans?

Now permanently on the fritz.

PSII’s just a fixer upper

It needs a protein exchange

Its phosphorylation is confirmation

That something is strange

So it’s a bit of a fixer upper

Plant cells know what to do

The way to fix this fixer upper

Is to make D1 anew

Damaged PSII is a bit of a fixer upper

That’s a minor thing

Just disassemble then reassemble

Voila just like recycling!

So PSII’s a fixer upper

It’s function is nixed

Get the damaged protein out of the way

And the whole thing will be fixed

We aren’t saying to remake the whole thing

‘Cause that’s just too much work

We’re only saying that light’s a force that drives PSII berserk

Electrons just excite the wrong things if they go off their normal path

Changing out the D1 protein erases all their wrath

New D1 clears the path!

All PSII’s are fixer uppers

They’re made to fizzle out

DegP, phosphorylation

FtsH, degradation

Get them on the repair route

All PSII’s are fixer uppers

When the electrons start to move

The only fixer upper fixer

That can fix a fixer upper is

To get the damage removed



Photosystem II performs the unique reaction of splitting water to form oxygen and extract electrons used to fuel photosynthesis. Not all of this energy goes in the direction that it should. When energy gets backed up in the system or electrons venture off of the designated path, irreversible damage to the proteins can occur. This damage means that PSII doesn’t work anymore. Because this photodamage is an unavoidable hazard, photosynthetic organisms have an efficient way of dealing with this problem.

For one thing, the D1 protein at the heart of the PSII complex bears the brunt of the irreversible damage. This makes sense because the D1 protein also coordinates many of the cofactors that comprise the electron transfer pathway through the system. On the one hand, damage to this one protein means function gets knocked out as well; on the other hand, it means that the damage is concentrated on just one protein. So, to fix it and restore function to the complex means photosynthesizers mainly focus on replacing one protein, not twenty. That’s what has to happen. The damaged protein must be replaced by a newly synthesized copy.

It sounds simple enough, but anyone who’s done any fixer-upper work knows there’s more to it than that. Repairing the damage starts with recognition; there must be systems in place to differentiate functional PSII from damaged PSII. Phosphorylation of certain residues on PSII subunits labels those complexes as targets for repair. These labels are interpreted by specific proteases, which then remove and chop up the damaged D1 protein. Next, a newly made D1 protein is inserted into the complex to restore function.

The proteases involved in removal of the damaged D1 protein are DegP and FtsH. Researchers still debate over which one is more important, but it is likely to be a combined effort by both. Also, because the D1 protein is located within the middle of the PSII complex, many other subunits and cofactors must partially or transiently disassemble as a result of D1 protein removal. Exactly how this works and what additional proteins are involved in this process are active areas of research in the photosynthesis community.

Photosynthesizers don’t give up on their PSII complexes just because they get a few dings. The constant recycling of PSII complexes through this repair process ensures that the light reactions of photosynthesis will continue to churn away, even in bright light (more energy, higher rates of damage). It may seem like a lot of trouble to run this elaborate repair shop, but it’s still easier than starting from scratch each time PSII is damaged.




References and Links:

PSII damage and repair


4 thoughts on “PSII is a Fixer-Upper

  1. Pingback: Frozen: A Plant Science Parody | New Under The Sun Blog

  2. ihstreet

    I love that you’ve done this whole series. I havent’ been able to follow all of them/read them, but what a great idea. Are you going to make your way through the whole Disney Canon? Circle of Life probably doesn’t need any changes…hmm.

  3. johnnaroose Post author

    Thanks, but I think I will be taking a break from parodies for a while. It really takes me quite some time to churn blog posts out when large parts of them must follow certain meters and rhyme schemes. If you haven’t read the ‘Stomata Open the Door,’ you should. That one is my favorite because it is the perfect storm of cheesiness and scientific accuracy. I’ll go back and make sure all these post are interlinked to one another so that the series can be found easily.


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