View source for Branched-chain alpha-keto acid dehydrogenase complex

{{Short description|Protein complex}}
The '''branched-chain α-ketoacid dehydrogenase complex''' ('''BCKDC''' or '''BCKDH complex''') is a multi-subunit complex of [[enzyme]]s that is found on the [[mitochondria]]l inner membrane.<ref>{{cite journal | vauthors = Indo I, Kitano A, Endo F, Akaboshi I, Matsuda I | title =Altered Kinetic Properties of the Branched-Chain Alpha-Keto Acid Dehydrogenase Complex Due to Mutation of the Beta-Subunit of the Branched-Chain Alpha-Keto Acid Decarboxylase (E<sub>1</sub>) Component in Lymphoblastoid Cells Derived from Patients with Maple Syrup Urine Disease | journal = J Clin Invest| volume = 80| issue = 1| pages = 63–70| year = 1987 | pmid = 3597778 | doi=10.1172/JCI113064| pmc = 442202}}</ref> This enzyme complex catalyzes the [[oxidative decarboxylation]] of branched, short-chain [[alpha-ketoacid]]s. BCKDC is a member of the mitochondrial α-ketoacid dehydrogenase complex family, which also includes [[pyruvate dehydrogenase]] and [[alpha-ketoglutarate dehydrogenase]], key enzymes that function in the [[Krebs cycle]].

==Biological function==
In animal tissue, BCKDC catalyzes an irreversible step<ref>{{cite journal | author = Yeaman SJ | title =The 2-oxo acid dehydrogenase complexes: recent advances. | journal = Biochem. J.| volume = 257| issue = 3| pages = 625–632| year = 1989 | pmid = 2649080 | pmc=1135633 | doi = 10.1042/bj2570625}}</ref> in the [[catabolism]] of the branched-chain amino acids [[L-isoleucine]], [[L-valine]], and [[L-leucine]], acting on their deaminated derivatives (L-alpha-keto-beta-methylvalerate, [[alpha-ketoisovalerate]], and [[alpha-ketoisocaproate]], respectively) and converting them<ref>{{Cite book|last=Rodwell|first=Vector|title=Harper's illustrated biochemistry|publisher=McGraw Hill|year=2015|isbn=978-0-07-182537-5|location=USA|pages=310|chapter=29}}</ref> to α-Methylbutyryl-CoA, Isobutyryl-CoA and Isovaleryl-CoA respectively.<ref>{{cite journal | vauthors = Broquist HP, Trupin JS | title = Amino Acid Metabolism | journal = Annual Review of Biochemistry| volume = 35| pages = 231–247| year = 1966 | doi = 10.1146/annurev.bi.35.070166.001311}}</ref><ref>{{cite journal | vauthors = Harris RA, Paxton R, Powell SM, Goodwin GW, Kuntz MJ, Han AC | title = Regulation of branched-chain alpha-ketoacid dehydrogenase complex by covalent modification. | journal = Adv Enzyme Regul.| volume = 25| pages = 219–237 | year = 1986 | pmid = 3028049 | doi=10.1016/0065-2571(86)90016-6}}</ref><ref>{{cite journal | vauthors = Namba Y, Yoshizawa K, Ejima A, Hayashi T, Kaneda T | title = Coenzyme A- and nicotinamide adenine dinucleotide-dependent branched chain alpha-keto acid dehydrogenase. I. Purification and properties of the enzyme from Bacillus subtilis. | journal = J Biol Chem| volume = 244| issue = 16| pages = 4437–4447 | year = 1969 | doi = 10.1016/S0021-9258(18)94337-1 | pmid = 4308861| doi-access = free }}</ref> In bacteria, this enzyme participates in the synthesis of branched, long-chain [[fatty acids]].<ref>{{cite journal | author = Lennarz WJ | title = The role of isoleucine in the biosynthesis of branched-chain fatty acids by micrococcus lysodeikticus.| journal = Biochemical and Biophysical Research Communications| volume = 6| issue = 2| pages = 1112–116| year = 1961 | pmid = 14463994 | doi = 10.1016/0006-291X(61)90395-3|display-authors=etal}}</ref> In plants, this enzyme is involved in the synthesis of branched, long-chain [[hydrocarbons]].

The overall catabolic reaction catalyzed by the BCKDC is shown in '''Figure 1'''.

[[File:Overall reaction catalyzed by the branched-chain alpha-ketoacid dehydrogenase complex.png|thumb|'''Figure 1:''' This is the overall reaction catalyzed by the branched-chain alpha-ketoacid dehydrogenase complex.]]

