BIOCHEMISTRY: A TEXTBOOK FOR UNIVERSITIES - E. S. Severin - 2004

CHAPTER 9. AMINO ACID METABOLISM AND FUNCTIONS

The Biological Significance of Amino Acids primarily stems from their utilization in Protein Synthesis. Protein METABOLISM occupies a central place in the Metabolic exchange between the Organism and the external environment. This is because Proteins are fundamental Structural components of Cells, Tissues, and Organs in humans and animals, perform enzymatic Functions, mediate transmembrane transport, and fulfill many other vital roles. Furthermore, proteinaceous Hormones play a crucial role in coordinating the functions of all cellular systems.

Amino acids are directly involved in The Biosynthesis of not only proteins, but also a wide array of other biologically active compounds that regulate metabolic processes, such as amino acid-derived Neurotransmitters and hormones. Moreover, amino acids serve as nitrogen Donors in the synthesis of all non-protein nitrogenous compounds, including NUCLEOTIDES, heme, creatine, Choline, and various other substances.

The Catabolism of Amino acids can serve as an energy source for ATP synthesis. The energetic function of amino acids becomes particularly significant during starvation, certain pathological states (such as Diabetes Mellitus), and predominantly protein-based diets. Indeed, Amino acid metabolism serves as the linking hub for the diverse chemical transformations occurring in living organisms.

I. Sources and Utilization Pathways of Amino Acids in Cells

The body's free amino acid pool is approximately 35 g. The concentration of free amino acids in Blood ranges on average from 35 to 65 mg/dL. The vast majority of amino acids are incorporated into proteins, the total amount of which in a normal adult human body is approximately 15 kg.

The primary sources of free amino acids in cells include dietary proteins, endogenous tissue proteins, and the synthesis of amino acids from CARBOHYDRATES. Many Cell types—with the exception of highly specialized ones, such as erythrocytes—utilize amino acids to synthesize proteins as well as numerous other substances: membrane Phospholipids, heme, purine and pyrimidine nucleotides, biogenic amines (catecholamines, histamine), and various Other Compounds (Fig. 9-1).

Class="center">Fig. 9-1. Sources and Pathways of amino acid utilization.

Unlike glucose (stored as Glycogen) or Fatty acids (stored as triacylglycerols), there is no specialized storage depot for amino acids. Consequently, the body's Functional and Structural tissue proteins serve as an amino acid reserve, predominantly Muscle Proteins, as they constitute the largest mass.

In The Human Body, approximately 400 g of protein are degraded into amino acids daily, and roughly the same amount is synthesized. Therefore, tissue proteins alone cannot compensate for the depletion of amino acids resulting from catabolism and their utilization in the Synthesis of Other substances. Carbohydrates also cannot serve as a primary source of amino acids, since they provide only the carbon Skeleton for the majority of amino acids, whereas the amino group must be derived from Other Amino Acids. Consequently, dietary proteins serve as the principal source of amino acids for the organism.