Lesson 4: Amino Acids and Proteins
4.1. Introduction
Adequate nutrition with sufficient intake of proteins and energy is essential to achieve a positive nitrogen balance and therefore growth and development. Proteins are the building blocks of the body, being the major structural component of all cells. Proteins are macromolecules, consisting of one or more long chains of amino acids subunits. To form a protein molecule, amino acids are joined together by peptide bonds. A polypeptide is a linear chain of amino acids. The chains can be formed by only a few amino acids (di, tri or oligopeptides) or by thousands forming polypeptides. X-ray crystallography has shown that proteins are not formed by linear chains, but they fold in a 3D structure (van Goudoever et al. 2014).
The mammalian proteins are formed by α-amino acids, which means that both the carboxyl group and the amino nitrogen group are attached to a central α-carbon:
Figure 3: α-amino acid: an amino (NH2) group, a carboxyl (COOH) group and a side chain attached to a central α-carbon.
Source: van Goudoever et al. 2014, reprinted with permission of S. Karger AG, Switzerland
Essential indispensable amino acids cannot be synthesized by humans but must come from exogenous sources (enteral or parenteral intake).
Non-essential dispensable amino acids can be made from other amino acids or amino acid precursors.
Another category of amino acids, the conditionally essential amino acids, can generally be synthesized by humans; however, extremely premature infants may be unable to synthesize these amino acids adequately, requiring exogenous administration (Table 2). Amino acids for parenteral administration to preterm infants are supplied in mixes which lead to free amino acid concentrations matching those of normally growing breast milk-fed infants (Koletzko et al. 2005).
Table 2: Amino acids by category
Source: Koletzko et al. 2005, reprinted with permission