Chapter 6: Energy Flow in the Life of a CellIssues in Biology |
One of the most important truths about proteins is that their function absolutely depends on their three-dimensional shape. Consequently, deciphering a protein’s structure is a kind of "Holy Grail" for a biochemist interested in how a particular protein carries out its role in the cell. To approach this problem, biochemists frequently use a technique called X-ray crystallography. In this technique, the biochemist purifies a single protein, places it into a solution and then allows the solution to slowly evaporate. As the water or other solvent molecules diffuse out of the solution, the protein molecules become more and more concentrated. Hopefully, the protein molecules will then interact with each other to form a solid crystal of pure protein. If the scientists are able to produce a protein crystal, they can bounce X rays off of it to get information about the protein’s three-dimensional shape. (This technique is not limited to proteins; the double helix nature of DNA was revealed by X-ray crystallography experiments carried out by Rosalind Franklin.) It is through such studies that we are beginning to understand the details concerning how a protein’s shape is critical for its function.
The ultimate determinant of a protein’s shape is the sequence of amino acids that make up the protein polymer (its primary structure). At critical sites in the amino acid chain, a single amino acid change can abolish the protein’s function. Many human diseases, including sickle cell disease and cystic fibrosis, result from the change of a single amino acid that prevents the protein from carrying out its normal function. In the case of cystic fibrosis (CF), the altered protein normally provides a channel for chloride ions to pass into or out of the cell, helping to regulate the amount of salt and water inside and outside the cell. However, in most CF cases, the 508th amino acid (out of 1480 total) is missing from the CF protein. As a result, the CF protein’s shape is slightly changed so that the cell sends it to the cellular equivalent of the recycling bin, rather than to the cell membrane. The absence of the CF protein in the cell’s membrane leads to abnormal salt regulation, which in turn leads to other symptoms of the disease and ultimately to death.
Protein structure can also be exquisitely sensitive to changes in its environment. Even slight changes in pH, temperature, or salt concentration can diminish or eliminate a protein’s activity. What does this mean for the proteins that we eat in our diet? If you do an Internet search using the keyword "enzyme," you are immediately plunged into a huge number of sites that espouse eating raw foods in order to maintain the “active enzymes” of the food. Recall that enzymes are simply a class of proteins that catalyze chemical reactions in cells. When foods are cooked, the increase in temperature causes proteins, including enzymes, to irreversibly unfold, losing their three-dimensional shape and inactivating them. To spare the enzymes in your food from this fate, the enzyme peddlers urge you to consume your foods raw. They suggest that the enzymes you eat are then free to carry out their functions in your stomach, thus saving your own body from producing digestive enzymes. Sounds intriguing, doesn’t it? Of course, since most of us like at least some of our foods cooked, they are willing to sell us "enzyme supplements" for a rather substantial price, although what these enzymes are and what chemical reactions they catalyze are generally not revealed.
Considering what we know about enzymes, do these ideas make sense? A typical cell may have 10,000 different enzymes, each performing a specific role in the cell. For example, enzymes may be making a DNA polymer, breaking down glucose, producing hormones, or synthesizing cholesterol; just about everything a cell does is catalyzed by an enzyme. A large proportion of the enzymes in a cell are in the cytoplasm or nucleus, where the pH is around 7. What is going to happen to these enzymes when they complete their journey from our mouths to our stomachs? Parietal cells secrete hydrochloric acid, producing a pH of 2, which is 100,000 times more acidic than the cytoplasm. Only a few digestive enzymes, specifically made to function in this extreme condition, are able to work in the stomach. When the enzymes in raw food or in enzyme supplements reach the stomach, the acidic pH will unfold them and permanently inactivate them. Thus, they will suffer the fate of all of the proteins that you eat: digestive enzymes that your cells secrete into the stomach and small intestine will break down the protein into amino acids that your cells can absorb and use to make their own proteins. The enzyme supplements might provide a source of dietary protein, but they are not going to be able to perform any enzymatic role. It would be much cheaper, and probably safer, to simply get the protein you need from a carton of yogurt or a can of beans.
Understanding biology not only makes the world around you more enjoyable as you begin to understand the intricacies of what it means to be alive. When it comes to "enzyme supplements," understanding the basic features of biology might just save you a bundle of cash!