Enzyme heterotetrameric

Phosphofructokinase (PFK) is a key regulatory enzyme of glycolysis that catalyzes the conversion of fructose-6-phosphate to fructose-1,6-diphosphate. The active PFK enzyme is a homo- or heterotetrameric enzyme with a molecular weight of 340,000. Three types of subunits, muscle type (M), liver type (L), and fibroblast (F) or platelet (P) type, exist in human tissues. Human muscle and liver PFKs consist of homotetramers (M4 and L4), whereas red blood cell PFK consists of five tetramers (M4, M3L, M2L2, ML3, and L4). Each isoform is unique with respect to affinity for the substrate fructose-6-phosphate and ATP and modulation by effectors such as citrate, ATP, cAMP, and fructose-2,6-diphosphate. M-type PFK has greater affinity for fructose-6-phosphate than the other isozymes. AMP and fructose-2,6-diphosphate facilitate fructose-6-phosphate binding mainly of L-type PFK, whereas P-type PFK has intermediate properties. 

The high-resolution crystal structure of MPT synthase [35, 36] revealed that the enzyme forms an elongated, heterotetrameric molecule (Figure 3.3A) with overall dimensions of 93 X 28 X 27 A. The small (MoaD) subunits are positioned on oppo-... 

The cytoplasmic NAD-reducing hydrogenase (SH) of the bacterium R. eutropha is a heterotetrameric enzyme, which contains several cofactors (Friedrich et al. 1996 Thiemermann et al. 1996). The Ni-containing subunit is called HoxH. This subunit plus the small subunit HoxY form the strictly conserved hydrogenase module with the Ni-Fe centre and a proximal [4Fe-4S] cluster. HoxF and HoxU represents the Fe-S/flavoprotein moiety which is closely related to a similar moiety in NADHrubiquinone oxidoreductase. The SH has been subject to molecular biological techniques in order to study its modular structure, mechanism and biosynthesis. 

SfeHNL (Sordan 79 variety) is a heterotetrameric enzyme (a/P)2. Both a- (33 kDa) and P-subunits (23 kDa) are glycosylated and linked by two disulfide bridges [31, 33]. The a- and p-subunit are synthesized in one pre-protein, which is cleaved proteolytically after protein biosynthesis (Fig. 2.2.3.3). 

Partially purified maize endosperm ADPGlc PPase (34U/mg) was found to be activated by 3PGA and Fru 6-P (25- and 17-fold, respectively) and inhibited by Pi.77 The heterotetrameric endosperm enzyme has been cloned and expressed in E. coli, and its regulatory properties were compared to an isolated allosteric mutant less sensitive to Pi inhibition.117 As indicated above, the increase of starch noted in the mutant maize endosperm ADPGlc PPase insensitive to Pi inhibition supports the importance of the allosteric effects of 3-PGA and Pi in vivo. Also as indicated above, it is believed that the major endosperm ADP-Glc PPase isoform is located in the cytosol.141... 

ADP-Glc PPase from potato tuber is composed of two different subunits, each of 50 and 51 kilodaltons, with ( 2 2 heterotetrameric-subunit structure. The potato tuber ADP-Glc PPase small subunit can be expressed as a homotetramer and is active in presence of high concentrations of the activator 3-PGA. The small subunit of many higher plant ADP-Glc PPases is highly conserved among plants with 85-95% identity (54). The homotetrameric potato enzyme that is composed exclusively of small subunits has a lower apparent affinity (Ao.s = 2.4 mM) for the activator 3-PGA than the heterotetramer (Aq.s =0.16mM),... 

The different large subunit isoforms of Arabidopsis thaliana ADP-glucose pyrophosphorylase confer distinct kinetic and regulatory properties to the heterotetrameric enzyme. J. Biol. Chem. 2003 278 28508-2515. 73. 

On the basis of the crystal structure of a Bacillus stearothermophilus pyruvate dehydrogenase subcomplex formed between the heterotetrameric El and the peripheral subunit binding domain of E2 with an evident stmctural dissymmetry of the two active sites, a direct active center communication via an acidic proton tunnel has been proposed (Frank et ak, 2004). According to this, one active site is in a closed state with an activated cofactor even before a substrate molecule is engaged, whereas the activation of the second active site is coupled to decarboxylation in the first site. Our own kinetic NMR studies on human PDH El (unpublished) support the model suggested, but similar studies on related thiamin enzymes, such as pyruvate decarboxylase, transketolase or pyruvate oxidase reveal that half-of-the-sites reactivity is a unique feature of ketoacid dehydrogenases. In line with this. X-ray crystallography studies on intermediates in transketolase catalysis indicated an active site occupancy close to unity in both active sites (Fiedler et al., 2002 and G. Schneider, personal communication). 

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