Figure 3

Ratios of maximum enzyme activities. Citrate synthase (CS), as the central hub in metabolism, connecting carbohydrates, lipids, and proteins, is used as denominator to decipher shifts in the use of metabolic fuels. Cytochrome-c-oxidase (COX) is a marker for aerobic capacity, pyruvate kinase (PK) and lactate dehydrogenase (LDH) are used as indicators of anaerobic ATP generation, 3-hydroxyacyl-CoA dehydrogenase (HADH) is essential for fatty acid metabolic processes, and glutamate dehydrogenase (GDH) represents a key link between catabolic and anabolic protein metabolism. A positive slope indicates a relatively increasing contribution of the respective enzyme (enzyme in the numerator position) to energy metabolism along with warming. Oppositely, a negative slope indicates a decreasing contribution of the enzyme in the numerator position to metabolic processes. For a comparison of fatty acid-based metabolism and glycolysis, HADH activity was related to PK and LDH activity. Measurements of enzyme activities in the tissue extracts of E. verrucosus, E. cyaneus, and G. lacustris were performed at the respective sampling temperatures. Individuals of all species were exposed to a gradual temperature increase (0.8 °C d⁻¹). Slopes and their 95% confidence intervals are indicated when simple linear regression analysis revealed that slopes were significantly different from zero (p < 0.05). Dots represent ratios of individual animals (one data point, i.e., HADH/PK ratio with the value of 6 in the data set G. lacustris—6 °C, is not depicted to avoid reducing the resolution of the panels but was not removed from any analysis); n = 4–7.