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In this minireview the main mechanism of control of mammalian pyruvate dehydrogenase complex (PDHC) activity by phosphorylation-dephosphorylation is presented in the first place. The information recently obtained in several laboratories includes new data about isoforms of the PDH converting enzymes (kinase and phosphatase) and their action in view of short-term regulation of PDHC. Moreover, interesting influence of exogenous thiamine diphosphate (TDP) and some divalent cations, especially Mn(2+), on the kinetic parameters of PDHC saturated with endogenous tightly bound TDP, is discussed. This influence causes a shortening of the lag-phase of the catalyzed reaction and a strong decrease of the K(m) value of PDHC mainly for pyruvate. There are weighty arguments that the effects have an allosteric nature. Thus, besides reversible phosphorylation, also direct manifold increase of mammalian PDHC affinity for the substrate by cofactors seems an important aspect of its regulation.

The multienzyme pyruvate dehydrogenase complex (PDHC) catalyzes the conversion of pyruvate, CoA and NAD + into acetyl-CoA, NADH and CO 2 (Reed, 2001).The main product, a CoA-activated two-carbon unit, can be condensed with oxalacetate in the first reaction of the citric acid cycle, or used for fa�y acid and cholesterol synthesis.So, the several-step process of oxidative decarboxylation of pyruvate occupies a central position in cellular metabolism, because it links glycolysis with the citric acid cycle and lipid biosynthesis.In eukaryotic cells, the PDHC is located in the mitochondria matrix and may be regulated in numerous ways (Wałajtys et al., 1974;Wieland, 1983).
Studies of the structural organization of E1 may be especially useful for understanding its catalytic and regulatory mechanisms.For example, the crystal structure of the holo-form of human pyruvate dehydrogenase at 1.95 Å resolution, recently determined by Ciszak et al. (2003), gives evidence

2005
S. Strumiło that a shu�le-like motion of E1 heterodimers (αβ) is possible.This is in accordance with a flip-flop mechanism of the catalytic action of animal E1 (Khailova et al., 1990).Site-directed mutagenesis, kinetic analysis and X-ray crystallography of human E1 revealed that Pro-188, Met-181 and Arg-349 in the α-subunit, and Trp-135 in the β-subunit play important structural roles.Moreover, the αPro-188 and αMet-181 residues were found to be essential for binding of TDP (Korotchkina et al., 2004).

CHARACTERIZATION OF PDH KINASES
PDHK is a specific kinase that phosphorylates three serine residues in three separate sites of the E1-component of the PDHC, using ATP as a do-nor of phosphate group (Wieland, 1983;Reed, 2001;Kolobova et al., 2001;Bao et al., 2004a).Mammalian PDHK has been shown to exist in four isoforms (Gudi et al., 1995;Bowker-Kinley et al., 1998;Harris et al., 2002).It should be noted that genes which encode PDHK 1 , PDHK 2 and PDHK 4 are more strongly expressed in the muscle tissues and liver.On the other hand, genes encoding PDHK 3 and PDHK 2 are expressed in the kidney, brain, and testis (Harris & Huang, 2001;Patel & Korotchkina, 2003).This determines the respective differences in tissue distribution of mammalian PDHK isoforms and their mRNAs.Recently the three-dimensional structure of rat PDHK (isoform 2) has been established (Steussy et al., 2001).Interestingly, the mammalian PDHKs exhibit low homology with other eukaryotic serine protein kinases, but are similar to bacterial histidine protein kinases (Harris et al., 2002).However, their catalytic mechanisms are different, because PDHKs phosphorylate a serine residue directly, without phosphorylation of a histidine intermediate (Steussy et al., 2001).Each PDHK isoform is a homodimer composed of subunits of molecular masses between 39 and 48 kDa (Patel & Korotchkina, 2003).

