乳糖操纵子

一、简述乳糖操纵子的结构和诱导机制（英文）-（大题）

Functional and regulatory components of the lac operon(作用)

Lac R = Regulatory gene，that encodes for the lac Repressor protein that is concerned with regulating the synthesis of the structural genes in the operon. Lac R is adjacent to the Promoter site of the operon. The lac repressor is inactivated by lactose, and is active in the absence of lactose.

O = Operator，specific nucleotide sequence on DNA to which an active Repressor binds.

P = Promoter，specific nucleotide sequence on DNA to which RNA polymerase binds to initiate transcription. If the Repressor protein binds to the operator, RNAp is prevented from binding with the promoter and initiating transcription. Under these conditions the enzymes concerned with lactose utilization are not synthesized. Structual gene Lac Z, Y and A = Structural Genes in the lac operon. Lac Z encodes for Beta-galactosidase; Lac Y encodes the lactose permease; Lac A encodes a transacetylase.（lac = lactose），the inducer molecule. When lactose binds to the Repressor protein, the Repressor is inactivated; the operon is derepressed; the transcription of the genes for lactose utilization occurs.

Response to lactose（作用机制）

①Lack of inducer: the lac repressor binds to operator and blocks all. This prevents binding RNAp to promoter subsequent transcription of lac genes but a very low level of trans-cription of lacZYA .

②Lactose is present, the low basal level of permease allows its uptake, andβ-galactosidase catalyzes the conversion of some lactose to allolactose.

③Allolactose acts as an inducer, binding to the lac repressor and inactivate it. RNAp initiates transcription of lac structual genes.

二、Microbes are preferred to plants and animals as sources of enzymes because：（英文）1）they are generally cheaper to produce.

2）their enzyme contents are more predictable and controllable,

3）plant and animal tissues contain more potentially harmful materials than microbes, including phenolic compounds (from plants), endogenous enzyme inhibitors and proteases.

三、固定化酶的优点（Advantages of Immobilized Enzymes）（英文）

Immobilised enzymes are very important for commercial uses as they possess many benefits which include:

①Convenience: Minuscule amounts of protein dissolve in the reaction, so workup can be much easier. Upon

completion, reaction mixtures typically contain only solvent and reaction products.

②Economical: The immobilized enzyme is easily removed from the reaction making it easy to recycle the biocatalyst.

③Stability: Immobilized enzymes typically have greater thermal（热的）and operational stability than the soluble form of the enzyme.