==Structure==
The mechanism of enzymatic catalysis by the BCKDC draws largely upon the elaborate structure of this large enzyme complex. This enzyme complex is composed of three catalytic components: 
{| class="wikitable"
|+
!Unit
!EC number
!Name
!Gene
!Cofactor
|-
|E<sub>1</sub>
|{{EC number|1.2.4.4}}
|[[alpha-ketoacid dehydrogenase]]
|BCKDHA
|[[thiamine pyrophosphate]] (TPP)
|-
|E<sub>2</sub>
|{{EC number|2.3.1.168}}
|[[dihydrolipoyl transacylase]]
|DBT
|[[lipoic acid]], [[coenzyme A]]
|-
|E<sub>3</sub>
|{{EC number|1.8.1.4}}
|[[dihydrolipoamide dehydrogenase]]
|DLD
|[[Flavin adenine dinucleotide|FAD]], [[Nicotinamide adenine dinucleotide|NAD]]
|}
In humans, 24 copies of E<sub>2</sub> arranged in octahedral symmetry form the core of the BCKDC.<ref>{{cite journal | vauthors = Aevarsson A, Chuang JL, Wynn RM, Turley S, Chuang DT, ((Hol WGJ)) | title = Crystal structure of human branched-chain α-ketoacid dehydrogenase and the molecular basis of multienzyme complex deficiency in maple syrup urine disease| journal = Structure| volume = 8| issue = 3| pages = 277–291| year = 2000 | pmid = 10745006| doi = 10.1016/S0969-2126(00)00105-2| doi-access = free}}</ref> Non-covalently linked to this [[polymer]] of 24 E<sub>2</sub> subunits are 12 E<sub>1</sub> α2β2 [[tetramers]] and 6 E<sub>3</sub> [[homodimers]].  In addition to the E<sub>1</sub>/E<sub>3</sub>-binding domain, there are 2 other important structural domains in the E<sub>2</sub> subunit: (i) a lipoyl-bearing domain in the [[amino-terminal]] portion of the protein and (ii) an inner-core domain in the [[carboxy-terminal]] portion. The inner-core domain is linked to the other two domains of the E<sub>2</sub> subunit by two interdomain segments (linkers).<ref>{{cite journal | author = Chuang DT. | title = Molecular studies of mammalian branched-chain alpha-keto acid dehydrogenase complexes: domain structures, expression, and inborn errors.| journal = Annals of the New York Academy of Sciences| volume = 573| pages = 137–154| year = 1989 | pmid = 2699394 | doi=10.1111/j.1749-6632.1989.tb14992.x| s2cid = 33210101}}</ref> The inner-core domain is necessary to form the oligomeric core of the enzyme complex and catalyzes the [[acyltransferase]] reaction (shown in the "Mechanism" section below).<ref>{{cite journal | vauthors = Chuang DT, Hu CW, Ku LS, Markovitz PJ, Cox RP | title = Subunit structure of the dihydrolipoyl transacylase component of branched-chain alpha-keto acid dehydrogenase complex from bovine liver. Characterization of the inner transacylase core. | journal = J Biol Chem| volume = 260| issue = 25| pages = 13779–86| year = 1985 | doi = 10.1016/S0021-9258(17)38793-8 | pmid = 4055756| doi-access = free }}</ref> The lipoyl domain of E<sub>2</sub> is free to swing between the [[active site]]s of the E<sub>1</sub>, E<sub>2</sub>, and E<sub>3</sub> subunits on the assembled BCKDC by virtue of the conformational flexibility of the aforementioned linkers (see '''Figure 2''').<ref>{{cite journal | vauthors = Reed LJ, Hackert ML | title = Structure-function relationships in dihydrolipoamide acyltransferases. | journal = J Biol Chem| volume = 265| issue = 16| pages = 8971–8974| year = 1990 | doi = 10.1016/S0021-9258(19)38795-2 | pmid = 2188967| doi-access = free }}</ref><ref>{{cite journal | author = Perham RN. | title = Domains, motifs, and linkers in 2-oxo acid dehydrogenase multienzyme complexes: a paradigm in the design of a multifunctional protein. | journal = Biochemistry| volume = 30| issue = 35| pages = 8501–8512| year = 1991 | pmid = 1888719 | doi = 10.1021/bi00099a001}}</ref> Thus, in terms of function as well as structure, the E<sub>2</sub> component plays a central role in the overall reaction catalyzed by the BCKDC.

