isoleucyl-tRNA synthetase has a nearby editing pocket (a deep cleft in the enzyme) that allows it to proofread the product of the adenylylation reaction. AMP-valine (as well as adenylylates of other small amino acids, such as alanine) can fit into this editing pocket, where it is hydrolyzed and released as free valine and AMP. In contrast, AMP-isoleucine is too large to enter the editing pocket and is therefore not subject to hydrolysis. As a consequence, isoleucyl-tRNA synthetase discriminates against valine twice: in the initial binding and adenylylation of the amino acid (discriminating by a factor of 100), and then in the editing of the adenylylated amino acid (again discriminating by a factor of 100), for an

overall selectivity of 10,000-fold (i.e., an error rate of 0.01%).

TLS Online TPP Program

#Id: 9726

#Unit 6. System Physiology – Plant

Hemlock (Cicuta sp.) produces cicutoxin, a polyacetylene that prolongs the repolarization of neuronal action potentials.

TLS Online TPP Program

#Id: 9727

#Unit 6. System Physiology – Plant

The active principle in foxglove (Digitalis sp.) is cardinolide digitoxin, a cardiac glycoside that inhibits ATPase activity and can increase myocardial contraction.

TLS Online TPP Program

#Id: 9728

#Unit 6. System Physiology – Plant

Laticifers are made of individual cells and can occur either as articulated systems (single cells connected by a small tube) or non-articulated systems (one large syncytial cell).

TLS Online TPP Program

#Id: 9729

#Unit 6. System Physiology – Plant

Latex are resin terpenoids produced by laticifers is released upon damage and often contains cardiac glycosides which fend off herbivores.

TLS Online TPP Program

#Id: 9730

#Unit 6. System Physiology – Plant

Conifer resin ducts produces bicyclic monoterpenes such as a-pinene and b-pinene, monocyclic terpenes like limonene and terpinolene, and tricyclic sesquiterpenes, including longifolene, caryophyllene, and d-cadinene, as well as resin acids, which are released immediately upon damage by herbivores

TLS Online TPP Program

#Id: 9731

#Unit 6. System Physiology – Plant

The specialist caterpillars of the monarch butterfly

(Danaus plexippus) are insensitive to the terpenoid toxins. They feed on milkweed leaves and retain the cardenolides. As a result, most insectivorous birds quickly learn to avoid eating monarch caterpillars and adult monarch butterflies.