Actomyosin ATPase

The ATP in fatigued muscle is, however, well above the K , for actomyosin ATPase activity (Naiminga and Mommaerts, 1960). This indicates that the decrease in force generation is not related to a lack of ATP for crossbridge formation but... 

In the so-called Lymm-Taylor model, the actomyosin ATPase reaction proceeds in a stepwise manner ... 

DEBYE-HUCKEL AW BIOMINERALIZATION ACTOMYOSIN ATPase MYOSIN... 

ACETATE KINASE ACETYL-CoA CARBOXYLASE ACETYL-CoA SYNTHETASE N-ACETYLCLUCOSAMINE KINASE ACTIN ATPase ACTOMYOSIN ATPase N-ACYLMANNOSAMINE KINASE ADENINE NUCLEOTIDE TRANSLOCASE ADENOSINE KINASE ADENYLATE KINASE (MYOKINASE) ADENYLYLSULFATE KINASE d-ALANINE-d-ALANINE LIGASE... 

Orthophosphate as substrate or product, ACETATE KINASE (PYROPHOSPHATE) ACETYL-CoA CARBOXYLASE ACID PHOSPHATASE ACTOMYOSIN ATPase ACYL PHOSPHATASE ASPARTATE-SEMIALDEHYDE DEHYDROGENASE ATPases... 

A combination of the novel flash photolysis technique enabling the rapid release of nucleotides such as ATP from inert, photolabile precursors (Kaplan et al., 1978 McCray et al., 1980 Gurney Lester, 1987) with the high x-ray intensities available from synchrotron sources has introduced the possibility of studying the kinetics of structural events associated with the actomyosin ATPase reactions in muscle fibres. The precursor or caged nucleotides can readily diffuse into skinned muscle fibres, where, in the case of caged-ATP, a pulse of ultraviolet (u.v.) light will photolyse... 

As an attempt to connect the first discussion, which was concerned with diffusion-reaction coupling, with Dr. Williams presentation of enzymes as dynamic systems, I wanted to direct attention to a number of specific systems. These are the energy-transducing proteins that couple scalar chemical reactions to vectorial flow processes. For example, I am thinking of active transport (Na-K ATPase), muscular contraction (actomyosin ATPase), and the light-driven proton pump of the well-known purple... 

Contraction in skeletal and cardiac muscles is triggered by the binding of Ca2+ to the troponin C (TnC) subunit (Fig. 5) of the troponin complex. The other two subunits are troponin I (Tnl), which binds to F-actin and inhibits actomyosin ATPase (for review, see Perry, 1999), and troponin T (TnT), which binds to tropomyosin, linking it to the rest of the troponin... 

Shivering thermogenesis using the actomyosin ATPase system while remaining stationary... 

By using actomyosin ATPase reaction, analysis of untreated rat slow muscle soleus (SOL) revealed predominantly type 1 fibers with a few type 2A and 2B fibers. In contrast, the fast muscle EDL is composed predominantly of type 2 fibers with few type 1 fibers. Total fiber numbers are approximately 1,800 in SOL and 2,500 in EDL (Gupta et al, 1989). 

It has generally been assumed that the energy required for muscular contraction was derived directly from the ATP split in the course of the reaction. The establishment of accurate relationships, however, has been hampered by the fact that the actomyosin ATPase continues to degrade the nucleotide long after contraction has reached an optimum. The availability of thrombosthenin with its much lower ATPase activity might be an interesting material for a comparative investigation of this basically important mechanism. 

Troponin I inhibits the ATPase and superprecipitation of actomyosin in the presence of tropomyosin. The extent of inhibition of actomyosin ATPase was reported to be maximally 80% in the presence of tropomyosin, but 20% in the absence of tropomyosin (Perry et al., 1973 Syska et al., 1976). Inhibition of superprecipitation of actomyosin by troponin I was found only in the presence of tropomyosin (Ebashi et al, 1974). The inhibition of contractile interaction by troponin-tropomyosin at low Ca concentration is due to this inhibitory action of troponin I. The inhibition of contractile interaction by troponin I is neutralized by the addition of stoichiometric amounts of troponin C, regardless of Ca concentration (Perry et al, 1973). 

The discussion above stresses the importance of the CN4 region (residues 96-116) of troponin I in the inhibition of contractile interaction of myosin-actin in the presence of tropomyosin. The amino acid sequence of the CN4 region is shown in Fig. 3. Talbot and Hodges (1981) synthesized 12 peptide analogs of the CN4 sequence and examined their inhibitory action on actomyosin ATPase activity. The absence of residues 115 and 116 did not affect the activity, whereas the absence of residue 114 significantly decreased the inhibitory action. As to the N-terminal portion of the peptide, residues 96-103 were not essential, but the absence of Lys-105 decreased the inhibitory activity. A peptide containing the region of residues Lys-105-Val-l 14 showed about half of the inhibitory action of troponin I. The authors concluded that Lys-105 and the bulky side chain at Val-114 are essential for the inhibitory action of troponin I (Fig. 3). 

Calmodulin has been reported to neutralize the inhibition of actomyosin ATPase by troponin 1 in the presence of Ca (Amphlett et al., 1976 Dedman et al., 1977 Yamamoto, 1983). However, the fact that both phosphorylation of Thr-11 and reactivity of Lys-18 to acetimidates are not affected by calmodulin (Perry, 1980 Moir et al., 1983) suggests that the N-terminal region of troponin I, which has an affinity for troponin C potentiated by Ca ", interacts only weakly with calmodulin. 

---