[[File:Swinging arm of BCKDC.png|thumb|'''Figure 2:'''This is a schematic of the "swinging" lipoyl domain. Note that this lipoyl domain is covalently attached to the E<sub>2</sub> subunit of the BCKDC, but is free to swing between the E<sub>1</sub>, E<sub>2</sub>, and E<sub>3</sub> subunits. As is described in the "Mechanism" section, the ability of the lipoyl group to freely swing between the active sites on each of the three subunits of the BCKDC plays a large and important role in the catalytic activity of this enzyme complex.<ref>Berg, Jeremy M., John L. Tymoczko, Lubert Stryer, and Lubert Stryer. Biochemistry. 6th ed. New York: W.H. Freeman, 2007. 481. Print.</ref>]]

The role of each subunit is as follows:

===E<sub>1</sub> subunit===
E<sub>1</sub> uses thiamine pyrophosphate (TPP) as a catalytic cofactor. E<sub>1</sub> catalyzes both the decarboxylation of the α-ketoacid and the subsequent reductive acylation of the lipoyl moiety (another catalytic cofactor) that is covalently bound to E<sub>2</sub>.

===E<sub>2</sub> subunit===
E<sub>2</sub> catalyzes a transfer of the acyl group from the lipoyl moiety to coenzyme A (a stoichiometric cofactor).<ref>{{cite journal | vauthors = Heffelfinger SC, Sewell ET, Danner DJ | title = Identification of specific subunits of highly purified bovine liver branched-chain ketoacid dehydrogenase. | journal = Biochemistry| volume = 22| issue = 24| pages = 5519–5522| year = 1983 | pmid = 6652074 | doi = 10.1021/bi00293a011}}</ref>

===E<sub>3</sub> subunit===
The E<sub>3</sub> component is a flavoprotein, and it re-oxidizes the reduced lipoyl sulfur residues of E<sub>2</sub> using FAD (a catalytic cofactor) as the oxidant. FAD then transfers these protons and electrons to NAD+ (a stoichiometric cofactor) to complete the reaction cycle.

==Mechanism==
As previously mentioned, BCKDC's primary function in mammals is to catalyze an irreversible step in the catabolism of branched-chain amino acids. However BCKDC has a relatively broad specificity, also oxidizing 4-methylthio-2-oxobutyrate and 2-oxobutyrate at comparable rates and with similar Km values as for its branched-chain amino acid substrates.<ref>{{cite journal | vauthors = Jones SM, Yeaman SJ | title = Oxidative decarboxylation of 4-methylthio-2-oxobutyrate by branched-chain 2-oxo acid dehydrogenase complex. | journal = Biochemical Journal| volume = 237| issue = 2| pages = 621–623| year = 1986 | doi = 10.1042/bj2370621 | pmc = 1147032 | pmid = 3800905}}</ref> The BCKDC will also oxidize pyruvate, but at such a slow rate this side reaction has very little physiological significance.<ref>{{cite journal | vauthors = Pettit FH, Yeaman SJ, Reed LJ | title =Purification and characterization of branched chain alpha-keto acid dehydrogenase complex of bovine kidney. | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 75| issue = 10| pages = 4881–4885 | year = 1978 | pmid = 283398 | pmc = 336225 | doi=10.1073/pnas.75.10.4881| doi-access =free }}</ref><ref>{{cite journal | vauthors = Damuni Z, Merryfield ML, Humphreys JS, Reed LJ | title =Purification and properties of branched-chain alpha-keto acid dehydrogenase phosphatase from bovine kidney. | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 81| issue = 14| pages = 4335–4338 | year = 1984 | pmid = 6589597 | pmc = 345583 | doi=10.1073/pnas.81.14.4335| doi-access =free }}</ref>

The reaction mechanism is as follows.<ref>Berg, Jeremy M., John L. Tymoczko, Lubert Stryer, and Lubert Stryer. Biochemistry. 6th ed. New York: W.H. Freeman, 2007. 478-79. Print.</ref> Please note that any of several branched-chain α-ketoacids could have been used as a starting material; for this example, α-ketoisovalerate was arbitrarily chosen as the BCKDC substrate.

:''NOTE: Steps 1 and 2 occur in the E<sub>1</sub> domain''

'''STEP 1:''' α-ketoisovalerate combines with TPP and is then decarboxylated. The proper arrow-pushing mechanism is shown in '''Figure 3'''.
[[File:BCKDC catalytic mechanism step 1.png|thumb|'''Figure 3:''' α-ketoisovalerate combines with TPP and is then decarboxylated]]

'''STEP 2:''' The 2-methylpropanol-TPP is oxidized to form an acyl group while being simultaneously transferred to the lipoyl cofactor on E2. Note that TPP is regenerated. The proper arrow-pushing mechanism is shown in '''Figure 4'''.
[[File:BCKDC catalytic mechanism step 2.png|thumb|'''Figure 4:''' The 2-methylpropanol-TPP is oxidized to form an acyl group while being simultaneously transferred to the lipoyl cofactor on E2. Note that TPP is regenerated.]]

