Enzymes, Nucleic Acids, and RNA Types: A Comprehensive Look
Feedback Inhibition
Feedback inhibition can be considered a form of allosteric effect. It occurs when the final metabolite of a metabolic pathway acts as an inhibitor of the first enzyme in that pathway, thereby preventing the metabolite’s own production. This effect is achieved because the final metabolite inhibitor is able to bind to the regulatory site of the first enzyme.
Enzyme Classification
- Oxidoreductases: Redox reactions involving electron loss and gain.
- Transferases: Catalyze the transfer of functional groups or radicals (other than hydrogen) between substrates.
- Hydrolases: Catalyze hydrolysis reactions, introducing OH and H groups. Examples include carbohydrases, esterases, peptidases, and nucleases.
- Lyases: Catalyze reactions involving the breakage and formation of bonds without the intervention of water.
- Isomerases: Conversion of one isomer to another.
- Ligases (Synthases): Join molecules or a functional group to a molecule, using energy provided by ATP.
Nucleic Acids
All organisms have molecules that direct and control the synthesis of proteins, providing information that determines their specificity and biological characteristics. These molecules, called nucleic acids, contain the instructions necessary for life processes and are responsible for all basic functions of living beings.
RNA Types
Messenger RNA (mRNA)
mRNA is a copy of a segment of DNA that is used by ribosomes to direct the order of amino acids in protein synthesis. mRNA chains have a short lifespan; if they were not degraded, protein synthesis would continue indefinitely, causing overproduction of proteins. Therefore, when a specific protein needs to be synthesized, the corresponding mRNA is produced again. mRNA constitutes approximately 3-5% of total cellular RNA.
Ribosomal RNA (rRNA)
rRNA is a component of ribosomes and participates in the binding of amino acids during protein synthesis. It is not molecule-specific, as it does not contain information about the type of protein being synthesized. Prokaryotes and eukaryotes have slightly different rRNA chains. rRNA is the most abundant type of RNA, accounting for 80-85% of total cellular RNA.
Nucleotides
Nucleic acids are polymers whose basic components are called nucleotides, formed by the chemical bonding of heterogeneous substances in a specific arrangement. Besides their structural function, nucleotides can play important roles as coenzymes and intermediates in energy metabolism.
A nucleoside is a molecule consisting of a nitrogenous base and a pentose sugar. Nitrogenous bases are compounds with nitrogen atoms in the ring and have basic character. There are two types:
- Purine bases: Derivatives of the purine core. The most abundant purine bases in nucleic acids are adenine and guanine.
- Pyrimidine bases: Derivatives of the pyrimidine core. The most abundant are cytosine, thymine, and uracil.
Pentoses are monosaccharides with five carbon atoms. Nitrogenous bases are joined to two possible aldoses: ribose and deoxyribose. The purine bases are attached to the pentose by an N-glycosidic bond between carbon 1 of the pentose and nitrogen 9 of the base. The pyrimidine bases form the N-glycosidic bond between carbon 1 of the pentose and nitrogen 1 of the base.
Nucleotides are compounds resulting from the union between a nucleoside and phosphoric acid molecules. This union is made by esterification of one alcohol group of the pentose, forming an ester bond between the pentose and phosphoric acid. A nucleotide may have 1, 2, or 3 phosphoric acid molecules bound to the 5′ carbon of the pentose. Therefore, when naming nucleotides, the number of phosphate groups is indicated. They are classified as deoxyribonucleotides and ribonucleotides based on the pentose and the nitrogenous base.
Transfer RNA (tRNA)
tRNA is responsible for transporting amino acids present in the cytoplasm to the ribosomes, where they are joined to form proteins. Each tRNA molecule carries a specific amino acid. These differences are due to a sequence of 3 nitrogenous bases called the anticodon, which varies among tRNAs. tRNAs are composed of short chains containing 10% nitrogen bases different from the 4 major ones. tRNA molecules have a characteristic secondary structure, with double-stranded regions called arms. There are typically 4 arms per molecule, but a fifth, shorter arm may also be present. At the ends of 3 arms are unmatched regions called loops.