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2. To explain the importance of the conserved sum of cycle metabolites as a parameter of the system, the cycle is first regarded as a 'black box'. The interactions of the cycle with the rest of the system are expressed in terms of 'cycle elasticities' and 'cycle control coefficients' by the usual connectivity properties. The conserved sum is seen to be an 'external' parameter in the sense that its effect is described by a combined response expression. All cycle coefficients can be written in terms of elasticities and concentrations of cycle metabolites.3. The treatment shows how connectivity expressions should be modified when moiety-conserved cycles are present and establishes new summation and connectivity properties. The analysis is applied to a two-member moiety-conserved cycle and its general application is discussed.An important feature of intermediary metabolism is the existence of moiety-conserved cycles. Two well known groups of metabolites participating in such cycles are ATP-ADP-AMP (the moiety being the adenylate group) and NAD-NADH (the oxidized and reduced forms of nicotinamide adenine nucleotide). The cycle is said to be 'conserved' when the group of interconvertible metabolites turn over rapidly, while the synthesis and degradation of the moiety is slow. The sum of the concentrations of these metabolites then remains, for all practical purposes, constant within the timescale of turnover of the individual metabolites.These cycles typically couple a variety of pathways: they may be said to be the 'cog wheels' around which metabolism is organised. Clear statements on the importance of these cycles come from the work of Atkinson [I] Some aspects of the control analysis of moiety-conserved cycles have been treated in the literature. The ATP-ADP cycle in mitochondria has been analysed in terms of concentration ratios [12, 131. Fell and Sauro [Ill found a way to express certain connectivity properties that link local and systemic changes in pathway fluxes, when a conserved cycle is present. These findings will be discussed in later sections. We shall show how new connectivity and summation properties go hand in hand with moiety-conserved cycles, and how one can assess the importance of the conserved sum in metabolic control. GENERAL BACKGROUNDMetabolic control analysis allows one to quantify the behaviour of a metabolic pathway in steady state in terms of dimensionless coefficients. A distinction is made between system parameters (such as external constant substrates or effectors, various kinetic constants of any enzyme or translocator, its concentration etc.) and system variables (metabolite concentrations, fluxes or any functions involving these). Two types of Coefficients, 'global' and 'local', are distinguished Global coefficients describe the change in the steady-state value of any system variable in response to an infinitesimal change in any system parameter. To eliminate dimensions, these changes are scaled relative to the steady-state values of the variable and the parameter respectively. The ...

The assignment of structure (1) to fusarin C, a mutagen isolated from cultures of Fusarium moniliforme is based on a detailed study of its high-field 1H and 13C n.m.r. spectra and X-ray crystallography of the 8Zisomer of (1) which defined the substitution pattern and relative configuration of the 2-pyrrolidone moiety; nuclear Overhauser enhancement experiments indicate that the 2€,4€,6€,8€,10€ polyene chromophore of (1) exists in solution as an equilibrium between two conformers with s-cis and s-trans topology of the C-5-C-6 single bond.