Interactions of glycolytic enzymes

Flight muscle function in Drosophila requires colocalization of glycolytic enzymes

Structural relationships between the myofibrillar contractile apparatus and the enzymes that generate ATP for muscle contraction are not well understood. We explored whether glycolytic enzymes are localized in Drosophila flight muscle and whether localization is required for function. We find that glycerol-3-phosphate dehydrogenase (GPDH) is localized at Z-discs and M-lines. The glycolytic enzymes aldolase and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) are also localized along the sarcomere with a periodic pattern that is indistinguishable from that of GPDH localization. Furthermore, localization of aldolase and GAPDH requires simultaneous localization of GPDH, because aldolase and GAPDH are not localized along the sarcomere in muscles of strains that carry Gpdh null alleles. In an attempt to understand the process of glycolytic enzyme colocalization, we have explored in more detail the mechanism of GPDH localization. In flight muscle, there is only one GPDH isoform, GPDH-1, which is distinguished from isoforms found in other tissues by having three C-terminal amino acids: glutamine, asparagine, and leucine. Transgenic flies that can produce only GPDH-1 display enzyme colocalization similar to wild-type flies. However, transgenic flies that synthesize only GPDH-3, lacking the C-terminal tripeptide, do not show the periodic banding pattern of localization at Z-discs and M-lines for GPDH. In addition, neither GAPDH nor aldolase colocalize at Z-discs and M-lines in the sarcomeres of muscles from GPDH-3 transgenic flies. Failure of the glycolytic enzymes to colocalize in the sarcomere results in the inability to fly, even though the full complement of active glycolytic enzymes is present in flight muscles. Therefore, the presence of active enzymes in the cell is not sufficient for muscle function; colocalization of the enzymes is required. These results indicate that the mechanisms by which ATP is supplied to the myosin ATPase, for muscle contraction, requires a highly organized cellular system.

Rotational diffusion of band 3 in erythrocyte membranes. 2. Binding of cytoplasmic enzymes

Time-resolved phosphorescence anisotropy has been used to study the rotational diffusion of eosin-labeled human erythrocyte band 3 in the presence of an enzyme bound at its cytoplasmic pole. With increasing amounts of G3PD (glyceraldehyde-3-phosphate dehydrogenase) added to ghosts, the infinite time anisotropy (r infinity) increases, and at saturating concentrations, very little decay of the anisotropy r(t) occurs at all. These phenomena are reversed by elution of the enzyme with 150 mM NaCl. Prior proteolytic removal of the N-terminal 41-kDa cytoplasmic fragment of band 3 also disenables the G3PD effect. When ghosts are stripped of their residually bound G3PD, a small reduction in the fraction of immobile band 3 is observed. Finally, titration of band 3 sites with aldolase shows effects on the r(t) qualitatively similar to those observed with G3PD. On the basis of our interpretation of the heterogenous anisotropy decay of eosin-labeled band 3 [Matayoshi, E. D., & Jovin, T. M. (1991) Biochemistry (preceding paper in this issue)], we conclude that the binding of G3PD and aldolase results in the immobilization of band 3 oligomers.

Enzyme-enzyme interactions and their metabolic role

There are continuing reports on the existence of complexes of sequential metabolic enzymes. New techniques for their detection have been described and include affinity electrophoresis and the use of anti-idiotypic antibodies. Channeling of substrates has been reported for several systems as well as direct substrate transfer through dynamic enzyme associations. Kinetic parameters of metabolic control of organized systems have been formulated and tested in several systems. These recent results are expanding our understanding of metabolic processes and their control.