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Biochemisty

The imidazole sidechain of histidine is a common coordinating ligand in metalloproteins and is a part of catalytic sites in certain enzymes. It has the ability to switch between protonated and unprotonated states, which allows histidine to participate in acid-base catalysis. In catalytic triads, the basic nitrogen of histidine is used to abstract a proton from serine, threonine, or cysteine to activate it as a nucleophile. In a histidine proton shuttle, histidine is used to quickly shuttle protons. It can do this by abstracting a proton with its basic nitrogen to make a positively charged intermediate and then use another molecule, a buffer, to extract the proton from its acidic nitrogen. In carbonic anhydrases, a histidine proton shuttle is utilized to rapidly shuttle protons away from a zinc-bound water molecule to quickly regenerate the active form of the enzyme. Histidine is also important in haemoglobin in helices E and F. Histidine assists in stabilising oxyhaemoglobin and destabilising CO-bound haemoglobin. As a result, carbon monoxide binding is only 200 times stronger in haemoglobin, compared to 20,000 times stronger in free haem.

Biosynthesis

Histidine, also referred to as L-Histidine, is an essential amino acid that is not synthesized de novo in humans. Humans and other animals must ingest histidine or histidine-containing proteins. The biosynthesis of histidine has been widely studied in prokaryotes such as E. coli. Histidine synthesis in E.coli involves eight gene products (His 1, 2, 3, 4, 5, 6, 7, and 8) and it occurs in ten steps. This is possible because a single enzyme has the ability to catalyze more than one step. For example, as shown in the pathway, enzyme His4 catalyzes 4 different steps in the pathway.

Histidine is synthesized from phosphoribosyl pyrophosphate (PRPP), a biochemical intermediate, which is made from ribose-5-phosphate by ribose-phosphate diphosphokinase during the pentose phosphate pathway. The first reaction in the pathway is the condensation of PRPP and adenosine triphosphate (ATP) by the enzyme ATP-phosphoribosyl transferase. ATP-phosphoribosyl tranferase is indicated by His1 in the image. Enzyme His4 then hydrolyzes the product of the condensation, phosphoribosyl-ATP, to phosphoribosyl-AMP (PRAMP), which is an irreversible step.

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L-histidinal is an amino aldehyde involved in the final production of histidine by acting as a precursor for L-histidine. The amino alcohol, L-histidinol, is oxidized to form L-histidinal, which is then converted to L-histidine.

Just like animals and microorganisms, plants need histidine for their growth and development. Microorganisms and plants are similar in that they can synthesize histidine. Both synthesize histidine from the biochemical intermediate phosphoribosyl pyrophosphate. In general, the histidine biosynthesis is very similar in plants and microorganisms.

Regulation of Biosynthesis

This pathway requires energy in order to occur therefore, the presence of ATP activates the first enzyme of the pathway, ATP-phosphoribosyl transferase (shown as His1 in the image). ATP-phosphoribosyl transferase is the rate determining enzyme, which is regulated through feedback inhibition meaning that it is inhibited in the presence of the product, histidine.

Degradation

Certain amino acids can be converted to intermediates of the TCA cycle. Carbons from four groups of amino acids form the TCA cycle intermediates α-ketoglutarate, succinyl CoA, fumarate, and oxaloacetate. Amino acids that form α-ketoglutarate are glutamate, glutamine, proline, arginine, and histidine. Histidine is converted to formiminoglutamate (FIGLU). The formimino group is transferred to tetrahydrofolate, and the remaining five carbons form glutamate. Glutamate can be deaminated by glutamate dehydrogenase or transaminated to form α-ketoglutarate.

Conversion to other Biologically Active Amines

• The histidine amino acid is a precursor for histamine, an amine produced in the body necessary for inflammation.

• The enzyme histidine ammonia-lyase converts histidine into ammonia and urocanic acid. A deficiency in this enzyme is present in the rare metabolic disorder histidinemia, producing urocanic aciduria as a key diagnostic symptom.

• Histidine is also a precursor for carnosine biosynthesis, which is a dipeptide found in skeletal muscle.

• In Actinobacteria and filamentous fungi, such as Neurospora crassa, histidine can be converted into the antioxidant ergothioneine.

Histidine, also referred to as L-Histidine, is an essential amino acid that is not synthesized de novo in humans. Humans and other animals must ingest histidine or histidine-containing proteins. The biosynthesis of histidine has been widely studied in prokaryotes such as E. coli. In E. coli, histidine synthesis involves eight genes (his A, B, C, D, F, G, H, I) and it occurs in ten steps. It is synthesized from phosphoribosyl pyrophosphate (PRPP), a biochemical intermediate. This pathway requires energy in order to occur therefore, the presence of ATP activates the first enzyme of the pathway, ATP-phosphoribosyl transferase. Different experiments have shown that ATP-phosphoribosyl transferase is an enzyme with a key role in the catalysis and regulation of the histidine biosynthesis pathway. ATP-phosphoribosyl transferase is the rate determining enzyme, which is regulated through feedback inhibition meaning that it is inhibited in the presence of the product, histidine. L-histidinal is an amino aldehyde involved in the final production of histidine by acting as a precursor for L-histidine. The amino alcohol, L-histidinol, is oxidized which leads to the formation of L-histidinal which is then converted to L-histidine.

Just like animals and microorganisms, plants need histidine for their growth and development. Microorganisms and plants are similar in that they can synthesize histidine. Both synthesize histidine from the biochemical intermediate phosphoribosyl pyrophosphate. In general, histidine biosynthesis is very similar in plants and microorganisms.

Once in the human/animal body, histidine can be found in hemoglobin and myelin sheets.

Catabolism

The histidine amino acid is a precursor for histamine, an amine produced in the body necessary for inflammation. Histidine is also a precursor for carnosine biosynthesis, the synthesis of carnosine which is a dipeptide found in skeletal muscle.

Chemical properties

This ability to switch between protonated and unprotonated states allows for histidine participation in acid-base catalysis.