It has often been claimed that random non-equilibrium mechanisms can result in apparent homotropic and heterotropic effects in steady-state kinetics of the kind more usually attributed to intersubunit allosteric interactions. However, it has never been shown whether any simple random mechanism could in fact give patterns of apparent interaction similar to those predicted by the well-known allosteric models. The patterns of apparent substrate co-operativity and affinity given by the steady-state of a standard simple random substrate-modifier mechanism in which catalytic velocity is proportional to substrate binding have been analysed mathematically and numerically. All patterns possible with this model are described. Some of them rather resemble those possible with standard allosteric models, in that there is a high-affinity and a low-affinity form at zero and infinite modifier concentrations (or vice versa) which show Michaelian behaviour, apparent co-operativity passing through a maximum or minimum at intermediate affinities. Unlike the allosteric models the family of curves is in principle not symmetrical. The random model can also give behaviour not possible with the standard allosteric models, such as higher substrate affinity at intermediate modifier concentrations than at either zero or infinite modifier, with concomitant negative apparent substrate co-operativity, or a single change of sign of apparent substrate co-operativity. The analysis uses recently discovered simplified forms of steady-state equations for random models.

The features that distinguish positive from negative co-operativity in double-reciprocal Eadie-Hofstee-Scatchard and Hanes plots, often incorrectly stated to be the sign of curvature or second derivatives, are explained. It is shown how to determine the ‘Hill exponent’ and interaciton free energies from curves in these plots, and in the simple plot of ligand binding or velocity against free ligand or substrate concentration. New types of plots, where the kind of co-operative behaviour is more obvious than in the traditional ones, are proposed.

Conference Article| June 01 1977 Patterns of Apparent Co-Operativity of the Steady-State of a Simple Non-Equilibrium Random Substrate-Modifier Mechanism EDWARD P. WHITEHEAD; EDWARD P. WHITEHEAD *Department of Biology, DG XII, Commission of the European Communities, Roma 00185, Italy†Istituto di Chimica Biologica and C.N.R. Centro di Biologia Molecolare, Universitá di Roma, Roma 00185, Italy Search for other works by this author on: This Site PubMed Google Scholar MAARTEN R. EGMOND MAARTEN R. EGMOND ‡ †Istituto di Chimica Biologica and C.N.R. Centro di Biologia Molecolare, Universitá di Roma, Roma 00185, Italy Search for other works by this author on: This Site PubMed Google Scholar Author and article information Publisher: Portland Press Ltd Online ISSN: 1470-8752 Print ISSN: 0300-5127 © 1977 Biochemical Society1977 Biochem Soc Trans (1977) 5 (3): 789–790. https://doi.org/10.1042/bst0050789 Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Facebook Twitter LinkedIn Email Cite Icon Cite Get Permissions Citation EDWARD P. WHITEHEAD, MAARTEN R. EGMOND; Patterns of Apparent Co-Operativity of the Steady-State of a Simple Non-Equilibrium Random Substrate-Modifier Mechanism. Biochem Soc Trans 1 June 1977; 5 (3): 789–790. doi: https://doi.org/10.1042/bst0050789 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAll JournalsBiochemical Society Transactions Search Advanced Search This content is only available as a PDF. © 1977 Biochemical Society1977 Article PDF first page preview Close Modal You do not currently have access to this content.

1. The activities of the purine phosphoribosyltransferases (EC 2.4.2.7 and 2.4.2.8) in purine-analogue-resistant mutants of Schizosaccharomyces pombe were checked. An 8-azathioxanthine-resistant mutant lacked hypoxanthine phosphoribosyltransferase, xanthine phosphoribosyltransferase and guanine phosphoribosyltransferase activities (EC 2.4.2.8) and appeared to carry a single mutation. Two 2,6-diaminopurine-resistant mutants retained these activities but lacked adenine phosphoribosyltransferase activity (EC 2.4.2.7). This evidence, together with data on purification and heat-inactivation patterns of phosphoribosyltransferase activities towards the various purines, strongly suggests that there are two phosphoribosyltransferase enzymes for purine bases in Schiz. pombe, one active with adenine, the other with hypoxanthine, xanthine and guanine. 2. Neither growth-medium supplements of purines nor mutations on genes involved in the pathway for new biosynthesis of purine have any influence on the amount of hypoxanthine-xanthine-guanine phosphoribosyltransferase produced by this organism.

Research Article| March 01 1964 The reaction of yeast alcohol dehydrogenase with iodoacetamide as determined with a silver-silver iodide electrode BR Rabin; BR Rabin Search for other works by this author on: This Site PubMed Google Scholar JR Cruz; JR Cruz Search for other works by this author on: This Site PubMed Google Scholar DC Watts; DC Watts Search for other works by this author on: This Site PubMed Google Scholar EP Whitehead EP Whitehead Search for other works by this author on: This Site PubMed Google Scholar Biochem J (1964) 90 (3): 539–542. https://doi.org/10.1042/bj0900539 Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Facebook Twitter LinkedIn MailTo Cite Icon Cite Get Permissions Citation BR Rabin, JR Cruz, DC Watts, EP Whitehead; The reaction of yeast alcohol dehydrogenase with iodoacetamide as determined with a silver-silver iodide electrode. Biochem J 1 March 1964; 90 (3): 539–542. doi: https://doi.org/10.1042/bj0900539 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAll JournalsBiochemical Journal Search Advanced Search This content is only available as a PDF. © 1964 The Biochemical Society1964 Article PDF first page preview Close Modal You do not currently have access to this content.

Research Article| March 01 1964 The thiol groups of yeast alcohol dehydrogenase EP Whitehead; EP Whitehead Search for other works by this author on: This Site PubMed Google Scholar BR Rabin BR Rabin Search for other works by this author on: This Site PubMed Google Scholar Biochem J (1964) 90 (3): 532–539. https://doi.org/10.1042/bj0900532 Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Facebook Twitter LinkedIn Email Cite Icon Cite Get Permissions Citation EP Whitehead, BR Rabin; The thiol groups of yeast alcohol dehydrogenase. Biochem J 1 March 1964; 90 (3): 532–539. doi: https://doi.org/10.1042/bj0900532 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAll JournalsBiochemical Journal Search Advanced Search This content is only available as a PDF. © 1964 The Biochemical Society1964 Article PDF first page preview Close Modal You do not currently have access to this content.