Growth rings and climate

The Botanical Review -     

Moon Dust… Key to Cosmic Secrets

Students examine the bacterial expansion pattern and attempt to relate what they discover to expanding powers of two. They also relate what they see and discover to increasing living space requirements in a world of infinite space and finite space to discover that living space decreases as a function of one over expanding powers of two. The population doubling effect is also related to refuse generation and food supply requirements as well as human drinking requirements. During their investigations, students engage in a series of concrete activities to demonstrate the various effects of unabated population expansion.     

Organization of the synthesis of glycolipid oligosaccharides in the Golgi complex

Glycolipids constitute a complex family of amphipathic molecules structurally characterized by a hydrophilic mono- or oligo-saccharide moiety linked to a hydrophobic ceramide moiety. Due to their asymmetric distribution in cell membranes, exposing the saccharide moiety to the extracytoplasmic side of the cell, glycolipids participate in a variety of cell-cell and cell-ligand interactions. Here we summarize aspects of the cell biology of the stepwise synthesis of the saccharide moiety in the Golgi complex of cells from vertebrates. In particular we refer to the participant glycosyltransferases, with emphasis on their trafficking along the secretory pathway, their retention and organization in the Golgi complex membranes and their dependence on the Golgi complex ultra structural organization for proper function.     

The crystal structure of the first enzyme in the pantothenate biosynthetic pathway,ketopantoate hydroxymethyltransferase,from M tuberculosis

Ketopantoate hydroxymethyltransferase (KPHMT) catalyzes the first committed step in the biosynthesis of pantothenate, which is a precursor to coenzyme A and is required for penicillin biosynthesis. The crystal structure of KPHMT from Mycobacterium tuberculosis was determined by the single anomalous substitution (SAS) method at 2.8 A resolution. KPHMT adopts a structure that is a variation on the (beta/alpha) barrel fold, with a metal binding site proximal to the presumed catalytic site. The protein forms a decameric complex, with subunits in opposing pentameric rings held together by a swapping of their C-terminal alpha helices. The structure reveals KPHMT's membership in a small, recently discovered group of (beta/alpha) barrel enzymes that employ domain swapping to form a variety of oligomeric assemblies. The apparent conservation of certain detailed structural characteristics suggests that KPHMT is distantly related by divergent evolution to enzymes in unrelated pathways, including isocitrate lyase and phosphoenolpyruvate mutase.     

Relationship between an altered membrane form and a low affinity form of the beta-adrenergic receptor occurring during catecholamine-induced desensitization. Evidence for receptor internalization

We have investigated the relationship between the catecholamine-induced occurrence in 1321N1 human astrocytoma cells of beta-adrenergic receptors that exhibit low apparent affinity for hydrophilic ligands in short-time assays with intact cells and a population of beta-adrenergic receptors that migrate in a light vesicle fraction on sucrose density gradients. Pretreatment of cells with concanavalin A prevents the generation of both of these forms of the receptor during incubation with agonists but does not prevent the agonist-induced decrease in isoproterenol-stimulated cyclic AMP production that also occurs during desensitization. Selective labeling of the low affinity beta-receptors with 125I-pindolol followed by centrifugation on sucrose density gradients revealed that all of the receptors in the light vesicle fraction from desensitized cells were of the low affinity type, but that a portion of the low affinity receptors also migrated in a heavier sucrose fraction together with the plasma membrane. In contrast, in control cells, no low affinity receptors were present in the heavy sucrose fractions. The agonist-induced occurrence of these various forms of the beta-adrenergic receptor can be explained on the basis of current models of desensitization involving agonist-induced internalization of beta-adrenergic receptors.     

Experimental concerns in tracer kinetic investigations of apolipoprotein metabolism

The complexity of the metabolism of the plasma lipoproteins makes it impossible to integrate the details of the reactions of specific apolipoproteins and their associated lipids without the use of computerized modeling methods. Because apolipoproteins impart specificity in the transport and chemical processing of plasma lipids, they have been the focus of many in vivo kinetic tracer investigations. The analysis of such kinetic data by modeling techniques has provided important advances in understanding lipoprotein metabolism. An example is the Delipidation Chain, an hypothesis explaining VLDL metabolism in terms of a sequential delipidation process. As a consequence of the advance in knowledge of apolipoprotein structure and metabolism, coupled with progress in computerized modeling of large systems, it has become important to refine the design of in vivo tracer kinetic investigations of the apolipoproteins. Considerations of particular importance include the selection of apolipoprotein tracers which can be shown to undergo the same reactions as the apolipoproteins whose metabolism they trace. If the physical and chemical processes which convert apolipoproteins from one metabolic pool to another are to be analyzed correctly, it is necessary to describe precisely and to measure accurately these pools. Current methods for delineating metabolic pools of apolipoproteins in vivo need to be refined. When accomplished, this will provide new opportunities to investigate the metabolic pathways of the apolipoproteins and their associated lipids. A very important challenge is to design experiments which will differentiate transfer processes, which result in net transport of a reactant, from exchange processes, whereby a tracer and a tracee are exchanged between pools without a net transport event occuring. Since both types of processes occur readily with apolipoproteins, it is important to develop methods to examine them separately. Computerized kinetic modeling provides a means for describing and understanding the complexities of lipoprotein metabolism. A major challenge is for the experimentalist to acquire data which accurately reflect the physiological processes involved in lipoprotein metabolism.