Proteins for Energy
Home Table of Contents Preliminary Information Common Amino Acids Structure Functions Proteins for Energy Protein in the Diet Wrap-Up

 

Proteins for Energy

You have seen how proteins can be built up from amino acids with a particular primary, secondary, tertiary and even quaternary structure to serve particular functions. If the proteins are damaged, or for some reason are no longer needed or no longer able to serve those functions, the proteins can be broken down by hydrolysis reactions to reform the amino acids from which they were made. In the body this hydrolysis process is catalyzed by enzymes. Yes, enzymes are proteins which can catalyze the hydrolysis of other proteins.

Strong acids or bases can also catalyze the hydrolysis reaction of protein both outside and unfortunately sometimes inside the body. This is what happens if you were to spill a strong acid or base on your skin or accidentally ingest it. They will cause the hydrolysis of the protein and thus break it down.

The amino acids that are formed by the hydrolysis of protein can be used to make new protein or they can have their amino groups removed and then the remainder of the molecule can be oxidized to provide energy.

These altered (deaminated) amino acids enter the citric acid cycle in a variety of ways but once they have done so, the result is the formation of carbon dioxide and the release of hydrogen with electrons that combine with oxygen to form water. With that, of course, comes the release of energy.

Some amino acids are converted to pyruvate or pyruvic acid. Some are converted into acetyl CoA. Others are converted into acetyl CoA through a previous step of acetoacetyl CoA. Some are converted into one of the chemicals in the cycle, oxaloacetate. Others are converted to other chemicals that are a part of the citric acid cycle. The nitrogen that is removed from the amino acids in this process is ultimately converted to urea and excreted in the urine.

Diagram of citric acid cycle showing amino acid input. [68075.jpg]

Whatever the particular pathway that the particular amino acids follow to get into the citric acid cycle and become oxidized, the process is somewhat wasteful of these particular very specialized and important compounds. It is, of course, useful to the body if the protein is no longer functioning to oxidize it and get some energy out of it. But the body can also oxidize protein if there is insufficient glucose or lipids (fats and fatty acids) available to provide the energy necessary for the body. In those cases, although it would prefer to use the glucose or the fats and fatty acids, the body can use functioning protein and break it down into amino acids in order to provide the amount of energy that is necessary for the body to continue to function. This is something to keep in mind when dieting and exercising, and I mean dieting in the larger sense of paying attention to what it is that you are eating. If energy is being expended more quickly than glucose is available to provide that energy, or using it faster than the fat reserves can provide the pyruvic acid and the acetyl CoA to enter the citric acid cycle, the body will break down functioning protein to provide the energy that is needed.

Top of Page

E-mail instructor: Sue Eggling

Clackamas Community College
2001, 2003 Clackamas Community College, Hal Bender