definition

Proteins, also known as polypeptides, are essential macromolecules composed of the basic elements carbon, hydrogen, oxygen, nitrogen, and sulfur. They are made up of amino acids linked by peptide bonds. In humans, the precise amino acid sequence of a protein is encoded in the genes of the nuclear DNA and, to a lesser extent, in the mitochondrial DNA.

background

Proteins are the central biochemical functional units of the body and make up about 15-17% of total body mass. They are essential for many processes:

• Provision of amino acids as well as nitrogen and sulfur compounds from food.
• Enzymatic catalysis of biochemical reactions.
• Transport and storage of molecules (e.g., oxygen by hemoglobin).
• Enabling movement (e.g., actin and myosin).
• Signal transmission via nerve impulses.
• Immune defense and cell growth.

Characteristics

Proteins are linear polymers consisting of 20 different amino acids with varying functional groups. Their specific amino acid sequence allows them to spontaneously fold into a three-dimensional structure, stabilized by hydrogen bonds. This structure is crucial for the diversity of their functions.

structure

Primary structure

The primary structure describes the linear sequence of amino acids linked together by peptide bonds. This sequence is genetically determined.

Secondary structure

Polypeptide chains can fold into regular structures such as the α-helix or the β-sheet :

α-Helix

The α-helix is ​​a helical structure stabilized by hydrogen bonds between the carbonyl oxygen atom of one peptide bond and the amino hydrogen atom of another. The side chains of the amino acids point outwards. Some amino acids, such as valine and threonine, can destabilize the helix.

β-sheet

The β-sheet consists of nearly elongated polypeptide chains that run antiparallel and are stabilized by hydrogen bonds. It is a central element of many proteins.

β-turns & Ω-loops

Beta turns allow for abrupt changes in the direction of the polypeptide chain. Omega loops, which lack regular structures, provide additional changes in direction.

tertiary structure

The tertiary structure describes the complete three-dimensional arrangement of a polypeptide chain. It is stabilized by hydrophobic interactions, hydrogen bonds, disulfide bridges, and ionic bonds.

Quaternary structure

The quaternary structure describes the spatial arrangement of multiple polypeptide chains (subunits) in a protein. For example, hemoglobin consists of four subunits and is called a tetramer.

Classification

Proteins can be classified according to various criteria:

• Occurrence of prosthetic groups: e.g., hemin proteins.
• Molecular modifications: e.g. glycoproteins.
• Occurrence: e.g., plasma proteins.
• Function: e.g., enzyme or structural proteins.

Nutritional needs

An adult's protein requirement is approximately 1 gram per kilogram of body weight per day. This requirement may be increased during periods of intense physical activity or certain life stages (e.g., growth, pregnancy).