REGULATORY PROPERTIES OF PDH KINASES
Mammalian PDHC contains only 1-3 molecules of PDHK (Yeaman, 1989) which bind mainly to the mobile lipoyl domains of E2-subunits (Liu et al., 1995).Only the PDHK 4 isoform prefers the lipoyl domain of the "E3-binding protein" (Roche et al., 2003).Such localization of PDHKs on the "swinging arms" and their transfer from one lipoyl domain to another gives the possibility of reaching almost all phosphorylation sites in the 30 molecules of E1 (Korotchkina & Patel, 2001).PDHKs are activated via conformational changes when bound to the lipoyl domains (Roche et al., 2003;Bao et al., 2004b).Moreover, the activity of PDHK is stimulated by the reduction and acetylation of the lipoyl residues occurring during PDC action (Harris et al., 2002).Numerous studies with PDHC isolated from various mammalian tissues indicate that PDHK is stimulated by NADH and acetyl-CoA (Wieland, 1983;Yeaman, 1989;Reed, 2001).A high NADH/NAD + ratio causes reduction of the lipoyl groups through the reversal of the reaction catalyzed by dihydrolipoamide dehydrogenase (E3), and a high acetyl-CoA/CoA ratio promotes acetylation of the reduced lipoyl groups by E2 (Patel & Korotchkina, 2003).High NADH and acetyl-CoA concentrations are found in mitochondria when fa�y acid oxidation is intensive, for example during starvation, in high fat diet and diabetes (Wu et al., 2000;Harris et al., 2002).Phosphorylation and concomitant inactivation of PDHC by stimulated PDHK is very typical for such metabolic states.On the oth- Mammalian pyruvate dehydrogenase complex er hand, high NAD + and CoA concentrations lead to PDHK inhibition, and the PDHC remains active (Wieland, 1983;Holness & Sugden, 2003).Pyruvate also exerts an inhibitory influence on PDHK (Bao et al. 2004a).Isoform PDHK 2 is more sensitive to inhibition by pyruvate and stimulation by NADH and acetyl-CoA than the other ones (Harris et al., 2002;Bao et al., 2004a).Competitive inhibition of PDHK by ADP versus ATP was also considered to be essential in the regulation of the PDHC (Strumiło et al., 1981;Wieland, 1983).Studies of PDHK 2 showed that ADP dissociation from active sites is a limiting step in E2-activated catalysis by isoform 2 (Bao et al., 2004b).Conversion of all lipoyl groups in the E2oligomer to the oxidized form greatly reduced k cat and the K m of PDHK 2 for ATP.On the other hand, reduction and, to a greater extent, reductive acetylation of the lipoyl groups increased PDHK 2 binding to E2 and stimulated it (Bao et al., 2004a).Thus, fluctuations of pyruvate concentration and the NADH/ NAD + , acetyl-CoA/CoA and ATP/ADP ratios in varied hormonal and nutritional states significantly change PDHK activity and, in an opposite manner, the activity of the PDHC.It should also be added that a lipoprotein which strongly inhibits the PDHC has been isolated from bovine adrenal mitochondria (Strumiło et al., 1984).The degree of inhibition was higher at lower ATP concentrations.It was proposed that the mitochondrial lipoprotein could specifically shield the active site of PDHK or the phosphorylation sites of E1 during catalytic action.
As presented above, the E1-component of mammalian PDHC has three phosphorylation sites.Isoforms of PDHK show different activity during phosphorylation of the sites.Their relative activities toward site 1 are: PDHK 2 > PDHK 4 > PDHK 1 > PDHK 3 ; toward site 2: PDHK 3 > PDHK 4 > PDHK 2 > PDHK 1 (Patel & Korotchkina, 2003).In the case of site 3, the differences in the activity of the PDHK isoforms are not so clear.According to Kolobova et al. (2001), PDHK 1 can phosphorylate all three sites, whereas PDHK 2 , PDHK 3 and PDHK 4 each phosphorylates only sites 1 and 3. Earlier, it was reported that only phosphorylation at sites 1 and 2 results in PDHC inactivation, yet additional phosphorylation of site 3 inhibits reactivation of E1 by PDH phosphatase (Wieland, 1983).However, according to new data, phosphorylation of either site leads to PDHC inactivation, but in a different way (Korotchkina & Patel, 2001;Kolobova et al., 2001).For example, phosphorylation of site 1 of human pyruvate dehydrogenase prevents its interaction with the substrate, pyruvate, and a lipoyl domain, whereas modification of site 3 hinders E1 interaction with TDP (Patel & Korotchkina, 2003).Conversely, exogenous TDP added to the reaction medium results in inhibition of PDHK activity (Walsh et al., 1976;Czygier & Strumiło, 1995a).TDP especially decreases the amount of phosphate that PDHK 1 incorporates in sites 2 and 3, and PDHK 2 in site 2 (Kolobova et al., 2001).Interesting data were obtained in kinetic investigation of aurochs heart PDHC: completely dephoshporylated PDHC showed a positive cooperativity of pyruvate binding sites at low substrate concentrations, yet partial phosphorylation of the complex (to 35% reduction of its activity) abolished the cooperativity (Czygier & Strumiło, 1995b).
Besides the short-term regulation by the metabolites, a long-term control of PDHK amount at the transcriptional level is very important.This aspect is presented thoroughly in the recently published reviews (Harris et al., 2002;Patel & Korotchkina, 2003).In this minireview, only several examples of long-term regulation will be quoted.Of the four PDHK isoforms greater changes in the amount of enzyme protein were observed for PDHK 4 and PDHK 2 .Starvation and diabetes increase the level of PDHK 4 expression in most tissues (Harris et al., 2001).Insulin treatment of diabetic rats reverses the changes (Harris et al., 2002).Starvation also increases PDHK 2 expression in the liver and kidney (Wu et al., 2000).

