New insights into the pathophysiology of methylmalonic acidemia

Abstract

Methylmalonic acidemia (MMA) is a severe inborn error of metabolism that is characterized by pleiotropic metabolic perturbations and multiorgan pathology. Treatment options are limited and non-curative as the underlying causative molecular mechanisms remain unknown. While earlier studies have focused on the potential direct toxicity of metabolites such as methylmalonic and propionic acid as a mechanism to explain disease pathophysiology, new observations have revealed that aberrant acylation, specifically methylmalonylation, is a characteristic feature of MMA. The mitochondrial sirtuin enzyme SIRT5 is capable of recognizing and removing this PTM, however, reduced protein levels of SIRT5 along with other mitochondrial SIRTs 3 and 4 in MMA and potentially reduced function of all three indicates aberrant acylation may require clinical intervention. Therefore, targeting posttranslational modifications may represent a new therapeutic approach to treat MMA and related organic acidemias.

Keywords: MMA; PTM; methylmalonyl-CoA mutase; organic acidemia; sirtuin.

FIGURE 1

Manifestations of methylmalonic acidemia. (A) pictorial representation of the pathophysiology of MMA and the organ/organ systems most affected.

FIGURE 2

Nonenzymatic and Enzymatic Acylation. (A) Nonenzymatic acylation of lysine substrate. In the alkaline environment of the mitochondria, free hydroxide can serve as the proton acceptor to initiate nucleophilic attack of the N^ε amino group of lysine on the reactive thioester of acyl-CoAs. The results are an acylated protein lysine residue, free CoASH, and reformation of hydroxide. (B) Enzymatic acylation of lysine substrate. Rather than free hydroxide, enzymatic acylation is initiated in the active pocket of a lysine acyltransferase or KAT through a catalytic amino acid (in this example it is glutamic acid) that initiates deprotonation. Deprotonation allows for nucleophilic attack of the N^ε amino group of lysine on the reactive thioester of acyl-CoA which is held in proximity via binding to the KAT.

FIGURE 3

SIRTIUN enzymes: compartmentalization and function. (A) Diagram demonstrating substrate preference and cellular compartment localization of each SIRT family member as well as established function of the mitochondrial SIRTs necessary for preservation of metabolic function.(B) SIRT3 regulated metabolic pathways. Green arrows indicate increased protein activity following modification by SIRT3. (C) SIRT4 regulated metabolic pathways. Green arrows indicate increased protein activity and red bars indicate reduced protein activity following modification by SIRT4. (D) SIRT5 regulated metabolic pathways. Green arrows indicate increased protein activity and red bars indicate reduced protein activity following modification by SIRT5.

FIGURE 4

Sirtuin enzymatic deacylation of substrate. (A) Nicotinamide cleavage. Nicotinamide adenine dinucleotide (NAD⁺) is shown in the purple and green pockets of the representative SIRT enzyme. Acylated lysine protein substrate is highlighted in the blue pocket. (B,C) Formation of the alpha-1’-O-alkylamidate intermediate. Nicotinamide is shown in the purple pocket, the alpha-1’-O-alkylamidate intermediate in the blue and green pockets, and the SIRT catalytic histidine in the yellow pocket. (D) Deacylation of lysine. The reaction yields deacylated lysine (blue pocket), nicotinamide (purple pocket), and 2’-O-acetyl-adenosine diphosphate ribose (green pocket).

FIGURE 5

Altered pathway functions in MMA. A summary of alterations seen in the TCA cycle, the urea cycle, and the glycine cleavage cycle in the context of MMA. The red symbol over MMUT is representative of loss of protein function in MMA. Green arrows indicate noted increase in protein abundance or increased metabolite level in MMA. Red arrows indicate decreased protein abundance or metabolite level in MMA. Yellow arrows indicate increased protein function when in the upward direction or decreased protein function when in the downward direction. Dashed line arrows indicate predicted change. Red acyl tags indicate the marked protein exhibits aberrant methylmalonylation, malonylation or both in MMA as found by mass spectrometry analysis in Head et al. Blue lipoyl tags indicate the marked protein exhibits lipoylation PTMs. The red phospho tags (P) indicate phosphorylation events. The orange star indicates the point in TCA cycle where synthetic anaplerotic supplementation could be beneficial to increasing TCA function in MMA.