Bio Chemistry

The quaternary protein structure is the spatial conformation due to interactions among polypeptide chains that form the protein.

Only those proteins made of two or more polypeptide chains have quaternary structure. Insulin (two chains), hemoglobin (four chains), and the immunoglobulins (antibodies, four chains) are some examples of protein having quaternary structure.

Protein Structure Review - Image Diversity: protein quaternary structure

Secondary, tertiary, and quaternary structures of proteins are spatial structures. Denaturizing is modification in any of these spatial structures that makes the protein deficient or biologically inactive.

After denaturizing, the primary protein structure is not affected.

Protein Structure Review - Image Diversity: denaturized protein

The process in which DNA is synthesized having as template a RNA chain is called reverse transcription. In cells infected by retroviruses (RNA viruses, like the AIDS or SARS viruses) reverse transcription occurs and DNA is made from information contained in the viral RNA.

Viral RNA within the host cell produces DNA with the help of an enzyme called reverse transcriptase. Based on that DNA the host cell then make viral proteins, new virus are assembled and viral replication occurs.

Nucleic Acid Review - Image Diversity: reverse transcription

Messenger RNA, or mRNA, transfer RNA, or tRNA, and ribosomal RNA, or rRNA, are the three main types of RNA.

The newly formed RNA molecule, a precursor of mRNA, is called heterogeneous RNA (hnRNA). The heterogeneous RNA bears portions called introns and portions called exons. The hnRNA is processed in many chemical steps, introns are removed, and mRNA is created formed only of exons, the biologically active nucleotide sequences.

DNA is the source of information for RNA production (transcription) and thus for protein synthesis. DNA is still the basis of heredity due to its replication capability.

The messenger RNA is the template for protein synthesis (translation). In this process, tRNA and rRNA also participate since the first carries amino acids for the polypeptide chain formation and the second is a structural constituent of ribosomes (the organelles where proteins are made).

Substances that “simulate” substrates can bind to the activation center of enzymes thus blocking the true substrates to bind to these enzymes and paralyzing the enzymatic reaction. Such “fake substrates” are called enzyme inhibitors.

The binding of enzyme inhibitors to enzymes can be reversible or irreversible.

Many medical drugs, for example, some antibiotics, antivirals, antineoplastics, antihypertensives and even sildenafil (trade name Viagra), are enzyme inhibitors that block enzyme activity.

Protein denaturizing can be caused by temperature variation, pH change, and changes in the concentration of surrounding solutes and by other processes. Most proteins are denatured after certain elevation of temperature or when in very acid or very basic solutions. This is one of the main reasons, why it is necessary for the organisms to keep adequate temperature and stable pH.

Any change of the protein structure is relevant if it alters its biological activity. Changes in the primary protein structure are more important because they are modifications in the composition of the molecule and such composition determines all other structures of the protein.

In sickle cell disease, there is change in the primary protein structure of one of the polypeptide chains that form hemoglobin: the amino acid glutamic acid is substituted by the amino acid valine in the ß chain. The spatial conformation of the molecule in addition is also affected and modified by this primary “mistake” and the modification creates a different (sickle) shape of the red blood cells.

Modified, sickled, red blood cells sometimes aggregate and obstruct the peripheral circulation causing tissue hypoxia and the pain crisis typical of sickle cell anemia.

Catalysts are substances that reduce the activation energy of a chemical reaction, facilitating it or making it energetically viable. The catalyst increases the speed of the chemical reaction.

There are two main models that explain the formation of the enzyme-substrate complex the lock and key model and the induced fit model.

In the lock and key model, the enzyme has a region with specific spatial conformation for the binding of the substrate. In the induced fit model, the binding of the substrate induces a change in the spatial configuration of the enzyme for the substrate to fit.

Enzyme Activity: lock and key model induced fit model

Substrates are reagent molecules upon which enzymes act.

The enzyme has spatial binding sites for the attachment of its substrate. These sites are called activation centers of the enzyme. Substrates bind to theses centers forming the enzyme-substrate complex.

Enzyme Activity: enzyme-substrate complex

Catalysts are not consumed in the reactions they catalyze.

The enzyme possibly works as a test tube within which reagents meet to form products. With the facilitation of the meeting provided by enzymes it is easier for collisions between reagents to occur and thus the activation energy of the chemical reaction is reduced. This is one of the explanatory hypotheses.

Enzymes are proteins that are catalysts of chemical reactions. From Chemistry, it is known that catalysts are non-consumable substances that reduce the activation energy necessary for a chemical reaction to occur.

Enzymes are highly specific to the reactions they catalyze. They are of vital importance for life because most part of chemical reaction of the cells and tissues are catalyzed by enzymes. Without enzymatic action, those reactions would not occur or would not happen in the required speed for the biological processes in which they participate.