Reaction reversibility updates

[z-haiman]

Brief description:

From Thuillier L. Purification and kinetic properties of human erythrocyte Mg2+-dependent inorganic pyrophosphatase. Biochim Biophys Acta. 1978;524(1):198-206. doi:10.1016/0005-2744(78)90118-3:

Biosynthetic reactions which produce pyrophosphate (polysaccharides, proteins, DNA and RNA synthesis} are subject to a "thermodynamic pull" [1--3] due to the subsequent complete and irreversible hydrolysis of pyrophosphate by widely distributed pyrophosphatases (standard free energy of hydrolysis: AG ° = --23.56 kJ • mo1-1 of PPi in the absence of divalent cations[4]).

The following enzyme catalyzed reactions would be considered to be reversible:

• Orotate phosphoribosyltransferase, catalyzed by UMPS
• Nicotinamide phosphoribosyltransferase, catalyzed by NAMPT
• Hypoxanthine phosphoribosyltransferase, catalyzed by HPRT1

Current status:

Identifier	Reaction equation	Genes
ORPT	orot5p_c + ppi_c <=> orot_c + prpp_c	UMPS
NMNS	h_c + ncam_c + prpp_c --> nmn_c + ppi_c	NAMPT
HXPRT	hxan_c + prpp_c --> imp_c + ppi_c	HPRT1

Proposed status/to-do list:

Identifier	Reaction equation	Genes
ORPT	orot_c + prpp_c <=> orot5p_c + ppi_c	UMPS
NMNS	h_c + ncam_c + prpp_c <=> nmn_c + ppi_c	NAMPT
HXPRT	hxan_c + prpp_c <=> imp_c + ppi_c	HPRT1

• Reverse stoichiometry for ORPT.
• Set NMNS as reversible.
• Set HXPRT as reversible.