In this study, triosephosphate isomerase was incubated in the presence of exogenous protein kinase A and S-nitrosoglutathione to elucidate the regulatory effects of phosphorylation and nitrosylation on the enzyme’s activity as a function of incubation time. High expression models of triosephosphate isomerase modification by phosphorylation and nitrosylation were constructed under different temperature conditions. The results showed that after 6 h incubation at 4 ℃, the triosephosphate isomerase activity, phosphorylation and nitrosylation levels of the control group were significantly lower than those of the constant temperature + modification group, suggesting that phosphorylation and nitrosylation worked together to improve the activity of triosephosphate isomerase. Compared with the constant temperature + modification group, the effect of phosphorylation on the activity of triosephosphate isomerase was weaker in the amplitude fluctuation + modification group, but stronger in the frequency fluctuation + modification group. After 6 h incubation, the relative content of β-sheet was significantly higher and that of random coil was significantly lower in both fluctuating temperature + modification groups than in the constant temperature + modification group. Atomic force microscopic (AFM) observation found that for all three treatment groups, the dimension of triosephosphate isomerase was smaller at 12 h than at 0 h, but no difference was observed among the three treatment groups. In conclusion, the increase in phosphorylation and nitrosylation levels slowed down the decline in the activity of triosephosphate isomerase in vitro at 4 ℃, and amplitude or frequency fluctuations of incubation temperature affected the regulatory effect of phosphorylation and nitrosylation on triosephosphate isomerase activity, leading to increased activity in the early stages of incubation and inhibiting the structural transition from order to disorder.

This study investigated the integrated atlas of protein phosphorylation, acetylation, ubiquitination and S-nitrosylation in pre-rigor (0.5 h) and post-rigor (5 days) lamb to interpret the roles of protein post-translational modifications (PTMs) during meat aging. The results showed that the most common PTMs in postmortem meat were phosphorylated proteins. PTMs regulation during the postmortem period altered proteins involved in metabolic pathways and muscle contraction. The phosphorylation and ubiquitination of proteins located in mitochondria and nuclear changed significantly between 0.5 h and 5 days postmortem. Gene set enrichment analysis revealed possible roles of total PTMs proteins, with a general downregulation of phosphorylation, acetylation and ubiquitination. There was a weak correlation in the lysine PTM of acetylation and ubiquitination at the same site in postmortem meat. Multiple PTMs of proteins in glycolysis, TCA cycle, muscle contraction and the calcium signaling pathway cooperatively regulate meat quality development from pre-rigor to post-rigor.

The pH value, glucose content, glycolytic potential, and expression levels of five glycolytic enzymes in lamb longissimus dorsi muscle at different time after slaughter were compared and analyzed, which treated at different cooling rates and stored at different temperatures. The impact of two steps in the process of ultra-fast chilling on the expression levels of glycolytic enzymes was determined. The regulatory mechanism of ultra-fast chilling on the glycolytic rate was elucidated from the perspective of protein expression. The results showed that ultra-fast chilling treatment significantly delayed the decrease of pH and the increase of glycolytic rate, promoted the expression levels of aldolase (ALDOA), glycogen phosphorylase (PYGM), and triosephosphate isomerase (TPI1), and inhibited the expression levels of phosphofructokinase (PFKM) and phosphoglycerate kinase (PGK). After cooling treatment, refrigeration and controlled freezing-point storage delayed glycolysis by inhibiting the expression level of PGK, without altering the effect of ultra-fast chilling. The expression level of PFKM was positively correlated with the rate of glycolysis at different temperatures. It was found that different glycolytic enzymes had different responses to temperature changes. Ultra-fast chilling affected energy supply and demand by changing the expression of enzymes involved in glycolysis. The high expression level of PFKM was associated with fast glycolysis. PFKM can be regarded as a key enzyme in the ultra-fast chilling process.

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.