When energy sources are high, both glucogenic and ketogenic amino acids are converted to fatty acids through the intermediate acetyl CoA. Other amino acids that are degraded to intermediates in the Krebs Cycle are siphoned off into the production of urea, a nitrogenous carboxyl compound that is filtered through the kidneys and secreted in the urine.

Maintaining fluid balance

Blood proteins such as albumin and globulin function to maintain fluid balance in the body. When the concentrations of proteins in the bloodstream are low, the fluid in the blood (serum) begins to seep into surrounding tissue. Proteins in the blood can counteract this effect by increasing the osmotic potential and forcing fluid back into the bloodstream. Therefore, low amounts of protein in the blood cause edema, a condition that is characterized by an abnormal amount of fluid in the tissue and extracellular space. Edema is seen in starvation, low calorie diets, and diseases like AIDS that decrease the amount of circulating antibodies and albumin.


Many hormones are composed of polypeptide chains. The beta cells in the pancreas produce the peptide hormone insulin. Insulin facilitates glucose uptake in cells and promotes the synthesis of glycogen and fatty acids. Diabetics must inject the peptide hormone insulin because it will be degraded into amino acids in the stomach and small intestine if taken orally. Other examples of peptide signal molecules include neurotransmitters, a class of molecules that are produced and released at nerve endings in the brain and autonomic nervous system.


Enzymes are an entirely different class of proteins. Enzymes catalyze biological reactions by increasing the reaction rates by factors of at least a million. Since most reactions in the body proceed at imperceptible rates without enzymes, it is critical that these proteins be present in sufficient quantities for proper functioning of cells.

How are enzymes so efficient and fast? First, enzymes are highly specific for their substrates. Substrates are any molecules for which the enzyme binds preferentially or with a high affinity. Enzymes are able to bind specific substrates because they form deep pockets or clefts that are complementary to the three dimensional substrate conformations. Since the enzyme pocket is complementary to the substrate, a large number of noncovalent, dipole-dipole, and Van der Waal interactions can occur, favoring enzyme-substrate binding.

The second reason for the high catalytic rate of enzymes is that they are able to stabilize transition state intermediates. By stabilizing these intermediates, enzymes are able to decrease the activation energy required for the reaction to occur. Upon reaching its high-energy transitional state, the bound substrate can be easily converted to the cell's desired product, which is then released to meet the needs of the cell. These reactions can take place on the order of microseconds to nanoseconds. In fact, many enzymes are so efficient and fast that they approach the diffusion-controlled limit, the rate at which substrate diffusion cannot keep up with the rate at which the enzyme catalyzes the reaction. Enzymes like these have reached catalytic perfection.

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