Proteins: Our Body's Bricks

In the realm of biology and nutrition, proteins hold a pivotal position. They are not just the building blocks of life, but also the fuel that powers our body's various functions. This article aims to delve into the depths of proteins, their chemical composition, their functions, and their biological value.

Proteins: The Building Blocks of Life

Proteins, also known as polypeptides, are macro-molecules or polymers composed of sequences of amino acids. These are quaternary molecules, made up of carbon, hydrogen, oxygen, and nitrogen. In the context of nutrition, proteins are considered energetic macronutrients, providing 4 kcal/g through various metabolic processes. However, their role extends beyond mere energy provision. Proteins boast numerous and specific functions that are critical for life.

Amino acids, the building blocks of proteins, contain different functional groups, including an amino group and a carboxylic acid group. There are approximately twenty different amino acids found in nature, which can combine in myriad ways to form tens of millions of unique proteins. Some of these amino acids, known as essential amino acids, cannot be synthesized by the body and must be obtained through diet.

The bond between amino acids, known as a peptide bond, is established through a condensation reaction, resulting in the loss of a water molecule. Depending on the number of amino acids linked together, these chains can be classified as oligopeptides, polypeptides, or proteins.

Proteins can be broken down into their constituent amino acids through a process called hydrolysis, which can occur during digestion or cellular catabolism. Each protein acquires specific properties based on its spatial arrangement, which is determined genetically.

The Multifaceted Functions of Proteins

Proteins play a myriad of roles in the body, thanks to their structural dynamism that allows them to interact with other molecules. They provide essential amino acids that facilitate several key functions, including:

• Regulatory function: Proteins serve as precursors for hormones and neurotransmitters.
• Catalysis function: Most enzymes, which facilitate biochemical reactions, are proteins.
• Plastic function: Amino acids serve as "building blocks" for the construction of new proteins in the body.
• Contractile function: Proteins enable the contraction of muscle fibers.
• Blood coagulation function: Certain proteins facilitate the formation of a clot to prevent excessive bleeding.
• Membrane channel function: Proteins form channels in the cell membrane, allowing the passage of ions and substances.
• Blood transport function: Proteins transport nutrients and oxygen in the blood.
• Immune function: Proteins form immunoglobulins or antibodies that defend the body against pathogens.
• Physical barrier function: Proteins form physical barriers in external lining epithelia.
• Genetic transmission function: Proteins play a role in the transmission of genetic characteristics.

Essential Amino Acids: The Indispensable Building Blocks

Out of the twenty amino acids found in nature, more than half are considered essential. This means that they cannot be synthesized by the body and must be obtained through diet. The essential amino acids for adults include phenylalanine, isoleucine, histidine, leucine, lysine, methionine, threonine, tryptophan, and valine. For growth and in certain pathological conditions, arginine, cysteine, and tyrosine are also considered essential.

The synthesis of a protein requires all essential amino acids. Therefore, the diet must provide these amino acids in the right quantities and proportions to ensure satisfactory protein synthesis. This is particularly crucial for athletes or bodybuilders, where protein synthesis plays a decisive role in muscle recovery and growth.

Biological Value: A Measure of Protein Quality

The biological value (BV) of a protein refers to the proportion of dietary protein that the body can utilize for protein synthesis. For instance, the BV of whole egg proteins is 97 out of a maximum of 100, implying that the body could synthesize 97g of its own proteins from 100g of egg protein.

Proteins from animal sources generally have a higher BV than those from plant sources. The BV is determined by the quantity and proportion of essential amino acids present in a protein, also known as the amino acid spectrum. If a protein lacks even one essential amino acid, protein synthesis is halted.

The BV is calculated by the ratio of nitrogen retained by the body for protein synthesis to the nitrogen absorbed from the diet. Nitrogen is an indicator of the amino acid spectrum of essential amino acids. The digestive utilization coefficient, which is the percentage of dietary protein absorbed as nitrogen, is also used in determining the BV.

In conclusion, proteins are an indispensable part of our diet, providing not just energy but also facilitating a host of crucial biological functions. Understanding their composition, function, and biological value can help us make informed dietary choices to support our health and well-being.