Peptide bond

A peptide bond is a covalent bond of the amide type that connects C1 (carbon atom number 1) of one -amino acid from a peptide or protein chain to two consecutive -amino acids of another -amino acid (second nitrogen atom).

It may also be called a eupeptide bond to separate it from an isopeptide bond (a different type of amide bond between two amino acids).

Synthesis

Peptide bond formation via dehydration reaction.

pic1. Peptide bond formation via dehydration reaction.

When two amino acids pass through a peptide bond to form a dipeptide, it is the type of condensation reaction.In this condensation, the two amino acids are close to each other, and the non-side chain (C1) carboxylic acid part of one is close to the non-side chain (N2) amino part of the other.One loses hydrogen and oxygen from its carboxyl group, and the other loses hydrogen from its amino group.This reaction produces water molecules (H2O) and two amino acids linked by a peptide bond (-co-nh -).Two linked amino acids are called dipeptides.

When the carboxyl group of one amino acid molecule reacts with the amino group of another amino acid molecule, causing the release of water molecule (H2O), the amide bond is synthesized, so the process is dehydration synthesis reaction.

The dehydration condensation of two amino acids to form a peptide bond (red) with expulsion of water (blue).

pic2. The dehydration condensation of two amino acids to form a peptide bond (red) with expulsion of water (blue).

The formation of peptide bonds consumes energy and is derived from ATP in organisms.Peptides and proteins are chains of amino acids held together by peptide bonds (sometimes by small amounts of isopeptide bonds).Organisms use enzymes to produce non-ribosomal peptides, and ribosomes produce proteins by different reactions with dehydration.

Some peptides, such as alpha-amanitin, are called ribosomal peptides because they are made from ribosomes, but many are non-ribosomal peptides because they are made from specialized enzymes rather than ribosomes.For example, GSH is composed of free amino acids and is synthesized in two steps by two enzymes: glutamate-cysteine ligase (which forms an isopeptide bond, which is not a peptide bond) and glutathione synthase (which forms a peptide bond).

The degradation

Peptide bonds can be broken by hydrolysis (addition of water).In the presence of water, they break down and release 8 to 16 kilojoules per mole (2 to 4 kcal/mole) of gibbs energy.This process is very slow, and each key at 25 ° C, the half-life of 350 to 600.

In organisms, this process is usually catalyzed by enzymes called peptidases or proteases, although it has been reported that peptide/protein folding into natural structures results in the hydrolysis of peptide bonds by conformational strains.[11] therefore, this non-enzymatic process is not accelerated by the transition state stability, but by the ground state instability.

spectrum

Peptide bonds absorb A at A wavelength of 190 to 230nm (which makes them particularly vulnerable to UV radiation).

Cis-trans isomer of the peptide group

The significant delocalization of the lone pair electron on the nitrogen atom characterizes the partial double bond of the group.Partial double bonds make the amide groups planar and exist as cis or trans isomers.In the unfolded state of the protein, the peptide groups can be isomerized freely and adopt two isomers.However, in the folded state, only a single isomer is used for each position (with very few exceptions).

Isomerization diagram of x-pro peptide bond.The figure shows the cis isomer on the left, the transition state in the middle, and the trans isomer on the right, with two-way arrows between each pair of states.

Conformational protein folding is generally much faster than cis-trans isomerization (10-100s) (usually 10-100ms).Some non-natural isomers of peptide groups can significantly disrupt conformational folding or slow down conformational folding or even occur before reaching the natural isomer.However, not all peptide groups have the same effect on folding;Extrinsic isomers of other peptide groups may not affect folding at all.

A chemical reaction

Because of their resonance stability, peptide bonds are relatively unreactive under physiological conditions, even less so than similar compounds such as esters.However, peptide bonds can undergo chemical reactions, usually through the attack of electronegative atoms on the carbonyl carbon, destroying the carbonyl double bond and forming tetrahedral intermediates.This is the pathway followed by proteolysis and, more generally, by n-o acyl exchange reactions, such as those involving peptides.This is acyclol, or, more specifically, thiacyclol, oxacyclol, or azacyclol.

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