Download of proteins

Biological molecules
Protein – Lesson 4 Psychobiology
Proteins are macromolecules that constitute the vast
majority of cellular and extra-cellular structures
Just as in the case of lipids and carbohydrates, they are
also formed from the polymerization of simple molecules
Proteins are characterized by a great structural
complexity, which gives life to a multitude of functions.
On the basis of these functions, proteins are generally
classified
•
Structural proteins
Enzimes
•
•
Membrane Proteins
•
Transcription factors
•
Etc…
Proteins are linear polymers of small molecules, called amino acids, linked by a
chemical bond said peptide bond
The amino acids joined together by this bond, form the polypeptide chain,
which becomes the protein only when its three-dimensional structure is stable
and functional (a wall is NOT a house!)
All proteins are formed by the different combination of only 20 amino acids
amino acids
amphoteric molecules
Amino group - basic
Carboxyl group - acid
carbon alpha
Has its 4 valences linked
to 4 atoms / groups of
different atoms, this
makes asymmetrical and
leads to the formation of
two optical isomers D
and L
Variable part, side chain residue, radical, R
group
The R group differentiates amino acids and
gives them the physical-chemical properties
Some amino acids besides acting as precursors of proteins, are also precursors to
some of the most important neurotransmitters or neurotransmitter themselves
In particular:
Serotonine
Dopamine
Norepinephrine
epinephrine
GABA
Peptide Bond
The peptide bond is a covalent bond, very durable that originates between the
amino group of an amino acid and the carboxylic acid group which precedes
it, with the elimination of a water molecule.
In the peptide bond C-N they are much closer than in any other type of C-N
bond, which makes the very strong bond and gives him a partial double bond
character, eliminating rotational functions.
However such a stable bond can create problems for the absorption of the
individual amino acids, for this reason, the organisms are equipped with
protease such as pepsin and trypsin (enzyme proteins) that cut the peptide
bond at normal conditions of temperature
The peptide bond form the polypeptides
Amino-group
START
Carboxyilic
group
END
The result will be a long flexible chain whose axial skeleton is composed by the
succession of atoms N-C-C-N-C-C-N ... etc ...
The axial skeleton is in turn covered by the various R groups of each amino acid
which protrude more or less depending on their length and which grant to the
chain its physical-chemical properties
The sequence of amino acids that comprises a polypeptide chain is defined as
the primary structure of the protein.
Secondary structures: alpha-helices
Although the peptide bonds are non-rotational, all other
bonds present in the axial skeleton, they are. This allows the
chain to bend at these points under the influence of the
thermal agitation.
This leads to the formation of a configuration similar to a
propeller blocked by hydrogen bonds that are established
between the N-H and O-C free of any amino acid.
These ties take place on a regular basis, i.e. every 4 amino
acids.
This conformation to the cylinder, takes the name of alphahelix and is covered by the various R of each amino acid, in
particular each turn is covered by 3.6 R
Secondary structures: the beta sheets-planar
In a similar way the
hydrogen bonds may
also bind between their
two arms of the same
chain, or the two paired
and parallel chains,
giving the structure a
laminar form
named
Beta-planar sheets
The arms forming the
leaflets may have a
parallel development,
anti-parallel or mixed
The ‘motifs' of proteins
alpha-helices combinations
Helix-loop-helix
Leucine-zip
The ‘motifs' of proteins
Beta-sheets combinations
Beta-barrel
Beta-sandwich
Tertiary structure
The total windings of the
polypeptide chain, stabilized
by the formation of bonds of
the R groups
it has been shown that the acquisition of secondary and tertiary structures,
depend exclusively on the primary structure of a protein.
The methodology for arriving at these conclusions is the denaturation of a native
protein (dissolution of all secondary and tertiary ties) and subsequent
renaturation
The quaternary structures
The quaternary structure is not present in all proteins, but is typical of multimeric
proteins, ie consist of more peptide chains or subunits, which may be the same
or different between them.
A typical example of a quaternary structure of the protein is hemoglobin or the protein
that is responsible to carry oxygen, contained in our red blood cells.
It consists of two different types of peptide chains (alpha and beta).
Furthermore, each chain contains in its interior a non-protein molecule called heme
group, with an iron atom that is the specific oxygen bond.
