Enzyme

Enzymes are bio-catalysts (biochemical catalysts) that are mostly composed of proteins but can also be RNAs. Unlike conventional catalysts, they are highly specific and selective of their target molecule (substrate). Most enzymes follow the nomenclature standard in which the suffix -ase is used e.g. dehydrogenase, lipase, amylase. But just like every catalyst, enzymes change the speed by which a reaction happens but not the outcome (equilibrium).

An example of an enzyme is alcohol dehydrogenase (ADH), which is responsible for the breakdown of hydroxyl groups (-OH, present in alcohols such as methanol, ethanol, isopropanol, among others) in the body, mostly ethanol (the alcohol present in alcoholic beverages). Different organisms tend to have similar enzymes to perform similar biochemical tasks, with selective pressures causing differences in efficiency and selectivity.

How do catalysts work?
It is important to understand some very basic chemical concepts. Firstly, and this is valid for every chemical reaction, there are educts, reaction intermediates and products. This will be illustrated by the example of burning methane (CH4, also known as cow fart).
 * Educts are the substances (molecules) we have at the beginning of the reaction. In our example, it would be methane (containing carbon atoms, C, and hydrogen atoms, H) and air (containing molecular oxygen, O2).
 * Products are the substances (molecules) we have at the end of the reaction. This would be smoke (carbon-dioxide; CO2) and water (this scary thing called dihydrogenmonoxide, H2O).
 * Reaction intermediates are the molecules in-between educts and products. Those are unstable and exist for tiny fractions of seconds. They only matter in a theoretical context to explain reaction mechanisms.

The complete reaction for burning methane, in this case, for the sake of reductionism simplified, is as following:

CH4 + 2 O2 → CO2 + 2 H2O

In order for a reaction to happen there needs to be an activation energy. In the example of burning methane, you would need to light up a match or generate a fire that would start the combustion (burning) of methane. In other words: You need to give energy (in this case, in the form of heat) to initiate (activate) a reaction. The higher the activation energy, the less likely it is that individual molecules will transition into the reaction intermediate and then into their products. Imagine it like a mountain range: the higher the mountain (activation energy), the less likely it will be that hikers will overcome the peak (reaction intermediate) in order to reach the other side of the mountain range. If you were the hiker, all that matters is that you reach the other side of the mountain range. But what does that have to do with catalysts? Imagine if a catalyst is the mountain guide that helps you to reach the other side of the mountain by giving you an alternative route e.g. a different, lower peak of a mountain within that mountain range. This is what catalysts essentially do! They reduce the activation energy by changing the path of the reaction and thus accelerating the speed by which the reaction happens. The chemical process in which catalysts are used is called catalysis.

An extreme example
An essential enzyme very important in cell communication (as in, cells sending signals to each other via chemicals), monoester phosphatase breaks down phosphate monoesters in around 10 milliseconds (10 ms is 0,01 s or 10-3 s, one hundredth of a single second), while the reaction without the enzyme (catalyst) could take 1 trillion years (1'000'000'000'000 years or 1×1012 years); this is older than the known age of the universe, 13 billion years (13'000'000'000 years or 13×109 years).