The Regulating Mechanisms Of Enzymes Co-transport Influenced By The Soil Active Components

Proteins are biological macromolecules,which present in all living organisms and are directly involved in the chemical processes essential for life.As a kind of catalytic protein,soil enzymes play an important role in the biogeochemical cycling of elements.However,due to human activities,some toxic and infectious proteins may also enter the environment by multiple pathways.Proteins have a strong affinity to bind mineral particles and natural organic matter(NOM),their interaction will affect the activity and stability of enzymes and proteins.Numerous studies have focused on biomolecule interaction in solution and their following interfacial behaviors at mineral surfaces.However,the knowledge of how and to what extent of naturally occurring organic and mineral colloids impact the transport of proteins in porous media is largely lacking.Investigations of the dynamic transport processes of protein in porous media at different conditions are required.The major objective of this study was to investigate the effects of soil active components(Humic acid,HA;Goethite)on the transport and fate of protein in saturated porous columns.Here,lysozyme(LSZ)and Acid phosphatase(ACP)were used as the typic protein and soil enzyme,purified Aldrich humic acid(PAHA)was chosen as model HA.By combining dynamic light scattering(DLS),Zeta potential,attenuated total reflectance Fourier transform infrared spectroscopy(ATR-FTIR),also DLVO theory and two-site kinetic retention model,the internal mechanism of HA and/or goethite impact on the transport and activity of enzymes were discussed.The main results are as follows:(1)Co-transport of PAHA decreases LSZ mobility and the activity of retained LSZPAHA decreased LSZ transport under all conditions.The breakthrough curves(BTCs)of LSZ were retarded and the breakthrough plateaus(C_i/C₀)were decreased with increasing PAHA/LSZ mass ratio.Correspondingly,the retention profiles(RPs)exhibited a hyper-exponential shape with greater retention being located near the column inlet when the PAHA/LSZ mass ratio was increased.The results indicated that the�size-selective retention�and concurrent homo-and hetero-aggregation induced straining,causing preferential retention of larger PAHA-LSZ aggregates in the column and elution of small ones.Due to differences in aggregate size,the enzyme activity of LSZ in the effluent was larger than that of retained LSZ and was smaller than that of the influent.(2)Goethite coating enhances the retention of LSZ,PAHA,and PAHA-LSZ complexesThe retention of pure LSZ and PAHA increased with increasing goethite-coated quartz sand(GQS)mass fractions in packed columns,where w=(GQS/(QS+GQS))from0 to 0.5,largely because of increased microscopic roughness and heterogeneity of packed columns.Furthermore,an overall decline of the breakthrough curve plateau was observed when mPAHA/mLSZ was fixed as 0.2 at different w.In these GQS/QS columns,positively charged LSZ broke through earlier because goethite of GQS tended to first capture negatively charged PAHA and PAHA-LSZ complexes/aggregations.At w=0.3,LSZ retention was increased with increasing mPAHA/mLSZ and the retention profiles exhibited a hyper-exponential shape.Due to the preferential retention of overall negatively charged and larger-sized PAHA-LSZ aggregates in the GQS/QS column,the enzyme activity of retained LSZ was lower than that in the effluent and decreased with increasing w.(3)The adsorption behavior,conformational change,and enzymatic activity of enzymes on goethite were affected by pH and initial enzyme concentrationACP has a stronger affinity for goethite;the adsorption rate of ACP and its adsorption capacity on goethite were higher than that of LSZ at different pH.The LSZ concentration and resultant surface coverage have a little impact on the conformation of adsorbed LSZ,but the contact efficiency between LSZ and substrate increases with increasing LSZ loading,and the relative activity of adsorbed LSZ increases significantly.Compared with LSZ,goethite has a more obvious disturbance to the secondary structure of ACP,the changes are related to the initial ACP concentration.At lower initial ACP concentrations,ACP unfolding is more pronounced on goethite surface with some active sites exposed,causing catalytic activity of ACP increases;at higher loading concentration,the adsorbed ACP were compactly arranged,its active site was masked that resulting in reduced enzyme activity.The activity of the immobilized enzyme is an integrated result of the interaction between enzymes and minerals.(4)The transport and enzyme activity are strongly related to the electrically charged characteristics and structural stability of enzymesWhen pH increased from 5 to 8,the transport of LSZ in the GQS/QS column decreased,while the mobility of ACP increased;the largest retention of enzymes observed at pH near the isoelectric point(IEP).The negatively charged PAHA will form larger-sized complexes/aggregations with the positively charged LSZ,which decreased the transport of LSZ,and the inhibitory effect of PAHA on LSZ transport diminished when mPAHA/mLSZ=0.4 at pH 8.In contrast,the transport of negatively charged ACP was strongly promoted by PAHA at both pH 5 and 8,which due to the competition for deposition sites and steric repulsion resulting fromPAHA adsorption onto porous media.LSZ has high structural stability,the enzyme activity of effluent LSZ was still high;however,the structural stability of ACP is lower,its enzyme activity has changed greatly during transport.The co-present of PAHA could protect ACP to some extent and reduce the impact of minerals on ACP activity.Therefore,the results could potentially provide fundamental information for understanding the effects of soil organic matter and minerals on the transport and enzyme activity in natural environments.It is also of great theoretical and practical significance to agricultural management(e.g.,fertilization,irrigation)and the bio-remediation of soil-groundwater contamination.