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.
-Based on the passage, the SIRT4 protein most likely contains which of the following?

Quizplus · Accepted Answer

11ecba2d_0ebe_c326_a3bf_d5941bd4e880_MD0008_00 In eukaryotes, different metabolic processes are carried out in different compartments of the cell. For example, glycolysis and the pentose phosphate pathway occur in the cytosol, whereas the citric acid cycle and fatty acid oxidation occur in the mitochondria. The enzymes required to facilitate each process must be localized to the appropriate compartments. Transportation of proteins synthesized in the cytosol is prompted by a short amino acid sequence at the N-terminus, which serves as a signal for cellular machinery to move the protein to its correct destination. The pyruvate dehydrogenase complex (PDHC) catalyzes the decarboxylation of pyruvate to form acetyl-CoA, which then enters the citric acid cycle. This process occurs in the mitochondrial matrix, so the PDHC is found in this organelle. According to the passage, SIRT4 colocalizes with the PDHC and must also be found in the mitochondrial matrix. Therefore, SIRT4 most likely has a mitochondrial targeting signal to induce its transport to the mitochondria. (Choice A) Nuclear localization sequences signal for the transport of proteins such as transcription factors to the nucleus. According to the passage, SIRT4 colocalizes with the PDHC, which is found in the mitochondria. (Choice B) Polyubiquitin tags target defective or unnecessary proteins for destruction by the proteasome. SIRT4 is used routinely in the cell, so it would have a polyubiquitin tag only if only it were defective. (Choice C) N-linked carbohydrate chains are added to certain asparagine residues of proteins in the endoplasmic reticulum. Mitochondrial proteins such as SIRT4 are translated completely in the cytosol and are not modified in the endoplasmic reticulum. Educational objective: In eukaryotes, different metabolic pathways occur in different subcellular compartments. The enzymes required for each pathway must move to the appropriate compartment, and N-terminal peptide sequences typically direct them to their correct destination. Proteins that colocalize with one another are found in the same compartment.

Passage The Sirtuins Are a Class of Enzymes That Regulate Various