Fundamentals of Molecular Biology. Part 1: Molecular Cell Biology - A. N. Ogodtsov 2011

Nucleic Acids and Proteins
Protein Functions

A protein is a single polypeptide or an aggregate of several Polypeptides that performs a biological function.

It should be emphasized that a polypeptide is a chemical concept, whereas a protein is a biological one.

For example, an immunoglobulin consists of four polypeptide chains that individually are not Proteins; a protein is only their functional aggregate (see sec. 6.6.1).

Nine key Functions of Proteins are distinguished.

1. Structural function. Proteins are essential components of all cellular Organelles: membrane-bound ones—Cell/30.html">The Plasma Membrane, nuclear envelope, Endoplasmic reticulum, Golgi apparatus, Lysosomes, Peroxisomes, vacuoles, Mitochondria, and Plastids—and non-membrane-bound ones—Chromosomes, Ribosomes, cell center (centrioles), Cilia and flagella, and microfilaments.

2. Catalytic function. All biocatalysts (except ribozymes) are proteins. In 1982, this function ceased to be considered unique when it was discovered that certain RNA molecules also possess catalytic activity, referred to as RNases or ribozymes.

3. Protective function (still unique). Antibodies are proteins. IMMUNOGLOBULINS "glue" Antigens together to form a precipitate.

4. Regulatory function. At THE CELLULAR LEVEL: repressor proteins and METABOLISM/31.html">Transcription activator proteins. At the Organism level: certain Hormones are proteins.

For instance, the pancreatic hormone Insulin regulates the passage of glucose across the plasma membrane. Insufficient insulin secretion leads to a severe condition known as Diabetes Mellitus. Growth Hormone, or somatotropin, is produced in the anterior Pituitary Gland. The same region produces adrenocorticotropic hormone (ACTH), which acts on the adrenal cortex to regulate the synthesis of Steroid Hormones.

5. Energy transformation. The retinal proteins rhodopsin and retinal convert light energy into electrical energy. Actin-Myosin complexes in Muscles transform chemical bond energy into mechanical energy. F₀F₁-ATP synthases convert the mechanical energy of the proton flow across the membrane into chemical energy by synthesizing ATP from ADP (see section 6.7.1).

6. Transport function. Hemoglobin carries out The transport of O₂ and CO₂, while transferrin transports iron. Permease systems are Membrane Proteins that transport polar compounds across the membrane both along and against the concentration gradient. Membrane ion-pumping ATPases facilitate the Transmembrane Transport of ions.

7. Energy function. 10 out of the 20 Amino Acids that make up proteins "burn" in The Human Body to release energy. These are non-Essential Amino Acids, which can be synthesized within The Cell from carbohydrate and lipid breakdown products.

8. Nutritional function.

a) Supplying essential amino acids. In humans, 10 out of the 20 amino acids—Arginine, valine, Histidine, isoleucine, leucine, Lysine, Methionine, Threonine, Tryptophan, and phenylalanine—cannot be synthesized by the body and must be obtained from external sources.

The distinction between "essential and non-essential amino acids" is species-specific and applies only to animals and Fungi. Furthermore, the adult organism does not require arginine (which is why arginine is often excluded from essential amino acids), while the presence of Cysteine and Tyrosine in the diet reduces the body's requirement for methionine by 80% and phenylalanine by 70%, respectively.

b) Reserve Proteins for embryonic development and nourishing the newborn. Examples include casein (milk protein), Ovalbumin (egg protein), and gliadin (wheat grain protein).

9. Buffering function. Every protein is an amphoteric polyelectrolyte. Proteins help maintain specific pH values within different cellular compartments, thereby ensuring proper compartmentalization.