physiology - Is there any use of CO₂ in human body?

We all know CO₂ as a waste product of metabolism . Does CO₂ have any helpful role , apart from having a role in pH of blood ?

Answer

Before I restrict the answer to human metabolism, I recon it is important to mention that CO₂ is the source of the carbon atoms of glucose in photosynthesis (in the Calvin cycle). [In photosynthesis CO₂ is 'fixed'].

Even with the above restriction, I am certain I cannnot do justice to every helpful aspect of CO₂ in mammalian metabolism, and I'll restrict myself to just one area that came to mind on reading your question: the requirement for carbon dioxide (in the form of bicarbonate) for fatty acid biosynthesis (FAS) and, in a bit more general sense, mammalian/bacterial biotin-dependent carboxylation reactions. Depeding on other contributions, I might be able to extend this a bit later.

Salih Wakil showed that CO₂ is an absolute requiement for fatty acid biosynthesis, but carbon atoms from CO₂ do not appear in the fatty acid product.

We now know that FAS begins with the carboxylation of acetyl-CoA to malonyl-CoA, catalyzed by the enzyme acetyl-CoA carboxylase. Acetyl-CoA, ATP and bicarbonate are the substrates for this enzyme, and malonyl-CoA is a key product. One of the many interesting properties of this enzyme is that it contains biotin, which (in this case) may be considered a carrier of 'active' CO₂.

This explains the requriement for carbon dioxide but why no carbon from CO₂ in the final product?

It is now known that in subsequent FAS reactions, a derivative of malonyl-CoA condenses with a derivative of acetyl-CoA (I am simplifying here) to give a four-carbon compound with loss of CO₂.

Thus, carbon dioxide (in the form of bicarbonate) is an obligate requiement for mammalian fatty acid biosynthesis, but no CO₂-derived carbon is incorporated into fatty acids.

Carbon dioxide is also required for oxaloacetate formation from pyruvate. This reaction may be though of a method of 'filling up' a key Krebs Cycle intermediate (a so-called anapleurotic reaction). The enzyme here is pyruvate carboxylase and the substrates for the reaction are pyruvate, bicarbonate and ATP, with oxaloacetate being a key product. This enzyme also contains biotin and (like acetyl CoA carboxylase), CO₂ becomes covalently bound to biotin during the reaction cycle.

Pyruvate-CoA carboxylase was discovered by Harland.G Wood and C. Werkman in bacteria (See here for a good reference on the early work on pyruvate carboxylase). Its discovery was very controversial because at the time it was thought that animal/bacterial cells could not 'fix' CO₂; that is it was though that CO₂ is only 'fixed' in photosynthesis. This discovery disproved that piece of dogmatism.

A third enzyme that requires CO₂ as substrate (in the form of bicarbonate) is propionyl-CoA carboxylase. This enzyme occurs in mitochondria and functions in odd-chain fatty acid metabolism. It also contains biotin.

I have concentrated on some biochemical aspects of your question. The three enzymes mentioned, acety-CoA carboxylase, pyruvate carboxylase and propionyl-CoA carboxylase all require CO₂ in the form of bicarbonate as substrate, all contain biotin, and (as far as I am aware) all play very central roles in mammalian metabolism. (They also all require ATP as substrate).

Of the many interesting aspects of biotin, I'll mention just one. Egg white contains a protein, avidin, which binds biotin very, very tightly. In fact the biotin-avidin interaction is one of the strongest non-covalent interactions known. As far as I am aware, no-one knows the function of avidin in egg white. Some bacteria contain a similar (but evolutionarily unrelated) protein called streptavidin. No one knows the function of steptavidin either (again, as far as I am aware).

The original Wood & Werkman paper: was published in the the Biochemical Journal in 1936

The utilisation of CO₂ in the dissimilation of glycerol by the propionic acid bacteria.

Harland Goff Wood and Chester Hamlin Werkman.

Biochemical Journal, Volume 30, January 1936, pp 48-53