Comprehensive Biochemistry of Proteins, Amino Acids, and Enzymes

Protein

As organic molecules, proteins are the most abundant and important, making up 50% or more of their dry weight. They are found in all parts of all cells, as they are fundamental aspects of all structure and function. There are many different protein species, each specialized for a diverse biological function. Furthermore, the majority of expressed information is dictated by protein genes.

They belong to the peptide class because they are formed by amino acids linked together by peptide bonds. A peptide bond is the union of the amino group (-NH₂) of one amino acid with the carboxyl group (-COOH) of another amino acid through the formation of an amide. They are a basic constituent of life, deriving their name from the Greek word "proteios," meaning "in first place." In animals, proteins correspond to about 80% of the dry weight of muscles, and about 70% to 90% of the dry skin and blood. Even in vegetables, protein is present.

The importance of proteins, however, is related to their functions and not their quantity. All known enzymes, for example, are proteins, and enzymes often exist in very small percentages. Even so, these substances catalyze all metabolic reactions and enable bodies to construct other molecules—proteins, nucleic acids, carbohydrates, and lipids—that are necessary for living.

Amino Acids

Amino acids are the building blocks of the body. Besides entering cells and tissues, they:

• Form antibodies to fight invading bacteria and viruses
• Participate in the enzyme and hormonal systems
• Construct nucleoproteins (RNA and DNA)
• Transport oxygen throughout the body and participate in muscle activities

When proteins are divided (or separated) by digestion, 22 known amino acids result. Eight amino acids are essential (cannot be manufactured by the body); the other amino acids are non-essential (can be manufactured by the body with a proper diet).

Non-Essential Amino Acids

Also known as non-essential amino acids, these are produced by the animal's own body. The animal organism is capable of synthesizing 12 of the 22 existing amino acids in nature, while the rest must be removed from the diet. Plants, on the other hand, are capable of synthesizing all necessary amino acids.

Essential Amino Acids

These are amino acids that animals cannot manage to produce, yet they are mandatory for manufacturing proteins; therefore, they must be obtained from food.

List of Natural and Essential Amino Acids

Natural: Glycine, Alanine, Asparagine, Serine, Cysteine, Proline, Tyrosine, Glutamic Acid, Aspartic Acid, Arginine.

Essential: Histidine, Phenylalanine, Valine, Glutamine, Tryptophan, Threonine, Lysine, Leucine, Isoleucine, Methionine.

Since not every food contains all amino acids, the diet must be well-diversified. The foods richest in essential amino acids are of animal origin, such as meat, eggs, milk, and cheese.

Non-essential plant-based foods do not possess all amino acids on their own, so a vegetarian diet must be properly diversified.

Peptides

Peptides are biomolecules formed by the union of amino acids. The union of peptides occurs between the carboxyl group of one amino acid and the amine group of another, always resulting in the release of a water molecule. The mechanism linking amino acids is called a peptide bond.

Peptides are classified according to the number of amino acids each compound contains:

• Two amino acids: Dipeptide
• Three amino acids: Tripeptide
• Four amino acids: Tetrapeptide

Furthermore, based on the number of amino acids in the molecule, peptides can be classified as oligopeptides when they present from two to ten amino acids, and polypeptides when they present more than ten amino acids.

Enzymes

Enzymes are protein complexes (protein derivatives or heteroproteins) that act as biological catalysts in processes. Thus, the reactions that occur in living organisms are catalyzed by enzymes. In many cases, these are intracellular enzymes or endoenzymes. In other cases, they act outside the cell in which they are produced, thereby being called extracellular enzymes or exoenzymes.

They are easily destroyed by high heat (temperatures above 70°C), intense agitation, ultraviolet waves, ultrasound, substances such as sodium cyanide or sodium fluoride, traces of heavy metals, or acids and bases, etc. Regarding their action, the enzyme and the substance upon which it will act (called the substrate) form an intermediate compound which then undergoes a split, regenerating the enzyme. Enzymes are highly specific catalysts, meaning there must be specific enzymes for each substrate. The nomenclature is formed from the name of the substrate upon which the enzyme acts, followed by the suffix -ase. For example: urease, which hydrolyzes urea, and maltase, which hydrolyzes maltose.

Nucleic Acids

General Concepts

These are molecules with the function of storing and expressing genetic information. There are basically two types of nucleic acids:

• Deoxyribonucleic acid - DNA
• Ribonucleic acid - RNA

Nucleic acids are macromolecules formed by the phosphodiester linkage of nucleotides between 5 different fundamental units.