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: 8408

#Unit 2. Cellular Organization

Tropomyosin (TM) is a ropelike molecule, about 40 nm in length, binds to 7 actin subunits in an actin filament.

TLS Online TPP Program

#Id: 8409

#Unit 2. Cellular Organization

Troponin, a complex of three polypeptides, 

TN-T (tropomyosin-binding)

TN-I  (inhibitory)

TN-C (Ca2+-binding)

TLS Online TPP Program

#Id: 8410

#Unit 2. Cellular Organization

TN-C is the calcium-binding subunit, controls the

position of TM on the surface of an actin filament through the TN-I and TN-T subunits

TLS Online TPP Program

#Id: 8411

#Unit 2. Cellular Organization

In the absence of Ca2+, muscle is relaxed

TM blocks myosin’s interaction with F-actin

TLS Online TPP Program

#Id: 8412

#Unit 2. Cellular Organization

Binding of Ca2+ ions to TN-C

triggers movement of TM to a new position on the filament, thereby exposing the myosin-binding sites on actin.

TLS Online TPP Program

#Id: 8413

#Unit 2. Cellular Organization

Troponin C is closely related to the ubiquitous 

Ca2+-binding protein calmodulin