Calmodulin, binding target enzymes/proteins

Calmodulin Nonspecific But Very Tight Binding of Numerous Target Enzymes/Proteins Through Unprecedentedly Large Conformational Change... 

Calcium can also directly affect signaling through another pathway. The major calcium binding protein in the cell is calmodulin (CAM). CAM is not an enzyme, but it will activate some enzymes when it binds to them. CAM binds to its target enzymes only when calcium is bound... 

The calcium mediated contraction of smooth muscle, which unlike striated muscle does not contain troponin, is quite different and requires a particular calcium-binding protein called calmodulin. Calmodulin (CM) is a widely distributed regulatory protein able to bind, with high affinity, four Ca2+ per protein molecule. The calcium—calmodulin (CaCM) complex associates with, and activates, regulatory proteins, usually enzymes, in many different cell types in smooth muscle the target regulatory proteins are caldesmon (CDM) and the enzyme myosin light chain kinase (MLCK). As described below, CaCM impacts on both actin and myosin filaments. 

An example for the reversible association of activator proteins with an enzyme is the Ca -calmodulin dependent enzymes. Calmodulin is a Ca -binding protein which can activate target enzymes, e.g. phosphorylase kinase (see 6.7.1 and 7.4) in its Ca -boimd form. Another example for activating proteins is the cyclins (see chapter 14). The cyc-lins are activators of protein kinases that regulate the cell cycle. 

Secondly, a calcium binding protein could function as a calcium dependent regulatory protein. A typical example is calmodulin (for a review see Ref. 17) at resting [Ca2+]j, this protein is inactive but during activation of the calcium signal the protein binds calcium and activates several target enzymes and proteins. 

Inositol trisphosphate opens a calcium transport channel in the membrane of the endoplasmic reticulum. This leads to an influx of calcium from storage in the endoplasmic reticulum and a 10-fold increase in the cytosolic concentration of calcium ions. Calmodulin is a small calcium binding protein found in all cells. Its affinity for calcium is such that, at the resting concentration of calcium in the cytosol (of the order of 0.1 /xmol per L), little or none is bound to calmodulin. When the cytosolic concentration of calcium rises to about 1 /xmol per L, as occurs in response to opening of the endoplasmic reticulum calcium transport channel, calmodulin binds 4 mol of calcium per mol of protein. When this occurs, calmodulin undergoes a conformational change, and calcium-calmodulin binds to, and activates, cytosolic protein kinases, which in turn phosphorylate target enzymes. 

As was pointed out above, inside the cell Ca levels are not usually transmitted directly to targets, but are first processed by sensor proteins. Many of them, like calmodulin (described later), bind Ca to characteristic EF-hand motifs undergoing in the process a conformational change, which is a prerequisite for their subsequent interaction with their respective target enzymes. However, some of these Ca sensors, like protein kinase C, calpains, and calcineurin, are themselves Ca -sensitive enzymes, while others like gelsolin and the annexins do not contain EF hands. 

FIGURE 24.12 The PIP2 second-messenger scheme. When a hormone binds to a receptor, it activates phospholipase C, in a process mediated by a G protein. Phospholipase C hydrolyzes PIPj to IP3 and DAG. IP3 stimulates the release of Ca from intracellular reservoirs in the ER. A complex formed between Ca and the calcium-binding protein calmodulin activates a cytosolic protein kinase for phosphorylation of a target enzyme. DAG remains bound to the plasma membrane, where it activates the membrane-bound protein kinase C (PKC). PKC is involved in the phosphorylation-channel proteins that control the flow of Ca + in and out of the cell. Ca from extracellular sources can produce sustained responses even when the supply of Ca + in intracellular reservoirs is exhausted. 

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