Tag Archives: catalysis

Fun Science: Enzymes

An enzyme. Spirals and sheets and strands indicate different kinds of structures. (from Wikipedia)

Enzymes are the catalysts of the body, helping to facilitate chemical reactions that would be very slow or unfavorable in their absence. In a previous post, I discussed how platinum lowers the activation energy barrier for desirable chemical reactions in a car engine, among other places. Enzymes do the same thing, but they are much more selective. Platinum can act on millions of molecules. Enzymes are shaped so specifically that they act only on one molecule. Because of this, enzyme catalysis is often compared to a lock and key–only one chemical is so perfectly shaped as to fit into the active site of the enzyme.

Enzymes are mostly proteins, which are made of hundreds of amino acids with several layers of structure. Our DNA is coded so that enzymes can be assembled from the instructions. The “primary structure” is the sequence of amino acids strung together. The shape of local groups of amino acids gives the “secondary structure”; some combinations tend to coil, others tend to be flat (see the picture at the top of this post). This is due to interactions between the amino acid groups; for example, ionic groups might attract or repel each other. The “tertiary structure” is the structure of the overall molecule, also called the “folding”. We can reproduce the primary and secondary structures in the lab; the folding is harder, because for most sequences of amino acids, there are several possible structures. In the body, the protein is assembled in such a way that it conforms properly. We are mostly still unable to synthesize proteins and enzymes. We usually use bacteria and fungi to make them, when possible.

Enzymes are essential to life. They aid in digestion. Many diseases are caused by the lack of a single enzyme. People with lactose intolerance lack lactase; the deadly Tay-Sachs disease is caused by the lack of hexosaminidase A. In Tay-Sachs, a waste product of cellular metabolism builds up in the brain. Without the enzyme to accelerate its break-down, the waste product builds up to intolerable levels. We can obtain hexosaminidase A, but we can’t therapeutically deliver it to where it is needed in the brain.

You probably already knew that the human body is a remarkable machine. But I hope this brief overview of enzymes gives an appreciation for this one small aspect. Happy digesting.

Fun Science: Why’s platinum so special?

In science, we tend only to learn about a small subset of the elements that populate our world. This is not unreasonable, since 96% of our bodies are composed of just hydrogen, water, carbon, and nitrogen. But there are over a hundred more elements, and they often influence life outside our bodies in ways we don’t hear about. So in today’s post I will talk about platinum.

Platinum is one of the rarest metals in the Earth’s crust. Only 192 tonnes of it are mined annually, where 2700 tonnes of gold are mined annually. When the economy is doing well, platinum can be twice as expensive as gold. So what’s so valuable about it?

Platinum is used a lot in jewelry. Platinum has the appearance of silver, but it doesn’t oxidize and become tarnished like silver. It’s harder than gold, and its rarity can be appealing.

But it’s the chemical properties of platinum that set it apart. Platinum is a great catalyst. This means that platinum facilitates chemical reactions, but is not consumed as the reaction proceeds. The catalytic converter in your car is a platinum catalyst. The catalytic converter helps eliminate a variety of undesirable compounds such as carbon monoxide, nitrous oxides, and incompletely combusted hydrocarbons. Platinum is also a critical part of current hydrogen fuel cells; it splits hydrogen into protons and electrons.

Platinum doesn’t force reactions to occur, but it makes them easier by reducing the energy required. The image below shows the reaction of carbon monoxide (CO) to carbon dioxide (CO2). The chart at the bottom shows the potential energy before, during and after the reaction. Imagine a ball rolling along the red curve (with platinum) and the black curve (without platinum). The ball on the black curve will need more speed to get over the hump. Any given ball is more likely to get over the red hump. Likewise, the presence of platinum lets CO get over the hump to become CO2. Platinum does this for all kinds of reactions.

activation energy

The reaction takes less energy because once a molecule bonds to the surface of platinum, the bonds within the molecule are a little weaker. Molecules like O-O and H-H can split into singletons, something they would never do off the surface. Below I show an example reaction for CO to COon platinum. This diagram is meant to be illustrative, a possible mechanism for the reaction and to show how platinum helps out. In reality these reactions occur very quickly, and careers can be spent figuring out exact reaction mechanisms.

catalysis

 

Platinum is a bit like velcro. Molecules become hooked to the surface, do their reaction, and unstick. If molecules stick and then refuse to unstick, this is called catalyst poisoning, and it’s a big issue in fuel cells. Like velcro, once the hooks are occupied, they can’t do anything else. Platinum is a good catalyst because a lot of things (like hydrocarbons) want to stick to it, but they don’t stick too hard. Other metals either are not attractive enough, or they are too attractive. Platinum is so valuable because, besides being rare, its properties happen to be balanced just right for the reactions we want.