自然连接

笛卡儿积

等值连接

θ连接

Acetoacetyl-CoA

HMG-CoA

Mevalonate

Squalene

Steroid ring

A reaction with positive free energy will proceed in the forward direction if the substrate concentration is raised high enough.

Under standard conditions, a reaction will proceed in the forward direction if the free energy Δ G0 ′ is positive.

The direction of a reaction is independent of the initial substrate and product concentrations because the direction is determined by the change in free energy.

The concentration of all of the substrates must be higher than that of all of the products for the reaction to proceed in the forward direction.

The enzyme for the reaction must be working at >50% of its maximum efficiency for the reaction to proceed in the forward direction.

自然连接

笛卡 尔 积

等值连接

θ连接

bone marrow

liver

kidney

myocardium

skeletal muscle

正确

错误

The pathway consists of two major parts, binding and initiation, each with multiple steps. First, the polymerase, directed by its bound σ factor, binds to the promoter. A closed complex (in which the bound DNA is intact) and an open complex (in which the bound DNA is intact but partially unwound near the −10 sequence) form in succession.

Second, transcription is initiated within the complex, leading to a conformational change that converts the complex to the elongation form, followed by movement of the transcription complex away from the promoter (promoter clearance).

Any of these steps can be affected by the specific makeup of the promoter sequences. The σ subunit dissociates stochastically (at random) as the polymerase enters the elongation phase of transcription. The protein NusA (Mr 54,430) binds to the elongating RNA polymerase, competitively with the σ subunit.

Once transcription is complete, NusA dissociates from the enzyme, the RNA polymerase dissociates from the DNA, and a σ factor (σ 70 or another) can again bind to the enzyme to initiate transcription.