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  1. Lac Repressor

    Linked via "tetramer"

    Structure and Composition
    The Lac Repressor is synthesized as a polypeptide of 360 amino acid residues. It predominantly functions as a tetramer in its fully active state, although the biologically relevant unit often observed interacting with DNA is the homodimer, formed by the association of two monomers [2]. The monomer structure is characterized by distinct functional domains:
    N-terminal DNA-Binding Domain ($\text{DBD}$): Res…

  2. Lac Repressor

    Linked via "tetramer"

    N-terminal DNA-Binding Domain ($\text{DBD}$): Residues $\sim 1-60$. This domain houses a canonical Helix-Turn-Helix ($\text{HTH}$) motif, allowing sequence-specific recognition of the operator DNA. The $\text{HTH}$ motif in Lac Repressor is notable for its affinity for regions of slightly altered helix periodicity, which some structural biologists attribute to its preference for the "bent" configuration of the lac operator [3].
    **Allosteric Core Dom…

  3. Lac Repressor

    Linked via "tetramer"

    The Lac Repressor exhibits extremely high specificity for the lac operator sequence, located immediately downstream of the transcription start site. This sequence, $\text{O}1$, is a quasi-palindromic stretch of 21 base pairs. The binding affinity is drastically increased when the repressor simultaneously engages the two secondary operator sites, $\text{O}2$ (downstream) and $\text{O}_3$ (upstream), forming a structured DNA loop.
    The [dissociation constan…

  4. Lac Repressor

    Linked via "tetramer"

    The inducer binds specifically to pockets located within the allosteric core domain. Binding is cooperative, meaning the binding of one inducer molecule to one subunit accelerates the binding of the second inducer molecule to the other subunit within the dimer.
    The structural change induced by ligand binding involves a rotation of the $\text{DBD}$ relative to the tetramer cor…

  5. Nitrogenase

    Linked via "tetramer"

    The MoFe Protein (Dinitrogenase)
    The MoFe protein is larger, typically a tetramer ($\alpha2\beta2$), with a combined molecular mass around $240 \text{ kDa}$. It houses the two critical metal centers necessary for $\text{N}2$ binding and reduction: the FeMo-cofactor ($\text{FeMoCo}$) and the unique $\text{Fe}-\text{S}$ cluster, sometimes designated the $\text{Fe}8$ cluster in comparative studies.
    The FeMo-Cofactor ($\text{FeMoCo}$)