2
Amino Acids, Peptides, and Proteins

2.1 Introduction

Aminocarboxylic acids occurring in nature have vital functions in living organisms. These amino acids are found as free substances or higher-molecular-weight compounds, where the amino acid building units are connected to one another by amide bonds, C(O)NH, termed peptide bonds. Depending on the number of bound amino acids, these compounds are divided into two main groups:

• peptides, usually composed of 2-100 monomers;
• proteins, which contain more than 100 monomers, but also hundreds or even thousands of amino acids.

Proteins and peptides may even contain some other compounds in addition to amino acids. Proteins are undoubtedly the most important amino acid derivatives: they are the basic chemical components of all living cells and therefore are also part of almost all food raw materials and foods of plant, animal, and microbial origin. In organisms, proteins perform a number of unique and extraordinary functions. Along with ribonucleic acids (RNA) and deoxyribonucleic acids (DNA), polysaccharides, some lipids, and other macromolecules, peptides and proteins are often known as biological polymers or biopolymers. Nucleic acids (RNA and DNA) have almost no significance as food components in human and animal nutrition, although they play fundamental roles in living systems. Plants and some microorganisms are capable of synthesising proteins from basic substrates such as carbon dioxide, water, and inorganic nitrogen compounds, but animals rely on getting their necessary vegetable or animal protein from their food. In the process of digestion, the food proteins are enzymatically broken down (hydrolysed) to their building blocks, from which animals may synthesise their own proteins or use them (along with carbohydrates and lipids) as a source of energy. Therefore, proteins, together with carbohydrates and lipids, belong to the category known as main (primary) nutrients.

The following sections deal with food-related important amino acids, peptides, and proteins, their structure, occurrence, properties, and fate in the human organism. Nutritional aspects and important interactions and reactions that fundamentally affect the nutritional value, organoleptic properties (odour, taste, colour, and texture), and toxicological quality of food commodities are also discussed.

2.2 Amino acids

Plants, animals, and other organisms have been shown to contain more than 700 different amino acids. Some of these are spread quite generally throughout nature, whilst others occur only in certain species of plants, animals, and other organisms. According to their origins, therefore, the following two groups of amino acids are recognised:

• amino acids found in all living organisms (bound in proteins or peptides or occurring as free amino acids);
• amino acids found only in some organisms (bound in peptides or present as free compounds) that are not protein constituents.

The amino acids bound in proteins (22 compounds) are called proteinogenic, encoded, basic, standard, or primary amino acids; 21 of them are constituents of proteins in food raw materials and foods. Of the 22 proteinogenic amino acids, 20 are encoded by the universal genetic code. The remaining two amino acids (selenocysteine and pyrrolysine) are incorporated into proteins by unique synthetic mechanisms. For example, pyrrolysine occurs as a component of enzymes involved in the production of methane in some methanogens, members of a group of single-celled microorganisms of the Archaea domain. Amino acids bound in peptides and free amino acids have the same nutritional value as proteins, but their importance in nutrition is usually negligible due to the small amounts in which free amino acids and peptides commonly occur in foods.

The process of protein biosynthesis is called translation. Posttranslational oxidation, alkylation, and esterification of some amino acids that are bound in proteins yield modified proteinogenic amino acids (see Section 2.2.1.1.2). Non-proteinogenic (non-encoded, non-standard, or secondary) amino acids do not function as building blocks of proteins, as they have other roles in organisms. Amino acids also have an influence on the organoleptic properties of food, especially on their taste. Products of reactions of amino acids are often important compounds influencing the odour, taste, and colour of foods.

2.2.1 Structure, terminology, classification, and occurrence

Amino acids are organic compounds that contain at least one amino group, in most cases primary NH2, together with at least one carboxyl group, COOH, and various side groups in the molecule. They can also be defined as carboxylic acids substituted by an amino group. According to the distance of the amino group from the carboxylic group, amino acids (2-1) are generally divided into:

• 2-aminocarboxylic acids (a-aminocarboxylic acids), which have the amino and carboxyl groups attached to the same carbon atom (such as a-alanine);
• 3-aminocarboxylic acids (ß-aminocarboxylic acids), which have the amino and carboxylic groups attached to adjacent carbon atoms (such as ß-alanine);
• 4-aminocarboxylic acids (?-aminocarboxylic acids), which have three (n = 2) carbon atom (group) between the amino and the carboxylic groups (such as ?-aminobutyric acid, GABA);
• 5-aminocarboxylic acids (d-aminocarboxylic acids), which have four (n = 3) carbon atoms (groups) between the amino and the carboxylic groups (such as d-aminolaevulinic acid);
• 6-aminocarboxylic acids (e-aminocarboxylic acids), which have five (n = 4) carbon atoms (groups) between the amino and the carboxylic groups (such as e-aminocaproic acid).

The amino acids also include carboxylic acids, which contain a secondary amino group NH in the molecule that is a part of three-, four-, five-, or six-membered rings. These amino acids are actually derivatives of the saturated nitrogen heterocycles aziridine, azetidine, azolidine (pyrrolidine), and azinane (piperidine), or of other more complex heterocyclic compounds. The only proteinogenic amino acid with a secondary amino group is proline, which comprises a pyrrolidine ring.

In most foods, about 99% of amino acids are bound in proteins and peptides. The rest (about 1%) are free amino acids. More free amino acids are found in foods in which proteolytic enzymes or chemical agents have hydrolysed proteins during manufacture or storage. Larger amounts of free amino acids can be found in some cheeses, beer, and wine. The enzymatic hydrolysates of proteins (such as soy sauce) or acidic protein hydrolysates (used as soup condiments) contain only free amino acids with small amounts of peptides, but no protein.

2.2.1.1 Amino acids of proteins

2.2.1.1.1 Proteinogenic amino acids

Proteinogenic amino acids bound in all proteins are exclusively 2-amino (a-amino) acids that have the primary or secondary amino and carboxyl groups attached to the same carbon atom in position 2 (a) to the carboxyl group.

The proteins in most organisms contain only 21 basic amino acids, of which 20 have the primary amino group, whilst one is an alicyclic amino acid with a secondary amino group (2-2). All amino acids except glycine (which is achiral) are optically active (chiral) compounds that contain at least one asymmetric centre (chiral carbon atom) and thus can occur in at least two non-superimposable mirror-image forms, D- and L-forms, known as optical isomers or enantiomers. Proteinogenous amino acids almost exclusively belong to the L-series, and thus have the L-configuration (see Section 2.2.3.2).

One or more hydrogen atoms of the substituent R of L-?-amino acids, known as a side chain, specific to each amino acid (2-2), may be substituted by a carboxyl group (called a distal carboxyl group), an amino group (distal amino group), or other functional groups, such as a hydroxyl group OH, a sulfhydryl (mercapto) group SH, a sulfide group SCH3, a guanidino group NHC(NH)NH2, or a phenyl group C6H5. The methylene groups or nitrogen atom of the secondary amino group of a-amino acids can also be substituted (2-2).

The proteinogenic amino acids (2-3) are often known by their trivial names, which are derived from their properties or the source from which they were first isolated. Systematic names of the proteinogenic amino acids are rarely used, but are more frequent in the case of non-protein amino acids. Each amino acid has both a three-letter code (mostly the first three letters of the trivial name) and a single-letter code, which is used for the registration of long sequences of amino acids in proteins (Table 2.1).

Table 2.1 Trivial and systematic names of proteinogenic amino acids and their codes.