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 the absence of SIRT4, the electrochemical potential across the inner mitochondrial membrane:

Quizplus · Accepted Answer

11ecba2d_0ec1_f779_a3bf_3b0251f78707_MD0008_00 The electron transport chain (ETC) pumps protons out of the mitochondrial matrix, creating an electrochemical potential across the inner mitochondrial membrane as protons accumulate in the intermembrane space. These protons flow back into the matrix through ATP synthase, and the process reaches a steady state as the ETC pumps protons out and ATP synthase allows them back in. The consumption of oxygen (O₂), which is the final electron acceptor in the ETC, can be used to monitor the rate of proton pumping. Disruptions in the amount of O₂ consumed by the ETC indicate a change in the steady state and the membrane potential. In Figure 2, researchers measured how the presence (WT) or absence (KO) of SIRT4 altered the consumption of O₂ in a variety of settings. In each case, O₂ was consumed at the same rate in both WT and KO livers, indicating that the ETC functions normally in KO mice. Therefore, KO mouse liver cells pump protons out of the matrix and let them back in at the same rate as WT cells, and no change in the electrochemical gradient across the inner mitochondrial membrane occurs. (Choices A and B) Figure 2 shows that O₂ consumption is not affected by SIRT4, indicating that the ETC is equally efficient in both WT and KO livers. Therefore, protons are being pumped out of the mitochondrial matrix and traveling back in through ATP synthase at the same rate in both cases, and the electrochemical gradient is neither increased nor decreased. (Choice D) Although Figure 2 shows that Cyt-C increases the rate of O₂ consumption, its effect is the same in both WT and KO livers. Educational objective: The electron transport chain pumps protons out of the mitochondrial matrix, and ATP synthase lets them back in. Healthy mitochondria reach a steady state in which proton concentration is higher outside than inside the mitochondrial matrix, producing an electrochemical potential across the membrane. Disrupting the steady state alters the electrochemical potential.

Passage The Sirtuins Are a Class of Enzymes That Regulate Various