There are several thousand types of different molecules in each cell (with some exceptions). These molecules are constantly changing and renewing themselves. Most of these molecules have a complex chemical structure resulting from a long molecular evolution.
In biochemistry, a biomolecule, or a biological molecule, is a molecule of living beings. Biomolecules are the essential building blocks of life and perform important functions in living organisms.
The six most abundant chemical elements or bioelements in living organisms are carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur. Biomolecules include nucleic acids, amino acids, proteins, lipids, carbohydrates and polysaccharides.
Biomolecules
- Nucleic acids: chemical substances carrying, in each cell, the coded hereditary instructions which allow the development of the organism.
There are two types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). In all plant and animal cells, the DNA is most of the time located in the nucleus, where it is compacted by histones and holds the coded genetic instructions. Whereas the RNA contributes to the transfer of those instructions into the cytoplasm of the cell, in order to be translated into a protein composed of amino acids.
- Amino acids: organic acids constituting the structural unit of proteins.
Hydrolysis of the proteins that make up living matter shows that they are all made up of an assembly of structural units called amino acids.
There are twenty different existing amino acids, namely : Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Phenylalanine, Tryptophan, Proline, Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine, Aspartic acid, Glutamic acid, Lysine, Arginine and Histidine.
- Proteins: macromolecules formed by the association of amino acids joined together by a peptide bond.
Proteins have very specific functions inside the body, for example haemoglobin is a protein involves in the carriage and delivery of oxygen in tissues.
A distinction is made between holoproteinas, which contain only amino acids, and heteroproteinas, which also contain a carbohydrate, lipid or mineral part.
- Lipids: substance containing fatty acids.
Blood serum lipids include free fatty acids, glycerol esters, or glycerides, comprising one or more fatty acids each attached to an alcohol group, cholesterol, free or esterified, as well as phospholipids, complex lipids which contain phosphoric acid and triglycerides.
- Carbohydrates: sugar molecules.
Your body breaks down carbohydrates into glucose. Glucose, or blood sugar, is the main source of energy for your body’s cells, tissues, and organs. Glucose can be used immediately or stored as glycogen in the liver and muscles for later use.
- Polysaccharides: complex sugar composed of several molecules of simple sugars.
The main polysaccharides are starch (carbohydrate reserve of plants), glycogen (carbohydrate reserve of animals, concentrated in the liver and muscles), and inulin (a dietary fibre that can be found in various plants and which is used as a storage medium containing energy).
They are transformed into simple sugars (glucose for starch and glycogen, fructose for inulin) during digestion.
Other polysaccharides, cellulose, hemicelluloses or even pectins, constitute dietary fibres, which cannot be assimilated by the human body.
- Enzyme: protein that speeds up chemical reactions in the body.
The general function of an enzyme is to catalyse a chemical reaction, meaning to accelerate it. Indeed, the enzyme and the substrate bind specifically to each other.
The enzyme remains unmodified at the end of the reaction process, contrary to the substrate which is generally modified to result in one or several new molecules called products.
Inside the cells, the enzymes are thus responsible both for the synthesis of new substances used to build the cell (anabolism) and for the degradation of substances used to produce energy (catabolism).
Their role is vital, because the physicochemical conditions (temperature, pH) that prevail in the body prevent most reactions from occurring at a sufficient rate.
There are thousands of enzymes, many of which are not yet known. Their most remarkable property is their double specificity : on the one hand, an enzyme is dedicated to a specific substrate. On the other hand, an enzyme is able to catalyse only one chemical reaction for a specific substrate.
- As being proteins, enzymes also have a similar composition : a very long chain of amino acids with various compositions. Meaning that each enzyme, as well as each protein, is composed of an amino acids sequence that is not the same from one to another.
Many also contain a non-protein part, which is called apoenzyme.
DNA
As previously mentioned, DNA is an acronym that stands for deoxyribonucleic acid. It is a molecule present in all living beings and which carries the genetic information necessary for the development and functioning of the organism.
This molecule has the shape of a very long ladder that would have been twisted on itself, which is why it is often nicknamed the double helix. The “uprights” of the scale are composed of alternating molecules of sugar (deoxyribose) and phosphate. The “runs”of the scale are attached to the sugars of each amount and are formed by two nitrogenous bases facing each other. They are, in a way, the building blocks that make up the DNA molecule.
There are four different nitrogenous bases: adenine (A), thymine (T), cytosine (C) and guanine (G). The creation of bonds occurs in a special way : the A bases always pair with the T bases, and the C bases always pair with the G bases. It is the succession of nitrogenous bases (ACCATTCGCT…) which determines the information contained in the DNA, according to a code called the genetic code.
DNA is associated with histones, basic proteins that play an important role in DNA packing and folding. The DNA double helix is a stable structure well suited to the transmission and storage of genetic information.
However, many aspects of DNA metabolism require access to a single-stranded form in which the nucleotide bases are accessible, allowing the intervention of remodelling proteins, among which are helicases and topoisomerases.
Helicase
Helicase is an enzyme that promotes the opening of the DNA helix, separating it into two single strands to make the replication occur. Indeed, before the cell division either for mitosis or meiosis, two copies of DNA are produced : this is called DNA replication.
Since DNA has a structure like a twisted ladder, it is first necessary to unwind it and then separate the two strands like a zipper before starting replication. When DNA is “closed”, the rungs of the ladder are made up of two nitrogenous bases facing each other. The helicase breaks the hydrogen bonds between the nitrogenous bases, leading to their separation and allows other enzymes to copy the DNA sequence. Thus from an initial molecule, this mechanism of DNA replication results in the creation of two identical DNA molecules.
In addition to DNA replication, many helicases participate in maintaining the integrity of the genome, but are also involved in numerous crucial phenomena in the human body such as :
- DNA recombination (the action of breaking and rejoining pieces of DNA) ;
- DNA repair (meaning the recognition of altered DNA to repair it, a natural protection of the genetic information against environmental damage and replication failure) ;
- Transcription (transfer of the genetic information from DNA to RNA) ;
- RNA splicing (it means that only coding regions of RNA remain, they are called exons, whereas the non-coding regions called introns are removed, to produce a mature mRNA molecule) ;
- And finally translation (transformation of the genetic code carried by mature mRNA molecule to create a particular sequence of amino acids, also called protein biosynthesis).
When helicases undergo mutations, it leads to disorders in the repairing system with severe clinical consequences.
In fact, about fifty helicases have been biochemically characterised, and about ten of them are associated with pathologies such as Werner’s syndrome (premature ageing causing a high risk of cancer), thalassemia (blood disease), breast cancer and even sclerosis.
