**Xinhong Liang, Wanli Zhang, Junjian Ran, Junliang Sun \*, Lingxia Jiao, Longfei Feng and Benguo Liu**

School of Food Science, Henan Institute of Science and Technology, Xinxiang 453003, China; liangxinhong2005@163.com (X.L.); zwl6996468@126.com (W.Z.); ranjunjian@126.com (J.R.); jiaolingxia@163.com (L.J.); flf18738380903\_007@163.com (L.F.); zzgclbg@126.com (B.L.) **\*** Correspondence: sjl@hist.edu.cn; Tel.: +86-135-0373-8786

Academic Editor: Stefano Serra Received: 19 September 2018; Accepted: 23 October 2018; Published: 24 October 2018

**Abstract:** The sweet potato β-amylase (SPA) was modified by 6 types of methoxy polyethylene glycol to enhance its specific activity and thermal stability. The aims of the study were to select the optimum modifier, optimize the modification parameters, and further investigate the characterization of the modified SPA. The results showed that methoxy polyethylene glycol maleimide (molecular weight 5000, Mal-mPEG5000) was the optimum modifier of SPA; Under the optimal modification conditions, the specific activity of Mal-mPEG5000-SPA was 24.06% higher than that of the untreated SPA. Mal-mPEG5000-SPA was monomeric with a molecular weight of about 67 kDa by SDS-PAGE. The characteristics of Mal-mPEG5000-SPA were significantly improved. The Km value, Vmax and *Ea* in Mal-mPEG5000-SPA for sweet potato starch showed that Mal-mPEG5000-SPA had greater affinity for sweet potato starch and higher speed of hydrolysis than SPA. There was no significant difference of the metal ions' effect on Mal-mPEG5000-SPA and SPA.

**Keywords:** sweet potato β-amylase (SPA) 2; methoxy polyethylene glycol maleimide (Mal-mPEG) 3; chemical modification 4; enzymatic characteristics
