Nuclear envelope

Double-membrane structure; the outer membrane is continuous with the endoplasmic reticulum and the two membranes are perforated by nuclear pore complexes

TLS Online TPP Program

#Id: 3648

#Unit 3. Fundamental Processes

In ValRS, the side chain of Asp 279 protrudes into the editing active site, where it hydrogen-bonds with the hydroxyl group of threonine. The isosteric (having the same shape) valine lacks this hydroxyl group and is thereby excluded from the editing pocket.

TLS Online TPP Program

#Id: 3649

#Unit 3. Fundamental Processes

Valine cannot coordinate the Zn2+ in this way and hence does not undergo adenylation by ThrRS. A separate editing site deal with misacylated Ser–tRNAThr.

TLS Online TPP Program

#Id: 3650

#Unit 3. Fundamental Processes

TyrRS distinguishes between tyrosine and phenylalanine through hydrogen bonding with tyrosine’s OH group. Since no other amino acid resembles tyrosine, the enzyme can do without an editing function.

TLS Online TPP Program

#Id: 3651

#Unit 3. Fundamental Processes

GlnRS is absent in gram-positive bacteria, archaebacteria, cyanobacteria, mitochondria, and chloroplasts.

TLS Online TPP Program

#Id: 3652

#Unit 3. Fundamental Processes

In many bacteria glutamate is linked to tRNAGln by the same GluRS that synthesizes Glu–tRNAGlu. The resulting Glu–tRNAGln is then converted to Gln–tRNAGln by Glu–tRNAGln amidotransferase in an ATP-requiring reaction in which glutamine is the amide donor.

TLS Online TPP Program

#Id: 3653

#Unit 3. Fundamental Processes

Some microorganisms that lack AsnRS use a similar transamidation pathway for the synthesis of Asn–tRNAAsn from Asp–tRNAAsn.