Analysis of Structure‐Function Relationships in Citrate Lyase Isolated from Rhodopseudomonas gelatinosa as Revealed by Cross‐linking and Immunoelectron Microscopy

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Abstract
Citrate lyase (EC 4.1.3.6) isolated from R. gelatinosa was analyzed using cross-linking experiments and immunoelectron microscopy. Different cross-linking reagents and antibodies directed against citrate lyase and specifically against all 3 subunit types (L, M and S) were applied. A structure-function model is proposed for citrate lyase from R. gelatinosa: the enzyme occurs in 2 configurations, ''rings'' and ''stars''. The ring contains 2 identical layers each consisting of 3 subunits L, with 1 subunit S as a polar cap sitting on each L, and 3 subunits M in alternating sequence (18 subunits altogether). In the star the same 18 subunits are arranged in a different way. The subunits L are located at the periphery and the subunits M are concentrated in the center of the particle. The subunits S are positioned relative to L as in the ring; their location relative to the subunits M is changed. By transition from ring to star, areas on S are brought into contact with areas on M by rotation of structural units; consists of 1 L, 1 M and 1 S subunit/layer against each other, with S of one structural unit close to M of the neighboring structural unit. This transition is assumed to work also in reversed direction. The observation of rings and stars as 2 distinct molecular forms is proposed to reflect the 2 states of citrate lyase: the ring is the form in which substrate is bound by acyl exchange, and the star in which the substrate is consumed by cleavage, i.e., the catalyzed reaction is completed.