The proteins

Biochemistry for nurses: Unit 1

THE PROTEINS


DEFINITION OF THE PROTEIN

• The proteins are the more complex molecules in the living cells.

• A human body has around 100.000 differents types of proteins.

• A protein is a biological macromolecule composed by one or several peptides.

• A peptide is a polymer (= chain) of amino acids (AA) linked between them by a

peptide bond.

•Protein = AA > 100; Peptide = 10 ≤ AA ≤ 100; Oligopeptide = AA ≤ 10 (including

Tripeptide (AA = 3); Dipeptide (AA = 2))


The amino acid

The amino acid is the basic unity of the protein.

The amino acid is composed by

a carbone (C) on which is linked:

• An amino group (NH2)

• An acid group (COOH)

• A variable chemical group (R)

• A hydrogen (H)


20 standard amino acids






Amino acids abbrevation table


The peptide bond

• The amino acid (AA) molecules are linked each other one by peptide bonds.

• A peptide bond is a covalent bond between the carboxylic group (CO) of one

amino acid and the amino group (NH2) of another amino acid molecule.

• The formation of a peptide bond releases a water molecule (H2O) = reaction

of condensation.

• The assembly of several AA molecules gives a peptide (AA ≤ 20)

• The assembly of several peptides gives a protein (AA ≥ 20)


The peptide bond


Protein synthesis (= Transcription + Translation)

• The protein synthesis has 2 steps: the transcription of the DNA into a mRNA,

and the translation of the mRNA into a protein.

•The proteins are assembled from amino acids according the informations

provided by the RNA messenger.


• The translation is the application of the genetic informations provided by the mRNA.

The amino acids are free inside the cytoplasm before to be assembled to form the

proteins during the translation step.

•Translation = Ribosome + amino acids + tRNA + mRNA


Essential and non-essential amino acids

• An essential amino acid is an indispensable amino acid that cannot be synthesized

by the human body: THEY ARE PROVIDED BY THE NUTRIENTS (FOODS)

• A conditionally non-essential amino acid is an amino acid which becomes

essential in some cases (premature baby).

• A non-essential amino acid can be synthesized by the human body.


The primary structure:

• The primary structure

corresponds to the amino

acids sequence according

the mRNA gene.

• We see the primary

structure only during a

short time, i.e, during the

elongation process of the

protein synthesis.

THE PROTEIN STRUCTURE


The secondary structure:

• The variable chemical groups (R) of the AA have differents properties:

Hydrophobic, hydrophilic, charges (+) or (-) , -SH groups…

Intramolecular chemical

interactions: Hydrogen

bonds, Disulfide bridges,

Ionic bonds, hydrophobic

and hydrophilic

interactions…

Alpha-helice and

Beta-sheet structures



The tertiary structure:

• The tertiary structure is the final form of the protein.

• The function of the protein is strongly related to its final

form (structure).

• The final structure can be destroyed in case of a heating

process or extrem pH = Denaturation process

Biological function is lost!


The quaternary structure:

• In case of proteins composed by

tertiary assembled subunits

(Hemoglobin, Immunoglobin…)


According their compositions:

1. Holoprotein = Protein composed only by amino acids.

2. Conjugated protein = protein composed by amino acids and by

something else (carbohydrates: Glycoprotein, lipids: lipoprotein).

Protein classification

According

their

functions:


What is an enzyme?

• Enzymes are proteins

produced in living cells.

• Enzyme are biological

catalysts = They speed up the

rate of chemical reactions.


What is an enzyme?

• Enzyme are specific to a particular substrate (Key-Lock:

sucrose-sucrase, maltose-maltase,…).


The cytoskeleton

The cytoskeleton is a dynamic

protein structure: It is a

skeleton and a musculature in

same time for the cells. The

cytoskeleton has the function to

maintain the shape of the cell,

to create a cell movement. It is

involved in the cell division.


Clinical correlation


• The Alzheimer’s disease is a chronic

neurodegenerative disease = a progessive

destruction of the brain neurones over time

until death.

• Most often begins in people over 65 years.

• It was first described in 1906 by the german

psychiatrist and pathologist Alois Alzheimer.

• The cause is almost unknown: The most

probable hypothesis is a genetic anomaly.

• No treatments stop or reverse its

progression.

• No medications or supplements decrease

the risk factor.

ALZHEIMER’S DISEASE

(AD)


AD is caused by 2 types of protein accumulation,

Intra and extracellular, which cause plaques and tangles in the Brain.

Disintegration of the microtubules

accumulation

= Tangles of Tau porteins

Destroying the structure of the

cell’s cytoskeleton = Death of the cell

Intracellular: Anomaly of TAU proteins


Accumulation

Senile plaques

Destruction of the

surrounding neurones

Extracellular: Anomaly of Amyloid proteins


The Alzheimer’s disease slowly invades the different areas of the brain

and gets worse over time. The disease causes progressively visibles

symptoms according the affected brain areas.

Education

The proteins