Re: Are there reactions in our body that can take place without enzymes?

Hi Dhanya,

You've asked some profound questions. Off the top of my head, I can think of one class of "biological" reactions that often occurs without enzymes – the breakdown of biological molecules and structures. Of course, these are undesired reactions from the perspective of the cell, but there is a wealth of literature on the effects of things like reactive oxygen species, free radicals, UV light, etc. on molecules like chlorophyll, DNA, RNA, and proteins. See, for example, Wikipedia's article on food "browning": http://en.wikipedia.org/wiki/Browning_(food_process)

It is much harder to find examples of "desirable" biological reactions that proceed nonenzymatically. In fact, I gave up after 10 minutes of Google searching. This MadSci answer from 1998 essentially agrees: http://www.madsci.org/posts/archives/1998-12/912787466.Bc.r.html. There probably are a small number of such reactions, perhaps some molecular rearrangements that proceed spontaneously and are sufficiently fast and selective that no enzyme was ever needed for them.

Something you're forgetting about in your question is the concept of cellular control. I would say that is just as important a reason for use of enzymes as the catalytic rate increase that enzymes provide. Cells need to control all the processes that occur in them. Loss of control leads to cancer and/or death. By relying on enzymes to carry out nearly all of the reactions inside, cells can exert powerful control over themselves, because enzymes can be regulated at several points – in the process of their synthesis, through allosteric interactions and things like product inhibition, and through their degradation. I found an article that discusses nonenzymatic reactions in biology in the context of the "prebiotic earth" that you may find interesting: http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060018

The specificity that many enzymes have to catalyze one reaction pathway but not other, similar ones, is another form of chemical control of which cells take advantage. The presence of enzymes in cells does not prevent chemical, nonenzymatic reactions from taking place, it's just that the enzymatic reactions are typically so much faster, that nearly all the substrate is channeled down the enzymatic reaction pathway, leaving very little to react nonenzymatically in some other way.

Your last question, "What brings about this kind of a difference in the reaction? Is it the additional structure of the protein (enzyme) that helps to hold the substrates together which facilitates the reaction faster than Fe could?" has, fortunately, been extensively studied and at least partially answered by biochemists. Enzymes are fascinating molecular machines that accelerate reactions by many orders of magnitude and often act very specifically, converting particular substrates to particular products. This link gives a very basic, and probably insufficient for your purposes, explanation: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_enzymes_work.html

From what I've learned about enzyme chemistry, your question is on the right track. Enzymes use their complex structure to bind, position and orient reactants very specifically in 3 dimensions with respect to each other and to key amino acid residues on the enzyme itself. Enzymes will also often "bend" or "stretch" substrates in a particular manner to "activate them" for a particular reaction, similar to the concept of stretching the rubber band on a slingshot. These complex steps really imply that enzymes are molecular machines, with moving parts, and not just "blobs" of atoms. I'll leave you with some links that describe how some enzymes work in more detail, but you should also consult a textbook or a local librarian for additional reading.
http://en.wikipedia.org/wiki/Enzyme_catalysis
http://www.madsci.org/FAQs/catalase.html
http://www.ncbi.nlm.nih.gov/pubmed/20951028
http://www.ncbi.nlm.nih.gov/pubmed/20822946
http://www.ncbi.nlm.nih.gov/pubmed/20729130
http://www.ncbi.nlm.nih.gov/pubmed/20484980

-Alex Tobias