Ribozimas.

Las ribozimas cabeza de martillo son pequeños RNAs catalíticos capaces de hidrolizar su propio esqueleto originando dos moléculas de RNA. Todas contienen 3 dominios bicatenarios y un núcleo de residuos muy conservado necesario para llevar a cabo la hidrólisis.

Esta reacción de ruptura implica un ataque del oxígeno del 2'-OH de la citosina del centro catalítico sobre el átomo de fósforo unido al C3' del mismo residuo, que rompe el esqueleto de azúcar-fosfato y produce un 2', 3' fosfato cíclico. Al igual que en las proteínas ribonucleases, presentan un ion metálico unido al centro activo (Mg2+) que estabiliza la forma ionizada del oxígeno en el 2'-OH, lo que promueve su ataque catalítico.
La estructura en forma de horquilla de la izquierda es una ribozima en la que parte del RNA ha sido una sustituido con un segmento de DNA que actúa como un inhibidor de la catálisis, debido a que carece del 2'-OH.

La ribozima corta un enlace fosfodiéster de este sustrato cuando está formado por ribonucleótidos (es un RNA); en este modelo dicho sustrato se ha sustituido por un DNA monocatenario, que, aunque se une a la ribozima, no es hidrolizado y así permite estudiar la estructura del complejo ribozima-sustrato.

Estructura de la ribozima cabeza de martillo de ARN-ADN

Stem I is formed by base pairing of complementary nucleotides through Watson-Crick hydrogen bonding. The 5' phosphate and 3' oxygen atoms mark the beginning and end of the ribozyme and are juxtaposed at the top of Stem I.

Stem II lies opposite Stem I in the other branch of the "wishbone." It also is formed by standard Watson-Crick base pairing and is capped by a four base loop at the top of the branch.

Stem III is formed by Watson-Crick base pairing and forms the base of the "wishbone." In a standard ribozyme, formed by a continuous RNA molecule instead of an RNA-DNA, two-chain hybrid, the nucleotides at the base of Stem III are connected by a short loop (see below).

The base sequences of the three stems can vary, depending on the hammerhead ribozyme under consideration. The only sequence restriction is that Watson-Crick base complementarity is maintained so that stem hydrogen bonding can form. The central core sequence, however, is highly conserved between ribozymes and is essential for ribozyme catalytic activity. The core contains two domains.

of the core comprises a CUGA sequence, the uridine turn, that follows Stem I.

Domain 2 is formed by non-Watson Crick base pairing, and connects Stem II and Stem III.

C17 is the conserved residue at which strand cleavage takes place.

Structure of an All-RNA Hammerhead Ribozyme.

Scott, et al. (1995) succeeded in determining the structure of a hammerhead ribozyme that comprises an RNA enzyme strand complexed with an RNA substrate strand, shown at left.

Domain 2 is formed by non-Watson Crick base pairing, and connects Stem II and Stem III.

C17 is the conserved residue at which strand cleavage takes place.


Créditos

Pley, H. W., Flaherty, K. M., McKay, D. B.: Three-dimensional structure of a hammerhead ribozyme. Nature 372: 68-74 (1994).

Scott, W.G., Finch, J.T., Klug, A.: The Crystal Structure of an All-RNA Hammerhaead Ribozyme: A Proposed Mechanism for RNA Catalytic Cleavage. Cell 81: 991-1002 (1995).

Dr. José Antonio Encinar. (IBMC-UMH)