Proteins as the Primary Carriers of Life Activities

Section 4: Proteins as the Primary Carriers of Life Activities

Proteins are the organic compounds most abundant in cells. From a chemical perspective, proteins are the most structurally complex and functionally diverse molecules known to date. Genetic information in the cell nucleus often needs to be expressed as proteins to function. Proteins are the primary carriers of life activities.

In general, proteins are the basic components of cells, playing crucial roles in structural formation, catalysis, transport, information transfer, defense, and other important functions. It can be said that every life activity of a cell relies on proteins.

The ability of proteins to perform such diverse functions is due to their diversity. There are tens of thousands of different proteins in the human body. It is estimated that there are as many as 10^10 to 10^11 types of proteins in the biological world.

Why are there so many types of proteins with such diverse functions? Is this related to their composition and structure?

Basic Units of Proteins - Amino Acids

Collagen, which is used as surgical sutures, can be absorbed by human tissues because it breaks down into amino acids that can be absorbed by the body.

In the human body, there are 21 types of amino acids that make up proteins. Amino acids are the basic units of proteins. What is the structure of amino acids?

Other amino acids have structures similar to the four amino acids mentioned above, meaning each amino acid has at least one amino group (—NH2) and one carboxyl group (—COOH), both connected to the same carbon atom. This carbon atom is also attached to a hydrogen atom and a side chain group represented by R (Figure 2-9). The difference between various amino acids lies in their R groups; for example, glycine has a hydrogen atom (—H) as its R group, while alanine has a methyl group (—CH3).

Although the types of amino acids are limited, they form a wide variety of functionally diverse proteins.

Structure of Proteins and Their Diversity

Proteins are large biomolecules composed of amino acid units. Initially, amino acid molecules link together through dehydration synthesis: the carboxyl group (—COOH) of one amino acid molecule bonds with the amino group (—NH2) of another amino acid molecule, releasing a molecule of water in the process, a bond called a peptide bond. Compounds formed by the condensation of two amino acids are called dipeptides.

Similarly, compounds formed by the condensation of multiple amino acids, containing multiple peptide bonds, are called polypeptides. Polypeptides usually have a linear chain structure known as a peptide chain. Due to hydrogen bonds and other interactions between amino acids, the peptide chain can coil and fold, forming a protein molecule with a specific spatial structure. Many protein molecules consist of two or more peptide chains, bonded together by chemical bonds such as disulfide bonds (Figure 2-11). These peptide chains are not linear and often do not lie in the same plane but instead form more complex spatial structures. For example, hemoglobin is a protein consisting of 574 amino acids, with four polypeptide chains, and its spatial structure is shown in Figure 2-12.

Within cells, a protein may consist of thousands of amino acids. The arrangement of different types of amino acids in a peptide chain can vary infinitely, leading to diverse coiling and folding patterns and resulting in a wide range of spatial structures for protein molecules. This diversity is the reason for the multitude of protein types in cells.

Each protein molecule has a unique structure adapted to its specific function. Changes in amino acid sequence or alterations in protein spatial structure can affect its function. For example, normal human hemoglobin has a spherical spatial structure, contributing to the biconcave disc shape of red blood cells it forms. If a glutamic acid in hemoglobin is replaced by a valine, abnormal hemoglobin may aggregate into fibrous shapes with properties different from normal hemoglobin, causing red blood cells to distort into sickle shapes (Figure 2-13) and greatly impairing their ability to transport oxygen.