Zombie Metabolism: Glycolysis

Glycolysis

As a pathway, you don't get much more fundamental than glycolysis.

Glycolysis, used to break down sugar to produce ATP, is probably used by all living cells - but what about 'unliving' cells? Yes, I am talking about the cells of the living dead, do they carry out glycolysis too?

Let's think about why glycolysis is such an important pathway...

1) The products can be used for synthesis.
2) It doesn't require mitochondria.
3) It doesn't require oxygen.

Point number three, my friends, is probably the important one. I believe that glycolysis is most likely an essential pathway for zombies, and here's why:

NO CIRCULATORY SYSTEM!!! 

 Zombie bodies show an amazing ability to function despite an incredible level of degradation. The other day, I saw this guy, I kid you not, stabbing a zombie with a sword - wait for it - in the heart! Any idiot knows that zombies don't need their hearts to beat! Even a decapitated zombie head will continue to move its mouth!

But this got me thinking... no blood, no pulse, no circulation. The tissue in these things must be seriously anaerobic! They have to be using glycolysis.

You could argue that the tissue is so badly decomposed that oxygen just diffuses in from the air, and I think that is quite likely, but there is a limit to how far diffusion will go. At the very least, the tissues further in, in particular, the skeletal muscle, must be using glycolysis.

 'But wait', I hear you cry (quietly, so the zombies aren't alerted to your presence), 'zombies move slowly, and they never get tired. That sounds like aerobic exercise, which relies mainly on fatty acid oxidation!'

In answer to that, I would like to make three main points:

1) Fatty acid oxidation IS useless for anaerobic exercise, but glycolysis is good for everything!
2) I wasn't saying they only use glycolysis, just that it would play an important role.
3) If you can get a zombie to sign the consent form, maybe we could measure his oxygen uptake and carbon dioxide production, calculate his respiratory quotient, and settle this argument once and for all?

Obviously I am being sarcastic - I didn't claim to have all the answers - I'm just theorising! While on the run. Sheesh!

Dear Miss Metabolism...

(Mouse over the purple text for explanations.)

Dear Miss Metabolism,

I don't know what to do with myself, I'm so lonely. I used to be carbon atoms 4, 5 and 6 of a happy fructose-1,6-bisphosphate compound, but I guess I forgot how lucky I was, got restless, and decided I wanted us to split up.

I had seen so many of my friends going off into glycolysis and it looked like fun, and I was curious. I had no idea what a big mistake I was making, but I told my other half I wanted a trial separation, and here I am.

We found an aldolase enzyme, it wasn't hard, there was practically one on every street corner, and she promised us that it would just be a reversible reaction, that we could get back together whenever we wanted. Technically, I guess she was telling the truth. Except that after I became a dihydroxyacetone phosphate, I never saw my glyceraldehyde 3-phosphate again.

I guess I thought she would wait for me to get it out of my system, but she didn't. She rushed off into glycolysis, maybe she was feeling hurt, and left me all alone. Glyceraldehyde-3-P dehyrogenase enzymes won't even look at me, so I'm just stuck here.


Please Miss Metabolism, I'm just a foolish intermediate who didn't know how good I had it, please tell me what I should do now?


Yours,
Sad in the Cytosol



Dear Sad in the Cytosol,

Don't be too hard on yourself, you weren't to know this, but those aldolase enzymes are a bunch of home-wreckers! They always say the separation is temporary, that you can both reconcile later if you find you're unhappy, but most of the time, the two compounds just get completely broken down in to carbon dioxide and float away. It's hard being single.

Listen, chances are, your darling GAP is long-gone. All you can do now is remember the good times, and get on with your metabolism. Basically, you have three main options:

1. Mind the GAP. Finding another glyceraldehyde-3-P to settle down with might be difficult, but if you are in a cell that's doing gluconeogenesis, it's not such a long shot.



2. Get fat. Why not try an alternative metabolic lifestyle, and get reduced to a glycerol-3-P. You could settle down with some nice fatty acids (not hard on today's modern 'diets'), and live the quiet life of a triacylglycerol molecule, or the fast-paced career of a phospholipid.



3. Switch teams. If life alone really is that unbearable, why not find a triose phosphate isomerase and become a GAP yourself. Maybe by following your ex's footsteps, you might even get some closure. Be careful though, if you go all the way to becoming an acetyl CoA, you will have given up on life as a carbohydrate forever.

So think carefully before you make your decision, even reversible reactions can have irreversible consequences.

Dynamically yours,
Miss Metabolism

Glucose the Barbarian

In a world where ATP synthesis is dominated by a handful of mitochondria, only glucose has the courage to produce ATP in the cytosol, free from the tyranny of oxygen.

Glucose the Barbarian - No compound should have to live in chains.