4-Hydroxyphenylpyruvate dioxygenase HPPD is an Fe

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

-containing enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

, that catalyzes the second reaction

Chemical reaction

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 in the catabolism

Catabolism

Catabolism is the set of metabolic pathways that break down molecules into smaller units and release energy. In catabolism, large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides, and amino...

 of tyrosine

Tyrosine

Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

 - the conversion of 4-hydroxyphenylpyruvate to homogentisate.

Function

The HPPD reaction occurs through a NIH shift

NIH shift

An NIH shift is a chemical rearrangement where a hydrogen atom on an aromatic ring undergoes an intramolecular migration primarily during a hydroxylation reaction. This process is also known as a 1,2-hydride shift. These shifts are often studied and observed by isotopic labeling...

 and involves the oxidative decarboxylation

Oxidative decarboxylation

Pyruvate decarboxylation is the far from equilibrium biochemical reaction that uses pyruvate to form acetyl-CoA, releasing NADH, a reducing equivalent, and carbon dioxide via decarboxylation...

 of an α-oxo acid as well as aromatic ring hydroxylation

Hydroxylation

Hydroxylation is a chemical process that introduces a hydroxyl group into an organic compound. In biochemistry, hydroxylation reactions are often facilitated by enzymes called hydroxylases. Hydroxylation is the first step in the oxidative degradation of organic compounds in air...

. The NIH-shift, which has been demonstrated through isotope-labeling studies, involves migration of an alkyl group to form a more stable carbocation

Carbocation

A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability . Therefore carbocations are often reactive,...

. The shift, accounts for the observation that C3 is bonded to C4 in 4-hydroxyphenylpyruvate but to C5 in homogentisate.

See also

• 4-hydroxyphenylpyruvate hydroxylase deficiency