CHARACTERIZATION OF PDH PHOSPHATASES
PDHP is a specific phosphatase that removes P i from phosphorylated sites of E1 and reactivates the component and the whole PDHC.Mammalian PDHP consists of two nonidentical subunits of 52 and 96 kDa (Chen et al., 1996).The smaller subunit performs the catalytic function, whereas the larger one is a flavoprotein performing a regulatory function (Lawson et al., 1997).There are two isoforms of PDHP encoded by different genes for the catalytic subunits (Huang et al., 2003).The isoforms PDHP 1 and PDHP 2 show some differences in activity, regulation, and tissue distribution (Karpova et al., 2003).It is known that binding of PDHP 1 to the mammalian PDHC occurs thorough the lipoyl domain LD 2 and requires the presence of Ca 2+ which plays a bridging role (Roche et al., 2003).As with the binding of PDHP 2 , the existing information is insufficient for unequivocal conclusions.

REGULATORY PROPERTIES OF PDHP
The reaction catalyzed by both isoforms of PDHP requires Mg 2+ concentrations in the millimolar range (Linn et al., 1969;Reed, 2001).Magnesium ions may be replaced by Mn 2+ .The K m values of pig heart PDHP for MgCl 2 (2.5 mM) and MnCl 2 (1.8 mM) are similar (Wieland, 1983), but in the case of bovine adrenal PDHP the constants (1.20 and 0.35 mM, respectively) differ over 3-fold, and a higher PDHP 762 2005 S. Strumiło activity was observed in the presence of Mn 2+ compared to Mg 2+ (Strumiło, 1983).In addition to Mg 2+ , the isoform PDHP 1 requires Ca 2+ at micromolar concentrations.Ca 2+ , besides its participation in the interaction of PDHP 1 with the LD 2 domain, strongly stimulates PDHP 1 activity (Huang et al., 1998;Roche et al., 2003).Moreover, a specific regulatory protein for PDHP 1 has been isolated and partially characterized (Lawson et al., 1997).According to the recently obtained data, PDHP 1 is more sensitive to Mg 2+ than is PDHP 2 (Harris et al., 2002).Polyamines, especially spermine, markedly decrease the K m values for Mg 2+ of both PDHP isoenzymes (Roche et al., 2003).In general, the short-term regulation of PDHP activity is not so varied as is the regulation of PDHK.PDHP activity is influenced mainly by divalent cations.For example, muscle contraction causes an increase in mitochondrial Ca 2+ which can then stimulate PDHP 1 activity and the whole PDHC (Harris et al., 2002;Patel & Korotchkina, 2003).Insulin can stimulate PDHPs via their phosphorylation by protein kinase Cδ, which is activated and translocated to mitochondria of muscle and liver cells by the hormone (Caruso et al., 2001).In additional to the short-term regulation by divalent ions, polyamines and, probably, insulin, a long-term regulation of PDHP via changes in its expression at the genetic level is a confirmed fact.Starvation and diabetes decrease PDHP 1 expression, especially in rat heart and kidney (Huang et al., 2003), whereas insulin increases it (Harris et al., 2002;Holness & Sugden, 2003).

