The Catabolism of Proteins

The catabolism of proteins is much more complex than that of fats and carbohydrates because each of the 20 amino acids is degraded through its own unique pathway. The general idea, however, is that the amino nitrogen atom is removed and the substance that remains is converted into a compound that enters the citric acid cycle. 

Because of the catabolism of proteins to (mostly) endogenous amino acids, classical biotransformation studies as performed for small molecules are not needed. Additionally, limitations of current analytical methods to detect and distinguish metabolites and the putative lack of pharmacological or toxicological activity of the metabolites, remain obstacles. Similarly mass balance studies... 

As the model suggests, the dietary need for amino acids is determined by the rates of depletion of the free amino acid pool by oxidation or synthesis of protein. During steady state conditions, the contribution to the free pool from dietary intake and protein breakdown should be exactly balanced by the flux out of the pool to synthesis and oxidation. Any condition that increases deposition of protein in the body or the rate of amino acid oxidation should produce an increased need for protein. For example, muscle hypertrophy is dependent on a positive balance of the protein turnover process. If synthesis of protein exceeds the catabolism of protein, then muscle mass will increase and the free amino acid pool would be depleted. Thus, a net increase in protein requires an increase in intake or a decrease in oxidation. Likewise, the same arguments hold for an increase in oxidation of amino acids. 

The urinary excretion of y-carboxyglutamate, as both the free amino acid and in small peptides, also reflects functional vitamin K status, because y-carboxyglutamate released by the catabolism of proteins is neither reutilized... 

In addition to containing carbohydrates and fats, diets may be rich in proteins. The catabolism of proteins results in the generation of their component parts, amino acids. When the dietary amino acid requirements of an individual are... 

The catabolism of amino acids provides pyruvate, acetyl-CoA, oxaloacetate, fumarate, a-ketoglutarate, and succinate, ail of which may be oxidized by the TCA cycle. In this way, proteins may serve as excellent sources of nutrient energy, as seen in Chapter 26. 

A substrate is a substance that is the basic component of an organism. Protein substrates are amino acids, which are essential to life Protein substrates are amino acid preparations that act to promote the production of proteins (anabolism). Amino acids are necessary to promote synthesis of structural components, reduce the rate of protein breakdown (catabolism), promote wound healing, and act as buffers in the extracellular and intracellular fluids. Crystalline amino acid preparations are hypertonic solutions of balanced essential and nonessential amino acid concentrations that provide substrates for protein synthesis or act to conserve existing body protein. 

Comparison of the model-derived DIFF with the experimentally evaluated DIFF show that protein routing can be described in terms of the amount of protein synthesized from catabolic activity that is, in general it is around 20% of the mean carbon flux of the diet. A simple explanation why dp > dw is also proposed. 

Figure 15-1. Outline of the pathways for the catabolism of dietary carbohydrate, protein, and fat. All the pathways lead to the production of acetyl-CoA, which is oxidized in the citric acid cycle, ultimately yielding ATP in the process of oxidative phosphorylation.

Lim CB et al Reduction in the rates of protein and amino acid catabolism to slow down the accumulation of endogenous ammonia a strategy potentially adopted by mudskippers during aerial exposure in constant darkness. J Exp Biol 2001 ... 

A protein that is unduT7 rich in the ten essential amino acids would not provide sufficient nitrogen for other metabolic processes without obligatory catabolism of the essential amino acids. Thus, the proportion of the total nitrogen intake that essential amino acids form indicate how a given protein fulfills nutritional requirements for proteins. This proportion, the E/T ratio (54), indicative of the amount of protein nitrogen supplied by essential amino acids, is (in g of essential amino acids per g of nitrogen)... 

The metabolic fate of the benzylpenicillin-human serum albumin conjugate was studied in rats [149]. The conjugate was taken up by the liver, where it underwent enzymatic cleavage to form benzylpenicilloic acid. Thus, the benzylpenicilloic acid excreted in urine may be formed either by direct hydrolysis of the /3-lactam ring, or by catabolism of protein conjugates formed in vivo. 

Synthesis of new proteins for growth or repair. This is discussed in Chapter 20. The rate of protein synthesis is a major factor determining the overall rate of amino acid metabolism the higher the rate of synthesis, the lower is the amino acid concentration which reduces the rate of catabolism. 

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