How do amino acids combine with each other



A peptide is an organic chemical compound that is the result of a combination of several amino acids. The individual amino acids are linked in a defined order (sequence) to form a mostly unbranched chain. The amino acids in a peptide are linked by an amide bond. Because of the great importance of peptides in biochemistry and organic chemistry, this type of bond is often also called a peptide bond. Peptides are relatively short amino acid chains of up to about 50 (usually a maximum of 100) amino acids.

Peptides differ from proteins solely in their size. The separation between peptides and proteins is not sharp and is more or less arbitrarily chosen. The limit is around 100 amino acids.

In nature, peptides are mainly formed through protein biosynthesis. The information about the sequence, i.e. the sequence of the amino acids, is encoded in the DNA. Peptides fulfill a large number of functions. In most cases, however, the mode of action is only partially understood. There are examples of peptides that act as hormones, others show an anti-inflammatory or anti-inflammatory effect, and there are also antibiotic and antiviral peptides. Some proteins that are frequently found in food, such as gluten, casein or proteins found in eggs or spinach, can be converted into so-called opioid peptides through the digestive processes. These peptides act on the body in a similar way to morphine. Individuals who are unable to further metabolize these peptides can develop signs of physical and mental illness.

The term peptide was coined by Emil Fischer (1906). peptide became out Pepton (peptos (Greek: digested)), the protein breakdown products of pepsines and the ending of poly-saccharid, because of the analogy to their structure from monomers.

structure

In the condensation of amino acids, the carboxyl group of one amino acid formally reacts with the amino group of the other amino acid to form the acid amide group -CO-NH- (peptide bond) with the escape of water. The free amino group at one end of the peptide is called the N-terminus, and the free carboxyl group at the other end is called the C-terminus.

Classification

In general, the number of amino acids that make up a peptide molecule is also called the chain length. The chain length is used to differentiate into

  • Oligopeptides - consisting of up to 10 amino acids
  • Dipeptides - from 2 amino acids
  • Tripeptides - from 3 amino acids

and

  • Polypeptides - consisting of more than 10 amino acids
  • Macropeptides - from more than 100 amino acids,

whereby the indented peptide groups represent only a finer classification of the oligo- and polypeptides. The demarcation between oligo- and polypeptides on the one hand and between polypeptides and proteins on the other hand is not sharp; the specified limits for the chain lengths are rather rough guide values.

Oligopeptides

As Oligopeptides are chemical compounds that consist of up to ten amino acids that are linked to one another via peptide bonds.

An oligopeptide is formed by the amino group of a first amino acid reacting with the carboxyl group of a second amino acid with elimination of water. The free amino group of the resulting dipeptide then reacts with the carboxyl group of another amino acid. The remaining amino acids are linked according to this pattern, so that a short chain of amino acids is created, which are connected to one another via peptide bonds.

Oligopeptides play e.g. B. play a role as components of enzymes in detoxification, transport and metabolic processes.

Dipeptides

see article Dipeptide

Polypeptides

A Polypeptide is a peptide that consists of at least ten amino acids; shorter polypeptides are called oligopeptides. The amino acids are linked by peptide bonds. Polypeptides can be of either natural or synthetic origin. Polypeptides with over 100 amino acids are usually referred to as proteins; however, other requirements are necessary for a protein, such as a defined protein folding.

In nature, polypeptides are formed by the mechanisms of protein biosynthesis on the basis of the building instructions that are encoded in the DNA or RNA.

If high molecular weight macropeptides are linked by hydrogen or disulfide bridges, these are called proteins. Depending on the historical classification, some amino acid chains with over 100 amino acids are also referred to as peptides.

Peptide synthesis

The synthetic method of choice for a peptide of a given sequence differs depending on its length:

  • Short peptides are built up step by step from the linkage of amino acids
  • Longer peptides are built from the linkage of shorter peptides

If an attempt is made to produce a certain dipeptide (e.g. Gly-Val) from two different amino acids (Gly + Val) by thermal dehydration, a number of undesirable products will arise in considerable quantities[1]:

In order to increase the selectivity (chemistry), the carboxyl and amino groups that are not to be linked are provided with a protective group (e.g. ester, Boc):


Because the protective groups can be split off by acids and bases, mild reaction conditions are necessary. Various coupling reagents are used which activate the carboxyl group and thus facilitate the coupling under mild conditions. There are several classes of such coupling reagents [2]:

  • Phosphonium reagents (e.g. BOP)
  • Uronium reagents (e.g. HBTU, HATU)
  • Immonium reagents
  • Carbodiimide reagents (e.g. DCC)
  • Imidazolium reagents
  • Organophosphorous reagents
  • Acid halogenating reagents
  • Chloroformates and others

In addition to this type of chemical synthesis, solid-phase syntheses are mostly used today. In addition, enzymes can also be used for peptide synthesis.

See also

swell

  1. K.Peter, C.Vollhardt, N.E. Schore:Organic chemistry, 4ed, Wiley-VCH, pp. 1399-1402, 2005
  2. S.Y.Han, Y.A.Kim, Tetrahedron, 60, pp. 2447-2467, 2004

Categories: Peptide | Substance group