Passage The sirtuins are a class of enzymes that regulate various metabolic processes by removing post-translational modifications from enzymes. SIRT4 is an autosomally encoded sirtuin protein that colocalizes with the pyruvate dehydrogenase complex. It deacylates lysine residues on certain metabolic proteins by removing methylglutaryl and hydroxymethylglutaryl units. Methylglutaryl-CoA and hydroxymethylglutaryl-CoA are intermediates in leucine catabolism, and scientists hypothesized that SIRT4 regulates this pathway. To test their hypothesis, livers from wild-type (WT) and SIRT4 knockout (KO) mice were treated with α-keto acid derivatives of several amino acids, including the derivative of leucine, α-ketoisocaproate (αKIC). Figures 1 and 2 show the rate of oxidation of α-keto acids and oxygen consumption in both WT and KO livers, respectively. Figure 1 Box plot showing the rate of oxidation of α-ketoisocaproate (αKIC), α-ketoglutarate (αKG), and pyruvate in SIRT4 knockout livers (Note: Asterisk indicates p

Question

Passage
The sirtuins are a class of enzymes that regulate various metabolic processes by removing post-translational modifications from enzymes. SIRT4 is an autosomally encoded sirtuin protein that colocalizes with the pyruvate dehydrogenase complex. It deacylates lysine residues on certain metabolic proteins by removing methylglutaryl and hydroxymethylglutaryl units. Methylglutaryl-CoA and hydroxymethylglutaryl-CoA are intermediates in leucine catabolism, and scientists hypothesized that SIRT4 regulates this pathway. To test their hypothesis, livers from wild-type (WT) and SIRT4 knockout (KO) mice were treated with α-keto acid derivatives of several amino acids, including the derivative of leucine, α-ketoisocaproate (αKIC) . Figures 1 and 2 show the rate of oxidation of α-keto acids and oxygen consumption in both WT and KO livers, respectively.

Figure 1 Box plot showing the rate of oxidation of α-ketoisocaproate (αKIC) , α-ketoglutarate (αKG) , and pyruvate in SIRT4 knockout livers (Note: Asterisk indicates p < 0.05 relative to WT level)

Figure 2 Oxygen consumption in WT and KO livers in the presence and absence of succinate and reduced cytochrome C (Cyt-C) Because leucine is known to regulate insulin secretion, scientists injected WT and KO mice with leucine and measured plasma insulin levels over time. They then injected mice with insulin and monitored blood glucose levels (Figure 3) .

Figure 3 (A) Plasma insulin levels after injection with leucine and (B) blood glucose levels after injection with insulin in both WT and KO mice
Adapted from Anderson KA, Huynh FK, Fisher-Wellman K, et al. SIRT4 Is a Lysine Deacylase that Controls Leucine Metabolism and Insulin Secretion. Cell Metab. 2017;25(4) :838-855.e15.
-In Figure 2, the most likely reason that researchers added succinate was to stimulate:

Quizplus · Accepted Answer

11ecba2d_0ec4_687b_a3bf_5d35c5d12a6e_MD0008_00 The electron transport chain (ETC) depends on the production of NADH and FADH₂ by the citric acid cycle and other metabolic pathways. Complex II is a flavoprotein (an oxidoreductase that contains an FAD or FMN prosthetic group) that is also known as succinate dehydrogenase. It is part of both the ETC and the citric acid cycle. In Complex II, electrons are transferred from succinate to an FAD molecule that is covalently attached to the protein complex, producing fumarate and FADH₂. The FADH₂ then transfers its electrons to ubiquinone, forming ubiquinol and regenerating FAD. Oxygen is the final electron acceptor in the ETC and is consumed only if sufficient NADH and FADH₂ are present. Increasing the rate of FADH₂ synthesis increases the rate of electron transfer and therefore increases the rate of oxygen consumption. In Figure 2, researchers measured oxygen consumption to assess ETC output under various conditions. They most likely added succinate to stimulate FADH₂ synthesis (and subsequent electron transfer) by Complex II. (Choice A) The addition of succinate can indirectly stimulate NADH synthesis at the downstream step catalyzed by malate dehydrogenase. However, Complex I only consumes NADH and does not synthesize it. (Choice B) Succinyl-CoA synthetase produces GTP only when it consumes succinyl-CoA to produce succinate. Adding succinate would shift the equilibrium to favor the reverse reaction and would not stimulate GTP synthesis. (Choice D) Ubiquinol is synthesized in Complex I and Complex II of the ETC. It is not synthesized by fumarase. Educational objective: The electron transport chain (ETC) depends on the production of NADH and FADH₂ to function. Complex II of the ETC is a flavoprotein (contains FAD) that is also known as succinate dehydrogenase. It is part of both the citric acid cycle and the ETC, and it oxidizes succinate to reduce FAD to FADH₂. FADH₂ is then oxidized back to FAD in the same enzyme, producing ubiquinol.

Passage The Sirtuins Are a Class of Enzymes That Regulate Various