Biochemistry can be defined as the science that deals with the study of the chemical constituents of living cells and the reactions and processes they undergo. Biochemistry also aims to explain the origin of life, a fascinating subject that is still in its early stages.
Life emerged on Earth about three billion years ago in the form of microorganisms with the ability to obtain energy from organic, inorganic, or even sunlight compounds. This energy, along with compounds present on the Earth's surface, allowed for the synthesis of a wide variety of complex organic molecules that, when properly combined, give rise to the complex properties of living organisms.
Chemical Composition of Living Beings
Living beings are composed of different organic and inorganic molecules, with water being the most abundant substance, constituting between 50% and 95% of the cell's weight. Sodium, potassium, magnesium, and calcium cations can represent up to 1%. The remaining weight of living organisms consists of macromolecules composed mainly of carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur, as well as small amounts of other metallic and non-metallic elements.
Most biomolecules have a carbon skeleton to which different groups, called functional groups, are attached, conferring characteristic properties and reactivities. For example, sugars have the carbonyl and hydroxyl functional groups in their structure, while amino acids have the amino and acid groups, and may contain other functional groups in their chain.
Main Types of Biomolecules
Within the cell, we find a large number of small molecules with molecular masses below 10,000 Daltons (D), which can be classified into four families: amino acids, sugars, fatty acids, and nucleotides. One of the functions of these small molecules is to act as monomers in the synthesis of larger molecules (polymers). Amino acids are the basic constituents of proteins; nucleotides make up nucleic acids (DNA and RNA), and small sugar molecules form the long chains of some carbohydrates such as glycogen.
Amino Acids and Proteins
Amino acids are classified as $\alpha$, $\beta$, or $\gamma$ depending on the position of the amino group relative to the carboxylic acid group. $\alpha$-amino acids have the amino group in the position adjacent to the carboxylic acid, from which the side chain also originates. Proteins are formed by 20 $\alpha$-amino acids, although some of them have additional functions, such as glycine and glutamic acid, which also act as neurotransmitters.
However, not all amino acids that are part of living beings are $\alpha$-amino acids. $\gamma$-aminobutyric acid (GABA) is a neurotransmitter found in the brain, and $\beta$-alanine is a precursor of pantothenic acid (vitamin B5).
Amino acids are joined by amide-type bonds, called peptide bonds, to form chains of variable length. Chains with fewer than fifty amino acids are called peptides, while longer polypeptides are called proteins. Proteins perform a wide variety of functions in the cell, including structural, enzymatic, and transport functions, among others.
Sugars and Carbohydrates
Sugars are organic compounds that contain carbonyl and alcohol groups in their structure. Those with an aldehyde function are called aldoses, while those with a ketone function are called ketoses. The most well-known sugar and one of the primary sources of energy for cells is glucose, which consists of six carbon atoms with an aldehyde group at position 1.
These small sugar molecules (glucose, fructose, galactose, ribose, etc.) are called monosaccharides and serve as the monomers for the formation of long-chain sugars or polysaccharides. Polysaccharides are polymers composed of thousands of monosaccharide units.
Glycogen is a polysaccharide formed by glucose units that animals use as an energy storage molecule. Glucose units are extracted from glycogen and "burned" to obtain energy. Other polysaccharides have structural functions, such as cellulose, which makes up the fibers in wood, or chitin, which forms the protective exoskeleton of crustaceans.
Sugars are also found as part of other biomolecules, such as ribose and deoxyribose, which are components of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Glycoproteins and glycolipids, abundant on the outer surface of cell membranes, are proteins and lipids bonded to sugar units.
Fatty acids are molecules consisting of a carbon chain and a carboxylic acid group at one end. They are monocarboxylic acids (R-COOH). The carbon chain can have double bonds between carbon atoms, in which case they are called unsaturated fatty acids, while those with only single carbon-carbon bonds are called saturated fatty acids.
The carboxylic acid group (-COOH) has a highly acidic hydrogen, and at physiological pH, it is ionized. Therefore, fatty acids are found in cells in the form of carboxylates. For example, oleic acid is found as oleate, and palmitic acid is found as palmitate. It is important to note that the amounts of fatty acids present in cells are minimal; most of them are part of more complex structures such as triglycerides and phosphoglycerides, which are structural components of cell membranes.
Fatty acids are insoluble in water due to their long carbon chain (apolar), although the carboxylic acid group can interact with water molecules, leading to the formation of micelles in the aqueous phase.
Nucleotides and Nucleic Acids
Nucleotides are the units that make up the long chains of nucleic acids. They consist of a sugar (ribose or deoxyribose), a nitrogenous base, and one or more phosphate groups. There are five nitrogenous bases, classified into two families: purines and pyrimidines. Purines are composed of two condensed rings and include adenine (A) and guanine (G). Pyrimidines consist of a single ring and include thymine (T), cytosine (C), and uracil (U).
Nucleotides serve as the structural units of nucleic acids (DNA and RNA), but they also have important energetic functions in the cell. For example, adenosine triphosphate (ATP) stores the energy released during food combustion in the form of phosphate bonds.
Deoxyribonucleic Acid (DNA)
DNA is composed of the sugar deoxyribose and the nitrogenous bases adenine, guanine, cytosine, and thymine. It also contains a phosphate group that connects two nucleotides through a phosphodiester bond. The structure of DNA is a double helix stabilized by hydrogen bonds formed between complementary bases. Adenine from one strand forms hydrogen bonds with thymine from the complementary strand, while guanine pairs with cytosine. A gene is a segment of DNA that contains the information to synthesize a protein. Although DNA is not directly involved in protein synthesis, its instructions are first copied into another molecule called RNA through a process known as transcription.
Ribonucleic Acid (RNA)
RNA is composed of the sugar ribose and the nucleotides adenine, cytosine, guanine, and uracil. Similar to DNA, nucleotides in RNA are connected by phosphodiester bonds, but unlike DNA, which is double-stranded, RNA consists of a single strand. RNA is synthesized by copying a segment of DNA in a process called transcription. The RNA molecule leaves the nucleus and travels to the ribosome, where it translates the information it contains into a sequence of amino acids or a protein.
There are three types of RNA that perform specific roles in protein synthesis: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). Messenger RNA is formed in the cell nucleus by copying a gene from DNA and carries the necessary information for protein synthesis. Ribosomal RNA, together with proteins, forms ribosomes, supramolecular complexes responsible for linking the amino acids that make up the protein. Finally, transfer RNA is responsible for carrying amino acids to the ribosome. Each transfer RNA molecule specifically binds to an amino acid and possesses a triplet of complementary bases with those of messenger RNA, allowing the pairing of both types of RNA to join the amino acids in the sequence encoded in the mRNA.