The quaternary structure is an important culmination of the process of molecular
evolution as it allows a cooperative effect between the different units
Amino acids
Polypeptidic chain, alpha-helix
and beta-sheet
«motifs» : proteins = gothic : architecture
Tertiary structure
Proteins
The post-translational modifications
After the synthesis and the complete acquisition of secondary and tertiary
structural levels, most of the protein undergoes structural changes said posttranslational modifcations.
Some of these modifications are permanent and necessary to ensure that the
protein can acquire the final three-dimensional conformation and thus its
biological functionality
Other modifications are reversible and have important functions in the control of
protein.
The post-translational modifications
Permanent changes can be classified into:
Proteolytic cuts (performed by specific enzymes called proteases)
•
Destroy defective proteins
•
Delete portions of the polypeptide chain no longer necessary (removing portions of the chain
which initially may be necessary for the correct winding of the protein, but that can later prove to
be unnecessary) - The insulin example
•
Protect themselves from potentially dangerous activity of the mature protein (for example in the
case of cells that create the enzymes capable of cleaving the proteins taken with food and
which have to protect themselves from the effects of proteases of their own products) - proenzymes Inactive
•
Cut a single macro-chain polypeptide in several fragments, each of which constitutes a protein
having its own function - The example of the pro-opiomelano-curtain (POMC)
The addition of non-protein portions
• Oligosaccharides - Glycoproteins
• Lipids - Lipoproteins
• prosthetic group
•
Delete portions of the polypeptide chain no longer necessary (removing portions of the chain which
initially may be necessary for the correct winding of the protein, but that can later prove to be
unnecessary) - The insulin example
The post-translational modifications
Permanent changes can be classified into:
Proteolytic cuts (performed by specific enzymes called proteases)
•
Destroy defective proteins
•
Delete portions of the polypeptide chain no longer necessary (removing portions of the chain
which initially may be necessary for the correct winding of the protein, but that can later prove to
be unnecessary) - The insulin example
•
Protect themselves from potentially dangerous activity of the mature protein (for example in the
case of cells that create the enzymes capable of cleaving the proteins taken with food and
which have to protect themselves from the effects of proteases of their own products) - proenzymes Inactive
•
Cut a single macro-chain polypeptide in several fragments, each of which constitutes a protein
having its own function - The example of the pro-opiomelano-curtain (POMC)
The addition of non-protein portions
• Oligosaccharides - Glycoproteins
• Lipids - Lipoproteins
• prosthetic group
•
Cut a single macro-chain polypeptide in several fragments, each of which constitutes a protein having its
own function - The example of the pro-opiomelano-curtain (POMC) produced by many parts of the body
but of which the most important is undoubtedly the pituitary gland
Corticotropin, stimulates the production of
corticosteroids in the adrenal gland
Regulation of conduct
Grooming in mice
Further cuts that
produce short
molecules called
enkephalins
endogenous opioids
The post-translational modifications
Permanent changes can be classified into:
Proteolytic cuts (performed by specific enzymes called proteases)
•
Destroy defective proteins
•
Delete portions of the polypeptide chain no longer necessary (removing portions of the chain
which initially may be necessary for the correct winding of the protein, but that can later prove to
be unnecessary) - The insulin example
•
Protect themselves from potentially dangerous activity of the mature protein (for example in the
case of cells that create the enzymes capable of cleaving the proteins taken with food and
which have to protect themselves from the effects of proteases of their own products) - proenzymes Inactive
•
Cut a single macro-chain polypeptide in several fragments, each of which constitutes a protein
having its own function - The example of the pro-opiomelano-curtain (POMC)
The addition of non-protein portions
• Oligosaccharides - Glycoproteins
• Lipids - Lipoproteins
• prosthetic group
The post-translational modifications
The reversible changes consist in the phosphorylation and de-phosphorylation
The supramolecular complexes
It often happens that the proteins do not perform their functions individually but in
conjunction with other identical proteins or different.
Taken together, these proteins form the supramolecular complexes.
These are recognizable by the presence of numerous ‘motifs' and are held together
by weak interactions or covalent bonds.
Each protein of a supramolecular complex, is defined sub-units.
Each sub-unit can perform various tasks.