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From Wikipedia, the free encyclopedia

l-Histidine

Skeletal formula of histidine (zwitterionic form)
Names
IUPAC name
Histidine
Other names
2-Amino-3-(1H-imidazol-4-yl)propanoic acid
Identifiers
3D model (JSmol)
84088
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.000.678 Edit this at Wikidata
EC Number
  • 200-745-3
83042
KEGG
UNII
  • InChI=1S/C6H9N3O2/c7-5(6(10)11)1-4-2-8-3-9-4/h2-3,5H,1,7H2,(H,8,9)(H,10,11)/t5-/m0/s1 checkY
    Key: HNDVDQJCIGZPNO-YFKPBYRVSA-N checkY
  • O=C([C@H](CC1=CNC=N1)N)O
  • Zwitterion: O=C([C@H](CC1=CNC=N1)[NH3+])[O-]
  • Protonated zwitterion: O=C([C@H](CC1=CNC=[NH1+]1)[NH3+])[O-]
Properties
C6H9N3O2
Molar mass 155.157 g·mol−1
4.19g/100g @ 25 °C [1]
Hazards
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
1
0
Supplementary data page
Histidine (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Histidine (symbol His or H)[2] is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO form under biological conditions), and an imidazole side chain (which is partially protonated), classifying it as a positively charged amino acid at physiological pH. Initially thought essential only for infants, it has now been shown in longer-term studies to be essential for adults also.[3] It is encoded by the codons CAU and CAC.

Histidine was first isolated by Albrecht Kossel and Sven Gustaf Hedin in 1896.[4] The name stems from its discovery in tissue, from ἱστός histós "tissue".[2] It is also a precursor to histamine, a vital inflammatory agent in immune responses. The acyl radical is histidyl.

YouTube Encyclopedic

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  • Special cases: Histidine, proline, glycine, cysteine | MCAT | Khan Academy
  • Histidine and histamine
  • Biosynthesis of Histidine
  • Histamine Synthesis and Metabolism Pathway
  • Histidine Biosynthesis/AICAR Production

Transcription

Hey. So welcome to the Amino Acids Show. And this show is going to be featuring just 4 of the 20 amino acids. And those amino acids are histidine, proline, glycine, and cysteine. And these four amino acids deserve sort of an extra time in the spotlight because they each have a side chain that sort of sets it apart from the rest. And so let's go through one-by-one and see what exactly these side chains are all about. So first up we have histidine, and I've drawn the structure of histidine for you here. And here is the backbone of the amino acid. So this is the same for all the amino acids. And then, you see here is the side chain of histidine. So what is so special about histidine, then, with this side chain? Well, as it turns out, this side chain has a pKa of around 6.5. And this turns out to be really close to physiological pH, which is right around 7.4. So what does this really mean-- to have a pKa that's close to physiological pH? Well, recall that, at a pH below an amino acid's pKa, the amino acid will exist in a protonated-- or positively charged-- form. And at a pH above an amino acid's pKa, it will exist in deprotonated form. Now, since the physiological pH-- which is the pH of the fluid within our own bodies-- is roughly equal to the pKa of histidine, then histidine's going to exist in both protonated and deprotonated forms. So this makes it a particularly useful amino acid to have at the active site of a protein where it can both stabilize or destabilize a substrate. So next step we have proline and glycine. If we go ahead and take a closer look at proline, we have the backbone structure here-- just like all the other amino acids. But then, you can see that the side chain is this alkyl group that wraps around and forms a second covalent bond with the nitrogen atom of the backbone. And so we say that proline has a secondary alpha amino group. And so this is just referring to the fact that the side chain forms a second bond with the alpha nitrogen-- the nitrogen in the backbone-- of this amino acid. Now, let's come over here and take a look at glycine. Here we have the backbone of the glycine molecule. And then, here we have the side chain. And the side chain for glycine is the simplest of all side chains. It is just 1 hydrogen atom. And I've drawn it out in wedge-and-dash form here to help emphasize how-- because the side chain of glycine is a hydrogen atom-- you have a duplication of atoms coming off of this carbon here-- the alpha carbon. And so now this carbon is no longer a chiral carbon. So we'll write that here. No chiral alpha carbon. And this kind of sets it apart from the rest of the amino acids because the rest of the amino acids do have a chiral carbon-- meaning optical activity under plane-polarized light. And glycine is also considered to be very flexible because it just has this little hydrogen atom as its side chain. And so there's a lot of free rotation around this alpha carbon. So we also consider it to be very flexible. So why are these two amino acids groups together? Well, they both play a role in disrupting a particular pattern found in secondary protein structure called the alpha helix. And an alpha helix is just a coiled up polypeptide chain that kind of looks like this. Now, because of its secondary alpha amino group, proline introduces kinks into this alpha helix. And it ends up looking like this. And also, since glycine is so flexible around its alpha carbon, it tends to do the same thing. And thus both of these amino acids are known as alpha helix breakers. So last but not least, we have cysteine. And here's the backbone again. And then, here is our side chain. And the side chain for cysteine has a special thiol group. And all thiol is really referring to is the sulfur and the hydrogen at the end there. So cysteines have this neat little trick where, if they're in close proximity with each other within a polypeptide chain or even between two different polypeptide chains, then their side chains can form a bond together between the two sulphur atoms called a disulfide bridge. So let's bring up 2 cysteine amino acids here. And I've shown them as isolated amino acids, but remember that they are part of a greater polypeptide chain. And the formation of the disulfide bridge occurs separate from the backbone. It is just between the side chains. The cysteine at the top is flipped over to bring its side chain in close proximity with the second cysteine below it. And then, the bridge forms between the two sulphur atoms. So before we go over how a disulfide bridge is formed, let's do a quick little review of redox reactions. And really, what you want to remember is the mnemonic OIL RIG. And that's to mind you that, in oxidation, you have a loss of electrons. So oxidation is loss. And in reduction, you have a gain of electrons. So reduction is gain. So remembering that will help you understand the disulfide bridge formation. So going back to our 2 cysteines. If you look closely at their side chains, the thiols are existing in reduced form. So you're going to find these tholes in a reducing environment. Now, say those cysteines end up in an oxidizing environment. In that case, you would see the loss of these hydrogens and then the formation of a bond between these two sulphur groups, which looks like this. So this here is your disulfide bridge. So when do you see cysteines going solo, kind of like you see here in the separate thiol group form? And when do you see them forming these disulfide bridges? Well, it turns out that it depends a little bit on what the rest of the environment around them is like. And as it turns out, the exterior of the cell or the extracellular space is an oxidizing environment. So I'll write that down here. So the extracellular space will favor the formation of disulfide bridges. But in the intracellular space, you're more likely to find a reducing environment. So I'll write that down here. And the way that I like to keep this straight is that I kind of think of how the interior of the cell has these little molecules called antioxidants. And these antioxidants, which-- you can kind of tell by the name of it-- stifle any oxidizing reactions. And so they keep the intracellular space a reducing environment. So you might have seen cysteine spelled without an e, like this. And you're probably thinking to yourself, is it cysteine with an e? Is it cysteine without an e? Is it cystine? Which one is it? I'm so confused. There are actually two official ways of spelling cysteine. The version with the e refers to cysteine when it's in its reduced form. And the version without the e refers to cystine when it has been oxidized. And the way I remember this is by picturing that the e stands for electrons. And so you have the electrons when you're in the reduced form. And then, you don't have the e for electrons when you're in the oxidized form. So hopefully that helps you keep things straight a little bit.

