Protein Pathways

Protein Pathways

When dietary proteins are digested, the resulting amino acids are absorbed into the blood and transported to various body cells. Many of these amino acids are joined to form protein molecules again, which then may be incorporated into cell parts or serve as enzymes. Still others may be decomposed to supply energy. When protein molecules are used as energy sources. they must first be broken down into amino acids.

The amino acids then undergo deamination a process that occurs in the liver and involves removing the nitrogen-containing portions ( — NH^ groups) from the amino acids. 

These — NH^ groups are later converted into a waste substance called urea. Depending upon the amino acids involved, the remaining denominated portions of the amino acid molecules are decomposed by one of several pathways—

All of which lead to the formation of acetyl coenzyme A. As before, the acetyl coenzyme. A can enter the citric acid cycle, and as energy is released some of it is captured in molecules of ATP. If energy is not needed immediately, the denominated portions of the amino acids may be changed by still other metabolic pathways into glucose or fat molecules.

Protein synthesis Pathway

Glucose can be changed back into some amino acids if certain nitrogen-containing molecules are available. However, about 8 necessary amino acids cannot be synthesized in human cells and so must be provided in the diet. For this reason, these are called essential amino acids.