ISOSTERIC INHIBITION BY END-PRODUCTS
In numerous kinetic studies of PDHC isolated from varied mammalian tissues, its competitive inhibition by acetyl-CoA and NADH versus CoA and NADH has been shown (Wieland, 1983;Yeaman, 1989;Czygier & Strumiło, 1994).The K i values for NADH were in the same range as the K m values for NAD + .It has been suggested that NADH inhibition of PDHC at the isosteric level (at the active sites of the E3 component) is important for the control of mammalian PDHC activity (Wieland, 1983).The inhibition of E2 caused by acetyl-CoA may be a little less significant for PDHC regulation since the K i values for acetyl-CoA were usually higher than the K m values for CoA (Wieland, 1983).For example, in the case of PDHC from the human heart the difference was nearly 3-fold (Kiselevsky et al., 1990).Yet in such states as starvation or diabetes when the mitochondrial ratios of acetyl-CoA/CoA and NADH/ NAD + are very high in some tissues, acetylation and reduction of lipoyl groups in backward reactions catalyzed by E2 and E3 can lead to an inhibition of pyruvate decarboxylation catalyzed by E1 (Wieland, 1983;Patel & Korotchkina, 2003).The end-prod-uct inhibition at the active centers of the E2 and E3 components seems to be an essential addition to the allosteric regulation of PDH kinase by acetyl-CoA and NADH, as discussed above.Both mechanisms concern the same, universal and clear principle of regulation -"negative feedback inhibition".

ALLOSTERIC REGULATION BY SOME COFACTORS
Many investigators have considered that TDP plays only a catalytic role in the active sites of E1.For a long time it has been suggested that mammalian PDHC loses TDP almost completely during isolation, because the measured PDHC activity was usually very low in the absence of added TDP (Walsh et al., 1976;Sumegi & Alkonyi, 1983).The measurements were based on the initial rate of the reaction catalyzed by PDHC.However, a far higher PDC activity was observed when the reaction rate was measured for a longer period, because in the absence of added TDP the maximum rate was achieved only a�er a lag-phase of several minutes (Strumiło, 1988).It was established that the duration of the lag-phase and subsequent reaction rate depends strongly on the concentration and type of divalent ions present (Mg 2+ , Ca 2+ , Mn 2+ ).The shortest lag-phase and the highest activity of PDHC with endogenous TDP is observed in the presence of Mn 2+ (Czerniecki & Czygier, 2001).Kinetics studies of bovine, bison, and pig heart PDHC saturated with endogenous TDP in the active sites of E1 showed that the addition of exogenous TDP leads to a disappearance of the lag-phase and manifold reduction of the K m value for pyruvate, but a more moderate decrease of the K m for CoA and NAD + with no considerable change in the maximum reaction rate (Strumiło et al., 1996;1999;Strumiło & Czygier, 1998).The apparent K m value for TDP is about 0.2 µM which means that the cofactor is very effective.Generally, an increase of an enzyme's affinity for substrates without alteration in the V max are typical signs of positive allosteric modulators in view of the model of Monod et al. (1965).TDP acts exactly in such a manner.For example, the K m values of pig heart PDHC for pyruvate in the absence and presence of added TDP are 76.7 ± 6.6 and 19.0 ± 2.7 µM, respectively (Strumiło et al., 1999).The increase of PDHC affinity for the substrate is 4fold.It has also been established that TDP added to PDHC changes both the UV and circular dichroism spectra, and lowers the fluorescence emission of the complex containing bound molecules of endogenous TDP in its active centers (Strumiło et al., 2002).The spectral data indicate conformational changes of the PDHC.Thus, TDP as an allosteric modulator can quickly induce conformational and functional changes of the multienzyme complex.The divalent ions

Mammalian pyruvate dehydrogenase complex
Mg 2+ , Ca 2+ , and especially Mn 2+ increase the effect of TDP (Czerniecki & Czygier, 2001).Participation of the main effectors in the control of PDHC activity is summarized in Fig. 2.
The data obtained suggest that TDP plays, in addition to its catalytic function, the important role of a positive regulatory effector of mammalian PDHC.Researchers who have the possibility of structural investigations by modern methods ought to notice the allosteric regulation of PDHC.Until now, the structural studies of pyruvate dehydrogenase have been limited to the processes in the active sites of the enzyme and the interactions among its subunits (Ciszak et al., 2003;Korotchkina et al., 2004).

Figure 1 .
Figure 1.Scheme of a fragment of the mammalian pyruvate dehydrogenase complex structure (from Harris et al., 2002, modified).Only two subunits of E2 with E1 and E3 components are shown.In complete PDHC 60 inner domains are arranged in a pentagonal dodecahedron.Abbreviations: ID, inner domain; BD, binding domain; LD, lipoyl domain.