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Connections of Proteins to Glucose Metabolism

Connections of Proteins to Glucose Metabolism

Proteins are hydrolyzed by a variety of enzymes in cells. Most of the time, the amino acids are recycled into the synthesis of new proteins. If there are excess amino acids, however, or if the body is in a state of starvation, some amino acids will be shunted into the pathways of glucose catabolism (see the figure below).

Each amino acid must have its amino group removed prior to entry into these pathways. The amino group is converted into ammonia. In mammals, the liver synthesizes urea from two ammonia molecules and a carbon dioxide molecule. Thus, urea is the principal waste product in mammals produced from the nitrogen originating in amino acids, and it leaves the body in urine.

This illustration shows that the amino acids alanine, glycine, threonine, cysteine, and serine can be converted into pyruvate. Leucine, lysine, phenylalanine, tyrosine, tryptophan, and isoleucine can be converted into acetyl CoA. Arginine, proline, histidine, glutamine, and glutamate can be converted into α-ketoglutarate. Isoleucine, valine, methionine, and threonine can be converted into succinyl CoA. Tyrosine and phenylalanine can be converted into fumarate, and aspartate and asparagine can be converted into oxaloacetate.

The carbon skeletons of certain amino acids (indicated in boxes) derived from proteins can feed into the citric acid cycle. (credit: modification of work by Mikael Häggström)

This lesson is part of:

Cellular Respiration

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