Modelling and analysis of energy metabolism in postmortem muscle

Abstract

Fresh-meat quality is largely determined by a number of biochemical reactions during the conversion of muscle to meat. To produce consistent high-quality meat, it is necessary to have an understanding of energy metabolism in postmortem muscle. Therefore, this research was conducted to model and analyze the energy metabolic process in postmortem muscle. The work included three main parts: A kinetic model structure was developed to describe the major variations in energy metabolism and to gain further understanding of pH changes in postmortem muscle experimentally observed with an in vitro glycolytic system. Simulation and analysis of pH variations based on the model suggested that phosphofructokinase (PFK) activity has the strongest impact on the rate and extent of postmortem pH decline. Postmortem pH is also influenced by rates of ATP hydrolysis and glycolysis, and to a much lesser extent, pH buffering capacity. Other reactions, including those mediated by creatine kinase, adenylate kinase, and AMP deaminase, have minimal effects on postmortem pH decline. The contributions of different pathways to ATP production and pH variations were analyzed by using a kinetic model based on data from beef Longissimus lumborum because the roles of energy pathways in postmortem muscles are still debated. Our results indicated that ATP is produced primarily by creatine kinase- and myokinase-mediated reactions, and anaerobic glycolysis. Aerobic respiration lasts up to 9 hours postmortem but contributes less than 1 percent of the total ATP production. Similarly, pH decline in postmortem muscles is mainly due to ATP hydrolysis and glycolysis (-0.52 and -0.6 after 24 hours). Aerobic respiration, on the other hand, leads to a pH increase but the amount is negligible (0.0043 after 24 hours). Analysis of the effects of various factors on pH variations showed that electrical stimulation affects pH primarily through ATP hydrolysis and anaerobic glycolysis. The initial muscle oxygen saturation and phosphocreatine stores showed minor impacts on pH. Since phosphofructokinase (PFK) plays a critical role in the regulation of glycolysis and a quantitative model for PFK activity is lacking, a new method by kinetic modeling was developed to estimate PFK activity and quantify the inhibitory effects of ATP and pH on PFK. In contrast to the conventional initial-velocity method, which gives a qualitative picture of the inhibition of PFK activity under varied conditions, the proposed model can separate the substrate effect from the inhibitor effect and thus quantify the inhibitor effect. The use of a kinetic model to evaluate enzyme activity and quantify the degree of inhibition can be extended to other enzymes as long as the underlying reaction mechanisms are known. Kinetic analyses of PFK based on the proposed model indicated that the affinity of the substrate binding to the catalytic sites is 4-fold of that to the inhibitory site. Increasing ATP concentration leads to a reduced overall reaction velocity as a result of increasing proportion of enzyme complexes with ATP bound to the inhibitory site.