Corpus: Protein

Proteins (proteins) are macromolecules whose basic substance consists of carbon, hydrogen, oxygen, nitrogen and sulphur. The molecules are made up of amino acids linked by peptide bonds; the exact amino acid sequence of proteins in humans and other animal organisms is encoded in the form of genes in the DNA of the cell nucleus and, to a very small extent, in the DNA of the mitochondria.

Proteins are the most important biochemical function carriers. They make up about 15-17% of the body mass. Dietary proteins provide amino acids and organic nitrogen and sulphur compounds, act as catalysts, transport and store other molecules (e.g. haemoglobin oxygen), enable movement, transmit nerve impulses, confer immunity and control the growth and differentiation of cells.

Proteins are linear polymers composed of 20 different amino acids and therefore also many different functional groups. They can spontaneously fold into a three-dimensional structure with their specific amino acid sequence, which is stabilised primarily by hydrogen bonds. This specific three-dimensional structure for each amino acid sequence gives proteins their variety of different functions.

Primary structure[Bearbeiten]

The primary structure of a protein indicates the sequence of amino acids linked by peptide bonds. This is known as the amino acid sequence. It is determined by the genes.

Secondary structure[Bearbeiten]

Polypeptide chains can fold into regular structures.

α-helix[Bearbeiten]

In the α-helix, the polypeptide chain is coiled in a ring and approximately every fourth amino acid is stabilised by hydrogen bonds between the carbonyl oxygen atom of one peptide bond and the amino hydrogen atom of the other. The residues all point outwards. The amino acids valine, threonine and isoleucine can destabilise this structure through their branching at the β-carbon atom due to steric hindrances. Asparagine, serine and aspartate, the deprotonated aspartic acid, may also disrupt this structure, as their strongly polar functional groups compete with those of the amides in the main chain for hydrogen bonds.

β-folder[Bearbeiten]

In contrast to the α-helix, the β-sheet consists of almost completely extended strands of the polypeptide chain, which mainly run anti-parallel and are stabilised by hydrogen bonds between the peptide bonds of all amino acids. The β-sheet is therefore an important structural element for many proteins (e.g. fatty acid-binding proteins consist almost exclusively of this structure).

β-sheets & Ω-loops[Bearbeiten]

As proteins usually have a very compact structure, there must be many changes of direction in the polypeptide chain. In the β-loop, the carboxyl oxygen atom of a peptide bond forms a hydrogen bond to the amino hydrogen atom of the next but one peptide bond, which leads to an abrupt change of direction. The Ω loops, whereby the "Ω" is only intended to emphasise the change in direction, form the remaining changes in direction and, in contrast to β-strands and α-helices, do not have regular structures.

Tertiary structure[Bearbeiten]

Tertiary structure refers to the overall arrangement of a polypeptide chain. This often results in a characteristic distribution of hydrophobic and hydrophilic residues, which leads to thermodynamic stability in certain environments. Hydrogen bonds, disulphide bonds, ionic bonds and van der Waals interactions are also involved in stabilisation.

Quaternary structure[Bearbeiten]

The quaternary structure describes the spatial arrangement of the subunits (polypeptide chains) and their interactions with each other in a protein. If a protein consists of two identical subunits, it is referred to as a dimer. Haemoglobin with its two pairs of subunits is a tetramer.

Proteins can be categorised according to various criteria, e.g. according to the presence of prosthetic groups (e.g. haemin proteins), molecular modifications (e.g. glycoproteins), according to their occurrence (e.g. plasma proteins) or according to their function (e.g. enzyme or structural proteins).

The daily protein requirement for an adult is 1g per kg body weight.

http://www.rcsb.org/pdb/