Proteins in general: properties and functions

Protein is often discussed in the context of nutrition, but its role is far broader. The diversity of proteins’ physical and chemical properties, which stems from their differing amino acid composition, is astonishing – few molecules can perform such a wide range of tasks.

Why do proteins come in such different forms?

Some proteins don’t dissolve in water at all, while others dissolve easily. Some are chemically inactive and resistant to all kinds of influences. Others are extremely unstable – they change at even the faintest light or the lightest touch.

In terms of shape, there are thread-like molecules that are hundreds of nanometers long, and globular ones with a diameter of just 5–7 nanometers. A simple truth: every protein’s structure matches its function exactly.

Structures made of keratin – horns, hooves, scales, nails, feathers, hair – are strong and durable, for example, because keratin molecules are built precisely for that purpose. Muscle proteins are thread-like and able to contract and extend. Transport proteins are small and round so that they can move quickly.

Proteins as building material: the foundation of the cell

First and foremost, proteins are the cell’s building material. They make up the membranes of cells and organelles, blood vessels, tendons, cartilage and many other structures. Without proteins, there would be neither cells nor organisms.

From proteins to enzymes: the catalytic function

The catalytic function of proteins is exceptionally important. The chemical activity of substances inside a cell is usually low, their concentration negligible and the temperature low – in theory, reactions should proceed very slowly. Yet they happen fast. Why?

Thanks to enzymes. Enzymes are the cell’s catalysts – protein molecules that speed up chemical reactions by tens and hundreds of millions of times. Almost every chemical reaction in a cell is catalyzed by its own specific enzyme, and there are several thousand such reactions in a cell – accordingly, several thousand different enzymes have been discovered.

The secret of how an enzyme works lies in its active site. Catalytic activity doesn’t come from the whole molecule, but from one small part of it. The enzyme’s active site and the reacting substance fit together like a lock and key – the geometric match is exact. The molecular mass of catalase is about 250,000, whereas that of hydrogen peroxide, which it breaks down, is just 34. When an enzyme is denatured, it loses its catalytic activity because the structure of the active site is destroyed.

Signaling, movement, transport and defense functions

Proteins also play a signaling role. Proteins in the cell’s outer membrane are able to change their tertiary structure in response to the external environment – this is how signals are received and commands are passed into the cell.

Movement is a hallmark of life. All types of movement, including muscle contraction in higher animals, are based on contractile proteins. Without these proteins, nothing would move.

Transport proteins bind various substances and carry them from one place to another within the cell. The blood protein hemoglobin binds oxygen and carries it throughout the body to all tissues and organs.

The defense function is also important. When foreign substances or cells invade the organism, it starts producing special defense proteins, which bind the invaders and render them harmless.

Proteins as an energy source

Proteins are also a source of energy – though not the primary one. Proteins are broken down in cells into amino acids. Some of the amino acids are used to synthesize new proteins, while the rest are broken down completely. The breakdown of 1 gram of protein releases 17.6 kJ of energy.

FAQ about proteins

What is the difference between a protein and an enzyme?
An enzyme is a catalyst made of protein that speeds up chemical reactions in the cell. All enzymes are chemically proteins, but not all proteins are enzymes.

Why does amino acid composition determine a protein’s properties?
Amino acids determine a protein’s three-dimensional shape and therefore its function. Different amino acid sequences produce molecules with different structures and different jobs.

How many different proteins are there in a cell?
Several thousand different enzymes have been discovered in cells – plus many other classes of proteins. The number of different reactions in a cell runs to several thousand, and each reaction has its own specific enzyme.

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Mikk-Alvar Olle (ref. Obshaja Biologija)

Source: WHO – healthy eating.

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