Reality → Life → Biochem → Proteins
Proteins are the 'nanomachines' of life, sustaining physiological processes and providing support and movement of the biological cell [1] . Genes carry the code for biosynthesis of proteins. After their synthesis, proteins spontaneously fold and undergo other modifications that give each protein its specific functionality [2] . Scientists hypothesize that the function of a protein is determined by the sequence of amino acids in the protein's constituent polypeptide chains [3] . The enormous number of possible variations explains highly specialized adaptations, as evidenced by antibodies and antigens of the immune system, or by enzymes and hormones that sustain and regulate cell metabolism [4] . Proteins play also a key role in the photosynthesis by plants [5] , a process fundamental to life .
Structure and function of globular, membrane, and fibrous proteins are extremely diverse. Some super-complex proteins and complexes are fantastic molecular machines (such as the ribosome, the ATP synthase, or Cas9). Ion channels regulate the passing of ions through the membrane of electrically excitable cells and nuclear pore complexes regulate the traffic of molecules through the inner and outer membrane of a cell's nucleus. The cytoskeleton's fibrous proteins provide stability and motility. Microtubules and myosin play a key role in the motor functions of cell division and muscle contraction. Synthetic models (see 2016 Nobel Prize in Chemistry) help to understand the structure and function of molecular machines.
Starting already during the process of translation, segments of newly formed peptide chains start to fold and clump together under the influence of intramolecular hydrogen bonding and van der Waal forces. The amazingly complex protein folding process is an area of ongoing research (see protein structure prediction and design).
Amino acids contain amine (NH2, basic) and carboxylic (COOH, acidic) functional groups that can form peptide bonds (CO-NH, water (H2O) released) between adjacent molecules of different amino acids and thus build long polypeptide chains that form the base of very complex protein molecules (for this, the acids have to be of the α-type, i.e., both functional groups are attached to the α-carbons (first position) of the chain's backbone). Human proteins contain 20 different amino acids (two more protein-building amino acids are known to occur in nature and many more are not protein-building). Polypeptides can consist of more than 20,000 amino acid molecules, though the chains in most proteins consist of 100-200 amino acids. An astronomical number of variations of the chain's fine structure is possible, e.g., a chain of only 100 units allows 20100 permutations, or different sequences of 20 amino acids, a figure far larger than the number of atoms in the universe.
Antibodies are Y-shaped proteins with highly specific 'locks' at the Y-tips that fit to corresponding 'keys' of antigens. This way, millions of different microbes or cells can be tagged for attack by other parts of the immune system. Thousands of different enzymes catalyze almost all chemical reactions in cells (enzymes increase the reaction rate by active sites of a protein). Hormones are messenger molecules that regulate growth, function, or metabolism of cells via receptors. Many hormones contain amino acids, have protein structure, and are generated through protein biosynthesis, often in specialized cells (glands). Proteins are constantly transforming and renewing in the cell plasma through cell metabolism sustained by food intake. The hugely complex protein structure raises the puzzling question of its evolutionary origins.
Light-dependent reactions take place in the membranes of thylakoids, sub-organelles of chloroplasts, which are organelles of plant cells. Protein complexes, integrated into the thylakoid membranes are central to the absorption of photons and the increase of electron energy.