Properties of the imidazole side chain

The conjugate acid (protonated form) of the imidazole side chain in histidine has a pKa of approximately 6.0. Thus, below a pH of 6, the imidazole ring is mostly protonated (as described by the Henderson–Hasselbalch equation). The resulting imidazolium ring bears two NH bonds and has a positive charge. The positive charge is equally distributed between both nitrogens and can be represented with two equally important resonance structures. Sometimes, the symbol Hip is used for this protonated form instead of the usual His.[5][6][7] Above pH 6, one of the two protons is lost. The remaining proton of the imidazole ring can reside on either nitrogen, giving rise to what are known as the N1-H or N3-H tautomers. The N3-H tautomer is shown in the figure above. In the N1-H tautomer, the NH is nearer the backbone. These neutral tautomers, also referred to as Nδ and Nε, are sometimes referred to with symbols Hid and Hie, respectively.[5][6][7] The imidazole/imidazolium ring of histidine is aromatic at all pH values.[8] Under certain conditions, all three ion-forming groups of histidine can be charged forming the histidinium cation.[9]

The acid-base properties of the imidazole side chain are relevant to the catalytic mechanism of many enzymes.[10] In catalytic triads, the basic nitrogen of histidine abstracts 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. In helices E and F of haemoglobin, histidine influences binding of dioxygen as well as carbon monoxide. This interaction enhances the affinity of Fe(II) for O2 but destabilizes the binding of CO, which binds only 200 times stronger in haemoglobin, compared to 20,000 times stronger in free haem.

The tautomerism and acid-base properties of the imidazole side chain has been characterized by 15N NMR spectroscopy. The two 15N chemical shifts are similar (about 200 ppm, relative to nitric acid on the sigma scale, on which increased shielding corresponds to increased chemical shift). NMR spectral measurements shows that the chemical shift of N1-H drops slightly, whereas the chemical shift of N3-H drops considerably (about 190 vs. 145 ppm). This change indicates that the N1-H tautomer is preferred, possibly due to hydrogen bonding to the neighboring ammonium. The shielding at N3 is substantially reduced due to the second-order paramagnetic effect, which involves a symmetry-allowed interaction between the nitrogen lone pair and the excited π* states of the aromatic ring. At pH > 9, the chemical shifts of N1 and N3 are approximately 185 and 170 ppm.[11]

Ligand

The histidine-bound heme group of succinate dehydrogenase, an electron carrier in the mitochondrial electron transfer chain. The large semi-transparent sphere indicates the location of the iron ion. From PDB: 1YQ3​.
The tricopper site found in many laccases, notice that each copper center is bound to the imidazole sidechains of histidine (color code: copper is brown, nitrogen is blue).

