Arabidopsis β-Amylase2 Is a K⁺-Requiring, Catalytic Tetramer with Sigmoidal Kinetics

Involvement of five catalytically active Arabidopsis β‐amylases in leaf starch metabolism and plant growth

“…The lack of any observable effect of BAM2 and BAM6 on plant growth as compared with B‐Null suggests that they do not contribute significantly to leaf starch degradation at the plant ages examined. BAM2 has highly unusual structural and catalytic properties (Monroe et al, ; Monroe & Storm ) suggesting that it may have a unique role in starch metabolism, and the results reported here suggest that its role does not overlap with the function of BAM3 or BAM1. Activity of BAM2 in assays of B2‐Q extracts was low compared with BAM3 activity in B3‐Q extracts, but they were significantly higher than that of B‐Null (Figure ).…”

“…Assays were conducted at 25°C and included 80 mg/ml soluble starch, which is near the Vmax for BAM2 and saturating for BAM1, ‐3, and ‐5. Assays also included 100 mM KCl, which is required for BAM2 activity (Monroe et al, ). Reducing sugars generated by BAM activity were measured using the Somogyi–Nelson method (Nelson, ).…”

“…Activity of BAM2 in assays of B2‐Q extracts was low compared with BAM3 activity in B3‐Q extracts, but they were significantly higher than that of B‐Null (Figure ). BAM2 activity has only been characterized using the purified enzyme (Monroe et al, ; Monroe & Storm ), so this is the first evidence that the enzyme is present in shoot extracts. In contrast, activity in B6‐Q extracts was not different than that of B‐Null extracts so it is possible that the BAM6 protein is not present in above‐ground parts of plants despite evidence of BAM6 mRNA in leaves (Winter et al, ).…”

“…Assays were conducted at 25°C and included 80 mg/ml soluble starch, which is near the Vmax for BAM2 and saturating for BAM1, -3, and -5. Assays also included 100 mM KCl, which is required for BAM2 activity (Monroe et al, 2017).…”

“…Of the five remaining BAM genes, four ( BAM1 , ‐ 2 , ‐ 3 , and ‐ 5 ) encode enzymes that are known to be catalytically active (Monroe & Preiss ; Monroe et al, ; Fulton et al, ; Lao et al, ; Li et al, ; Sparla, Costa, Schiavo, Pupillo, & Trost, ) and one ( BAM6 ) encodes a protein that is predicted to be active based on its amino acid sequence (Monroe & Storm, ). Of these five BAM proteins, three (BAM1, ‐2, and ‐3) were shown to be located in plastids where they could participate directly in starch degradation (Fulton et al, ; Lao et al, ).…”