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The postmortem conversion of muscle to meat goes through a series of complex physiological and biochemical changes, among which protein post-translational modifications (PTMs) are key factors affecting meat quality. Glycolysis is the major energy supply pathway in postmortem muscle. PTMs can change the function and structure of pyruvate kinase, a rate-limiting enzyme in the glycolytic pathway, which can in turn affect meat quality. Beginning with an overview of the structure and function of pyruvate kinase, this article reviews the recent research progress on the effects of PTMs of pyruvate kinase on its function and structure and the effects of pyruvate kinase phosphorylation, acetylation and S-nitrosylation on meat quality.

In order to clarify the relationship between the degree of glycolysis and water-holding capacity during the maturation of Tan sheep meat, the M. longissimus dorsi of six-month-old Tan sheep was aged at 4 ℃ postmortem and evaluated for drip loss, purge loss, calpain-1 activity, myofibril fragmentation index (MFI), ATP content, glucose content, glycogen content, lactic acid content, lactate dehydrogenase (LDH) activity, pyruvate kinase (PK) activity and pH after 0, 1, 2, 3, 4 and 8 days. The results showed that as maturation time increased, drip loss, purge loss and centrifugal loss increased first and then decreased (P < 0.05), reaching their maximum values on day 4. ATP and glycogen contents continued to decrease (P < 0.05). The contents of glucose and lactic acid increased first and then decreased (P < 0.05), and so did LDH activity (P < 0.05), reaching its maximum level on day 4. PK activity showed a constant decreasing trend (P < 0.05). The correlation analysis showed that glycogen content was negatively correlated with drip loss (P < 0.05) as well as purge loss and centrifugal loss (P < 0.01), and positively correlated with nuclear magnetic resonance (NMR) transverse relaxation times (T21, T22 and T23) (P < 0.05 or P < 0.01). LA content was significantly positively correlated with drip loss (P < 0.001) as well as purge loss and centrifugal loss (P < 0.01), but had no correlation with T21, T22 or T23. Therefore, the glycolysis process of Tan sheep meat was activated rapidly after slaughter, which caused change in the activity of glycolysis-related enzymes and the contents of metabolites, thereby resulting in changes in the water adsorption capacity of muscle proteins, and ultimately affecting the water-holding capacity of Tan sheep meat after slaughter.

This study was performed in order to uncover the molecular mechanism by which postmortem glycolysis affects meat quality traits for the purpose of promoting the research and development of prediction technologies for the quality change of chilled Tan lamb and promoting the improvement of its processing, storage and transportation technologies. High-throughput sequencing technology was used for transcriptome sequencing and analysis of the hind leg meat from Yanchi Tan lambs after 0, 4 and 8 days of chilling. The results showed that the differential expression genes (DEGs) were mainly enriched in cellular amide metabolism, ATP metabolism, skeletal muscle contraction and ribosomes and involved in glycolysis, ribosomes and the p53 signaling pathway, and these DEGs constituted a key part of muscle cell function. The phosphofructokinase (PFKM), glyceraldehyde-3-phosphate dehydrogenase (GADPH), phosphoglycerate mutase 2 (PGAM2), enolase 3 (ENO3) and pyruvate kinase (PKM) genes were the core genes involved in the glycolytic metabolism of Tan lambs, which changed meat quality mainly by affecting the structure of muscle fibers, the degradation degree of structural proteins, the activity of glycolytic enzymes, the production of ATP and the activity of complexes.

Our purpose was to clarify the effect of CaCl2 treatment on the energy level and quality of Tan sheep meat during postmortem aging. Tan sheep hind leg meat was injected with 200 mmol/L CaCl2 solution, and aged at 4 ℃ for 0, 2, 4, 6, and 8 days. The protein expression of phosphofructokinase (PFKM) was detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot to verify whether the glycolysis pathway could be activated by CaCl2. At the same time, the energy level and meat quality indexes were measured during postmortem aging to analyze the mechanism by which the glycolysis pathway affects the metabolic level and quality of Tan sheep meat. The results showed that the protein expression of PFKM decreased during postmortem aging, indicating activation of the glycolytic pathway. CaCl2 treatment accelerated the glycolysis process after slaughter, thereby promoting the decomposition of glycogen, increasing the production of lactic acid, and resulting in a rapid decrease in pH. Water-holding capacity (WHC), myofibril fragmentation index (MFI) and b* value were higher and shear force and a* were lower in the treatment group than in the control and blank groups. Therefore, CaCl2 could affect the color, WHC and tenderness of Tan sheep meat at different aging stages. By accelerating the decrease of pH and the accumulation of energy metabolites, the glycolytic pathway increased the level of energy metabolism during postmortem aging, while inhibiting the production of oxymyoglobin, activating the activity of calpain, changing the structure of myofibrils and shortening the space between muscle protein molecules, which accelerated the process of myofibril fragmentation and the hydrolysis of myofibrillar protein, and led to a deterioration in meat color and an increase in water loss and meat tenderness.

Tan lamb is highly recommended breed in China. It is of great significance to understand the underlying mechanism of how water‐soluble flavor precursors metabolize in Tan lamb muscles during the postmortem aging period. In this study, we investigated the muscle pH, lactate dehydrogenase (LDH) activity, and the variations in water‐soluble flavor‐related metabolites. The proteome changes were profiled to provide insights into the biochemical changes affecting accumulation of water‐soluble flavor precursors in different aging stages (days 0, 4, and 8). The results indicated that pH value considerably decreased from day 0 to day 4, and increased from day 4 to day 8 (p <.05). The activity of LDH significantly increased from day 0 to day 4, and decreased from day 4 to day 8 (p <.05). Postmortem glycolysis was activated in 4 days, which directly affected the variations in metabolic enzymes and triggered the accumulation of flavor‐related carbohydrates. The free amino acids accumulated due to hydrolysis of structural proteins, with 3‐hydroxy‐L‐proline, aspartic acid, and methionine increasing from day 0 to day 4, and aspartic acid, serine, threonine, tyrosine, phenylalanine, and D‐phenylalanine from day 4 to day 8. The inosine and hypoxanthine accumulated due to the degradation of ATP. The results of the present study provide insightful information, revealing the differences in biochemical attributes in Tan lamb muscles caused by postmortem aging. [ABSTRACT FROM AUTHOR]