Histidine forms complexes with many metal ions. The imidazole sidechain of the histidine residue commonly serves as a ligand in metalloproteins. One example is the axial base attached to Fe in myoglobin and hemoglobin. Poly-histidine tags (of six or more consecutive H residues) are utilized for protein purification by binding to columns with nickel or cobalt, with micromolar affinity.[12] Natural poly-histidine peptides, found in the venom of the viper Atheris squamigera have been shown to bind Zn(2+), Ni(2+) and Cu(2+) and affect the function of venom metalloproteases.[13]

Metabolism

Biosynthesis

Histidine Biosynthesis Pathway Eight different enzymes can catalyze ten reactions. In this image, His4 catalyzes four different reactions in the pathway.

l-Histidine is an essential amino acid that is not synthesized de novo in humans.[14] 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 (His1, 2, 3, 4, 5, 6, 7, and 8) and it occurs in ten steps. This is possible because a single gene product has the ability to catalyze more than one reaction. For example, as shown in the pathway, His4 catalyzes 4 different steps in the pathway.[15]

Histidine is synthesized from phosphoribosyl pyrophosphate (PRPP), which is made from ribose-5-phosphate by ribose-phosphate diphosphokinase in the pentose phosphate pathway. The first reaction of histidine biosynthesis is the condensation of PRPP and adenosine triphosphate (ATP) by the enzyme ATP-phosphoribosyl transferase. ATP-phosphoribosyl transferase is indicated by His1 in the image.[15] His4 gene product then hydrolyzes the product of the condensation, phosphoribosyl-ATP, producing phosphoribosyl-AMP (PRAMP), which is an irreversible step. His4 then catalyzes the formation of phosphoribosylformiminoAICAR-phosphate, which is then converted to phosphoribulosylformimino-AICAR-P by the His6 gene product.[16] His7 splits phosphoribulosylformimino-AICAR-P to form d-erythro-imidazole-glycerol-phosphate. After, His3 forms imidazole acetol-phosphate releasing water. His5 then makes l-histidinol-phosphate, which is then hydrolyzed by His2 making histidinol. His4 catalyzes the oxidation of l-histidinol to form l-histidinal, an amino aldehyde. In the last step, l-histidinal is converted to l-histidine.[16][17]

The histidine biosynthesis pathway has been studied in the fungus Neurospora crassa, and a gene (His-3) encoding a multienzyme complex was found that was similar to the His4 gene of the bacterium E. coli.[18] A genetic study of N. crassa histidine mutants indicated that the individual activities of the multienzyme complex occur in discrete, contiguous sections of the His-3 genetic map, suggesting that the different activities of the multienzyme complex are encoded separately from each other.[18] However, mutants were also found that lacked all three activities simultaneously, suggesting that some mutations cause loss of function of the complex as a whole.

Just like animals and microorganisms, plants need histidine for their growth and development.[10] Microorganisms and plants are similar in that they can synthesize histidine.[19] Both synthesize histidine from the biochemical intermediate phosphoribosyl pyrophosphate. In general, the histidine biosynthesis is very similar in plants and microorganisms.[20]

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 on the right). 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.[21]

Degradation

Histidine is one of the amino acids that can be converted to intermediates of the tricarboxylic acid (TCA) cycle (also known as the citric acid cycle).[22] Histidine, along with other amino acids such as proline and arginine, takes part in deamination, a process in which its amino group is removed. In prokaryotes, histidine is first converted to urocanate by histidase. Then, urocanase converts urocanate to 4-imidazolone-5-propionate. Imidazolonepropionase catalyzes the reaction to form formiminoglutamate (FIGLU) from 4-imidazolone-5-propionate.[23] The formimino group is transferred to tetrahydrofolate, and the remaining five carbons form glutamate.[22] Overall, these reactions result in the formation of glutamate and ammonia.[24] Glutamate can then be deaminated by glutamate dehydrogenase or transaminated to form α-ketoglutarate.[22]

Conversion to other biologically active amines

Conversion of histidine to histamine by histidine decarboxylase

Requirements

The Food and Nutrition Board (FNB) of the U.S. Institute of Medicine set Recommended Dietary Allowances (RDAs) for essential amino acids in 2002. For histidine, for adults 19 years and older, 14 mg/kg body weight/day.[29] Supplemental histidine is being investigated for use in a variety of different conditions, including neurological disorders, atopic dermatitis, metabolic syndrome, diabetes, uraemic anaemia, ulcers, inflammatory bowel diseases, malignancies, and muscle performance during strenuous exercise.[30]

See also

References

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  2. ^ a b "Nomenclature and Symbolism for Amino Acids and Peptides". IUPAC-IUB Joint Commission on Biochemical Nomenclature. 1983. Archived from the original on 9 October 2008. Retrieved 5 March 2018.
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External links

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