Complement factor H (FH) attenuates C3b molecules tethered via their thioester domains to self-surfaces and thereby protects host tissues. FH is a cofactor for initial C3b proteolysis that ultimately yields a surface-attached fragment (C3d), corresponding to the thioester domain. We used NMR and X-ray crystallography to study the C3d:FH19–20 complex in atomic detail. NMR further identified glycosaminoglycan-binding residues in FH module 20 of the C3d:FH19–20 complex. Mutagenesis justified the merging of the C3d:FH19–20 structure with an existing C3b:FH1–4 crystal structure. The merged structure was concatenated with the available FH6–8 crystal structure and new SAXS-derived FH1–4, FH8–15 and FH15–19 envelopes. The combined data suggests a bent-back FH molecule, binding via its termini to two sites on one C3b molecule and simultaneously to adjacent polyanionic host-surface markers.

We show that the M2 isoform of pyruvate kinase (M2PYK) exists in equilibrium between monomers and tetramers regulated by allosteric binding of naturally occurring small-molecule metabolites. Phenylalanine stabilizes an inactive T-state tetrameric conformer and inhibits M2PYK with an IC 50 value of 0.24 mM, whereas thyroid hormone (triiodo-L-thyronine, T3) stabilizes an inactive monomeric form of M2PYK with an IC 50 of 78 nM. The allosteric activator fructose-1,6-bisphosphate [F16BP, AC 50 (concentration that gives 50% activation) of 7 μM] shifts the equilibrium to the tetrameric active Rstate, which has a similar activity to that of the constitutively fully active isoform M1PYK. Proliferation assays using HCT-116 cells showed that addition of inhibitors phenylalanine and T3 both increased cell proliferation, whereas addition of the activator F16BP reduced proliferation. F16BP abrogates the inhibitory effect of both phenylalanine and T3, highlighting a dominant role of M2PYK allosteric activation in the regulation of cancer proliferation. X-ray structures show constitutively fully active M1PYK and F16BP-bound M2PYK in an R-state conformation with a lysine at the dimer-interface acting as a peg in a hole, locking the active tetramer conformation. Binding of phenylalanine in an allosteric pocket induces a 13°rotation of the protomers, destroying the peg-in-hole R-state interface. This distinct T-state tetramer is stabilized by flipped out Trp/Arg side chains that stack across the dimer interface. Xray structures and biophysical binding data of M2PYK complexes explain how, at a molecular level, fluctuations in concentrations of amino acids, thyroid hormone, and glucose metabolites switch M2PYK on and off to provide the cell with a nutrient sensing and growth signaling mechanism.allosteric regulation | nutrient sensor | thyroid hormone T3 | Warburg effect T he last of 10 enzymatic steps used to convert glucose to pyruvate is carried out by pyruvate kinase (PYK), which transfers a phosphate from phosphoenolpyruvate to ADP to generate ATP. There are four human PYK isoforms (1); RPYK is restricted to erythrocytes, LPYK is found predominantly in liver and kidney, M1PYK is in muscle and brain, and M2PYK is found in fetal tissues and in proliferating cells. All four isoforms are active as tetramers; M1PYK is constitutively fully active, whereas R-, L-, and M2PYKs are activated by the effector molecule fructose-1,6-bisphosphate (F16BP) (2). M2PYK is a splice variant of the nonallosteric M1PYK isoform and differs by 22 amino acid residues (3). Recent quantification of the concentrations of constitutively fully active M1PYK and allosterically regulated M2PYK isoforms in both cancerous and control tissue samples has revealed that M2PYK is almost always the most abundant isoform in cancer cells, although it can also be predominant in matched control tissues (4). The up-regulation of the M2PYK isoform plays a key role in cancer metabolism (3) and explains the Warburg effect, in which proliferating cancer cells metabolize increas...

Allosteric regulation provides a rate management system for enzymes involved in many cellular processes. Ligand-controlled regulation is easily recognizable, but the underlying molecular mechanisms have remained elusive. We have obtained the first complete series of allosteric structures, in all possible ligated states, for the tetrameric enzyme, pyruvate kinase, from Leishmania mexicana. The transition between inactive T-state and active R-state is accompanied by a simple symmetrical 6 o rigid body rocking motion of the A-and C-domain cores in each of the four subunits. However, formation of the R-state in this way is only part of the mechanism; eight essential salt bridge locks that form across the C-C interface provide tetramer rigidity with a coupled 7-fold increase in rate. The results presented here illustrate how conformational changes coupled with effector binding correlate with loss of flexibility and increase in thermal stability providing a general mechanism for allosteric control.Allosteric regulation ("the second secret of life" quotation ascribed to Jacob Monod) (see Ref. 1) controls many important cellular processes, including signal transduction, transcription, and metabolism (2). It describes the effect of binding one ligand on the subsequent binding of a second ligand at a topographically distinct site. Human pyruvate kinase (hPYK) 2 provides a striking example of the significance of allosteric regulation: a splicing switch of the primary RNA transcript to yield the M1 or M2 isoenzymes is now known to be responsible for the Warburg effect in cancer (3, 4) and opens up the possibility of developing isoform specific therapeutics (5). Additional mechanisms of activity regulation, including the binding of amino acids, phosphorylation, and the binding of oncoproteins, may provide further therapeutic approaches for targeting hPYK (6, 7).Most allosterically regulated proteins are enzymes in which the binding of an activator or inhibitor to the effector site can affect the binding of a substrate at the active site. The MonodWyman-Changeux model of allostery (8) suggests that oligomeric enzymes undergo symmetrical transitions (classically between the T-and R-states) 3 (36) that can be stabilized by ligand binding. However, there are now examples of allosteric control that do not show obvious conformational change (9), and a growing body of work exists to support the idea that flexible regions of molecules (which exist as an ensemble of conformers) may undergo allosteric regulation by changes in the conformer population (10). There are examples of allosteric enzymes in which the same protein has been captured in both a T-state (which has low affinity for substrate) and an R-state (which has higher affinity for substrate), and some structural insight has been obtained by the study of at least one of the allosteric states of Ͼ50 proteins (10 -12). However, a full understanding of the allosteric effect requires structural information on each of four states: (i) apoenzyme, (ii) active site complex, (iii) eff...