ACTIVITIES

a) List the four levels of protein structure.

The levels of structure of proteins are the primary structure, the secondary one in which we can distinguish the α-helix, the β conformation or folded sheet and the collagen helix; the tertiary where we find globular and fibrillar or filamentous proteins, and the quaternary.

 

b) Indicate what types of links are involved in the stabilization of each of these structural levels.

In the primary structure, peptide bonds are established, which are rigid and very stable bonds, which is why they are said to have a double bond character. On the other hand, covalent bonds and hydrogen bridges intervene in the secondary, which in the case of the α-helix are intrachain and in the β conformation interchain. Regarding the tertiary structure, the bonds can be hydrogen bonds between peptide groups, electrostatic attractions between groups with opposite charge, hydrophobic attractions, Van der Waals forces or disulfide bonds between thiol groups (-SH) of two cysteines, which is a strong bond. Finally, in the quaternary structure, the most common bonds are weak (non-covalent) or, sometimes, covalent disulfide bonds.

 

c) Specify the structure that characterizes α-keratins.

The characteristic structure of α-keratins is the α-helix of the secondary structure. In addition, they have cysteine monomers in their amino acid chains, which form disulfide bridges.

 

d) Describe two general properties of proteins.

Specificity: The proteins of living things are characteristic of each species, but they still vary within a species due to the order of amino acids. Proteins that fulfill the same function in different living beings, but are not identical, are called homologous.

This specificity is important, since if a protein is introduced from one body into another, it can recognize it as a foreign body and react against it (as occurs in organ transplant rejections)

Buffering capacity: Due to their amphoteric behavior they can behave as acids or bases, releasing or removing protons from the medium. Thus, they buffer the pH variations of the medium in which they are found.

 

e) Describe two functions of proteins. Give an example.

Reserve function: They store amino acids in ovalbumin from egg white or casein from milk etc.

 

Structural function: Proteins (especially filamentous) form the majority of cellular and organic structures. For example, glycoproteins are part of the plasma membrane, actin and tubulin form cilia, flagella, cytoskeleton, etc. Histones and protamines, collagen, elastin, keratin, etc. also have this function.

 

f) Define the denaturation process. What types of links are not affected?

Denaturation consists of the loss of the quaternary, tertiary or even secondary structure of proteins. It can occur due to changes in temperature, pH, salt concentration, or agitation. When this structure is lost, many bonds are broken (but not the peptides)

When a protein is denatured, it precipitates, since it acquires a filamentous appearance (the radicals are not surrounded by water)

Protein renaturation can occur if conditions are optimal again, since peptide bonds have not been altered.

 

g) What does it mean that an amino acid is amphoteric?

That an amino acid is amphoteric means that it can act as an acid and as a base, depending on the conditions of the environment. This allows regulation of the pH.

 

When the medium is acidic, the amino acid behaves as a base and the carboxyl group captures protons, while if it is in a basic medium it behaves as an acid and the amino group releases protons to the medium.

 

If the amino acid is at a pH in which it assumes a neutral dipole form, that pH level is called the isoelectric point, it is the ideal medium.

 

I hope it is useful to you and, until the next post! ;-)

 

Information obtained from the syllabus and class notes.