Proteins perform diverse roles and are involved in virtually all life processes in biological organisms.
Introduction
Proteins are among the most remarkable molecules in life sciences.
Why? First, they are the most abundant macromolecules in cells, accounting for up to ~50% of a cell’s dry mass. Second, cells and tissues contain a far greater variety of functional proteins than any other class of macromolecules. Finally, proteins participate in virtually every aspect of cellular structure and function.
Cell
Earth is populated by a huge diversity of organisms, the number of which is estimated to be in the millions.
Despite this diversity, which is manifested in morphology, behavior, diet, and modes of reproduction, there is one universal trait shared by all organisms; they are all made of cells.
The cellular structure is super important for maintaining life because it enables the organisms they build to distinguish themselves from the environment. That is to say, the cellular structure creates an inner environment that differs in its physical and chemical properties from the outer environment.
The manifestation of this distinction is what we call ‘life processes’, i.e., the ability of the cell to extract energy from its environment, build complex materials and degrade waste, grow, divide, move, etc.
Given that living organisms are made of the same atoms as inanimate matter, it may seem strange that cells are chemically unique. Indeed, carbon, hydrogen, oxygen, nitrogen, and sulfur, which are the common atoms of living tissues, all come from either the crust or atmosphere of our planet. However, there is a difference between chemical composition and molecular composition. That is, the uniqueness of biological cells is not expressed in their atomic composition, but rather in the way these atoms are organized in the form of molecules. Whereas the cells’ inanimate environment is made of simple molecules such as water (H2O), gases (O2, N2, CO2), metals and minerals, cells include, in addition to the above, complex molecules.
In particular, cells are rich in highly complex molecules termed macromolecules, which may contain thousands to millions of atoms. As we will see later, macromolecules are built from basic organic building blocks, all of which have unique properties that make the existence of macromolecules (and life) possible — the tendency to self-assemble. That is, these small organic molecules tend to chemically react and physically interact with each other to form larger and more complex molecules.
Macromolecules
There are three types of macromolecules: proteins, nucleic acids, and carbohydrates. These are responsible for the most basic aspects of life processes.
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Nucleic acids,i.e., DNA and RNA, function in the encoding and expression of the cell’s own genetic information.
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Complex carbohydrates function as energy stores in animals (glycogen) and plants (starch); as constituents of the cell wall in plants (cellulose) and of the exoskeleton of insects (chitin); and as a sophisticated means of molecular recognition.
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Proteins also play a variety of important roles in cells and tissues, and their unique properties distinguish them from nucleic acids, lipids, and carbohydrates.
The human body is estimated to contain only 20,500 genes but ~100,000 different proteins, and biochemical methods of protein detection suggest that each cell may express up to 15,000 distinct proteins.
Biological roles
Proteins carry out many roles and are involved in virtually all life processes in biological organisms. These roles can be grouped into a few types, which will be briefly discussed in the following subsections. Some functions overlap, as can be expected when dealing with such a complex system.
Catalysis of metabolic processes
Living organisms maintain a wide range of metabolic processes that allow them to grow or divide, extract energy from foodstuff, build complex materials, decompose waste products, detoxify harmful substances, etc. These metabolic processes, which are responsible for sustaining life in all organisms, involve thousands of chemical reactions that cells and tissues execute both simultaneously and consecutively.
Many of these reactions occur readily, as their products are more stable (i.e., have less free energy) than their reactants. However, the molecular needs of the organism dictate that these reactions must be completed within a timescale of 10−5 to 102 seconds. In stark contrast, many chemical reactions have much longer half-lives, which may span from minutes to millions of years.
During their long evolution processes, living organisms have developed means of speeding up the chemical reactions occurring within them.