Protein charge ladders reveal that the net charge of ALS‐linked superoxide dismutase can be different in sign and magnitude from predicted values

• Published in: Protein science Vol. 23; no. 10; pp. 1417 - 1433
• Main Authors: Shi, Yunhua, Abdolvahabi, Alireza, Shaw, Bryan F.
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.10.2014; BlackWell Publishing Ltd
• Subjects: Acetylation; Amino acids; amyloid; Amyotrophic lateral sclerosis; Amyotrophic Lateral Sclerosis - genetics; Binding; Calorimetry; Capillary electrophoresis; Catalytic Domain; Cu,Zn superoxide dismutase; Deuterium; Deuterium Exchange Measurement; Differential scanning calorimetry; Dimers; Electrophoresis; Electrophoresis, Capillary; Histidine - metabolism; Humans; Hydrogen-Ion Concentration; Inductively coupled plasma mass spectrometry; Ladders; Lysine; Mass spectrometry; Mass spectroscopy; Metal ions; Metals; Missense mutation; motor neuron disease; Mutation; Mutation, Missense; Organic chemistry; Oxidation; pH effects; Phenotypes; protein aggregation; Protein Folding; Protein Stability; Proteins; SOD1; Stoichiometry; Structural stability; Superoxide dismutase; Superoxide Dismutase - chemistry; Superoxide Dismutase - genetics; Superoxide Dismutase - metabolism; Superoxide Dismutase-1; Zinc; Zinc - chemistry; Zinc - metabolism; motor neuron disease; amyloid; amyotrophic lateral sclerosis; protein aggregation; Cu,Zn superoxide dismutase; SOD1
• ISSN: 0961-8368; 1469-896X
• DOI: 10.1002/pro.2526

This article utilized “protein charge ladders”—chemical derivatives of proteins with similar structure, but systematically altered net charge—to quantify how missense mutations that cause amyotrophic lateral sclerosis (ALS) affect the net negative charge (Z) of superoxide dismutase‐1 (SOD1) as a function of subcellular pH and Zn2+ stoichiometry. Capillary electrophoresis revealed that the net charge of ALS‐variant SOD1 can be different in sign and in magnitude—by up to 7.4 units per dimer at lysosomal pH—than values predicted from standard pKa values of amino acids and formal oxidation states of metal ions. At pH 7.4, the G85R, D90A, and G93R substitutions diminished the net negative charge of dimeric SOD1 by up to +2.29 units more than predicted; E100K lowered net charge by less than predicted. The binding of a single Zn2+ to mutant SOD1 lowered its net charge by an additional +2.33 ± 0.01 to +3.18 ± 0.02 units, however, each protein regulated net charge when binding a second, third, or fourth Zn2+ (ΔZ < 0.44 ± 0.07 per additional Zn2+). Both metalated and apo‐SOD1 regulated net charge across subcellular pH, without inverting from negative to positive at the theoretical pI. Differential scanning calorimetry, hydrogen‐deuterium exchange, and inductively coupled plasma mass spectrometry confirmed that the structure, stability, and metal content of mutant proteins were not significantly affected by lysine acetylation. Measured values of net charge should be used when correlating the biophysical properties of a specific ALS‐variant SOD1 protein with its observed aggregation propensity or clinical phenotype.