:''NOTE: The acylated lipoyl arm now leaves E<sub>1</sub> and swings into the E<sub>2</sub> active site, where Step 3 occurs.''

'''STEP 3:''' Acyl group transfer to CoA. The proper arrow-pushing mechanism is shown in '''Figure 5'''.
[[File:BCKDC catalytic mechanism step 3.png|thumb|'''Figure 5:''' Acyl group transfer to CoA]]

:''*NOTE: The reduced lipoyl arm now swings into the E<sub>3</sub> active site, where Steps 4 and 5 occur.''

'''STEP 4:''' Oxidation of the lipoyl moiety by the FAD coenzyme, as shown in '''Figure 6'''.
[[File:BCKDC catalytic mechanism step 4.png|thumb|'''Figure 6:''' Oxidation of the lipoyl moiety by the FAD coenzyme.]]

'''STEP 5:''' Reoxidation of FADH<sub>2</sub> to FAD, producing NADH:
:FADH<sub>2</sub> + NAD<sup>+</sup> → FAD + NADH + H<sup>+</sup>

==Disease relevance==
A deficiency in any of the enzymes of this complex as well as an [[enzyme inhibitor|inhibition]] of the complex as a whole leads to a buildup of branched-chain amino acids and their harmful derivatives in the body. These accumulations lend a sweet smell to bodily excretions (such as ear wax and urine), leading to a pathology known as [[maple syrup urine disease]].<ref>{{cite journal |pmid=10234605 |date=April 1999 |vauthors=Podebrad F, Heil M, Reichert S, Mosandl A, Sewell AC, Böhles H |title=4,5-dimethyl-3-hydroxy-25H-furanone (sotolone)--the odour of maple syrup urine disease |volume=22 |issue=2 |pages=107–114 |journal=Journal of Inherited Metabolic Disease |doi=10.1023/A:1005433516026|s2cid=6426166 |doi-access=free }}</ref>

This enzyme is an [[autoantigen]] recognized in [[primary biliary cirrhosis]], a form of acute liver failure. These [[antibodies]] appear to recognize oxidized [[protein]] that has resulted from inflammatory immune responses. Some of these inflammatory responses are explained by [[gluten sensitivity]].<ref name="pmid17657817">{{cite journal | vauthors = Leung PS, Rossaro L, Davis PA | title = Antimitochondrial antibodies in acute liver failure: Implications for primary biliary cirrhosis | journal = Hepatology| volume = 46| issue = 5| pages = 1436–42| year = 2007 | pmid = 17657817 | doi = 10.1002/hep.21828 | pmc=3731127|display-authors=etal}}</ref> Other mitochondrial autoantigens
include [[pyruvate dehydrogenase]] and branched-chain [[oxoglutarate dehydrogenase]], which are antigens recognized by [[anti-mitochondrial antibodies]].

Mutations of the [[BCKDK]] gene, whose protein product controls the activity of the complex, may result in over-activation of the complex and excessive catabolism of the three amino acids. This leads to [[branched-chain keto acid dehydrogenase kinase deficiency]], a rare disease first described in humans in 2012.<ref name="pmid22956686">{{cite journal | vauthors = Novarino G, El-Fishawy P, Kayserili H, Meguid NA, Scott EM, Schroth J, Silhavy JL, Kara M, Khalil RO, Ben-Omran T, Ercan-Sencicek AG, Hashish AF, Sanders SJ, Gupta AR, Hashem HS, Matern D, Gabriel S, Sweetman L, Rahimi Y, Harris RA, State MW, Gleeson JG | title = Mutations in BCKD-kinase lead to a potentially treatable form of autism with epilepsy | journal = Science | volume = 338 | issue = 6105 | pages = 394–7 | date = October 2012 | pmid = 22956686 | pmc = 3704165 | doi = 10.1126/science.1224631 | url = }}</ref>

==References==
{{Reflist}}

==External links==
* [https://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=gene&part=msud GeneReviews/NCBI/NIH/UW entry on Maple Syrup Urine Disease]
* {{MeshName|Branched+Chain+Ketoacid+Dehydrogenase}}
* {{EC number|1.2.4.4}}
* [http://ars.els-cdn.com/content/image/1-s2.0-S0300571202000507-gr3.gif]

{{Autoantigens}}
{{Aldehyde/Oxo oxidoreductases}}
{{Amino acid metabolism enzymes}}
{{Multienzyme complexes}}

{{DEFAULTSORT:Branched-Chain Alpha-Keto Acid Dehydrogenase Complex}}
[[Category:Autoantigens]]