Despite their common roles in catalyzing biochemical reactions, the diversity of enzymes is massive and daunting. In fact, not all enzymes are even proteins! Some of the "oldest" enzymes around are thought to be RNA enzymes, called ribozymes. (RNA is a nucleic acid called ribonucleic acid. Refresh your memory in the Biomolecules unit.) While RNA is often thought of as a messenger and an intermediate between DNA and proteins, scientists now believe that early in the history of Life, RNA played a much more central role.
Scientists discuss the possibility of an "RNA world" where neither DNA nor proteins existed. In this scenario, RNA molecules acted both as storage of genetic material and as enzymes catalyzing important biological reactions. According to this hypothesis, fittingly called the RNA world hypothesis (scientists are so creative), only later on in evolution did DNA and proteins predominantly take over these roles.
Even in organisms today, ribozymes play critical roles in the cell. While the ribosome consists of many proteins, it is the ribosomal RNA in the large subunit that catalyzes the joining of amino acids in the synthesis of proteins. You’ll learn a lot more about ribosomal "RNAs" later, but right now, you can appreciate the fact that RNA can take on complicated structures just like proteins can, and can catalyze important biological reactions to boot. Pretty good, RNA, pretty, pretty, pretty good.
Even if you only focus on protein enzymes, the diversity of the reactions catalyzed by these enzymes is mind-mushing. Take digestion as an example. You know the saying, "you are what you eat?" While it’s true that our bodies rely on food to keep our cells running and to create new cells, it doesn’t mean that if we eat enough Domino's pizza, we’ll turn into a ExtravaganZZa Feast pizza of a different sort. You can thank enzymes for that. Go on; thank them. We'll wait. Welcome back. Enzymes in your digestive tract break down what you eat into useful biological building blocks to keep you alive and kicking.
Now that we mention it, enzymes in your digestive tract determine what nutritional benefits you get from what you eat. The rather shocking part about this is that these enzymes often are not found within cells of your body. That's right; you read us right. Instead, specialized bacteria in your gut determine what your body can or cannot digest. Not all humans have the same bacteria, either.
For example, a special microbe called Bacteroides plebeius is found predominantly in the digestive tract of people of Japan, and it contains enzymes that help them digest seaweed.2 We can also influence the types and number of bacteria that comprise our guts. Some food products, such as yogurts, contain live bacterial cultures that can occupy our guts and influence digestion. Activia, anyone?
Microbes, microorganisms, germs, or whatever you want to call them, in other organisms also extract nutrients. Have you ever wondered how termites digest wood? No? It turns out that they can’t! However, the enzymes provided by the microbes in those termite tummies do all the hard work of digestion.3 The diversity of microbes, and the enzymes that they contain, have allowed termites to use that super tasty wood as a food source.