Cross references:   Enzyme-linked Receptors   

Enzyme (Wiki)   
Enzymes are proteins that catalyze (i.e., increase the rates of) chemical reactions.[1][2] In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates sufficient for life. Since enzymes are selective for their substrates and speed up only a few reactions from among many possibilities, the set of enzymes made in a cell determines which metabolic pathways occur in that cell.
Most enzyme reaction rates are millions of times faster than those of comparable un-catalyzed reactions. As with all catalysts, enzymes are not consumed by the reactions they catalyze, nor do they alter the equilibrium of these reactions. However, enzymes do differ from most other catalysts in that they are highly specific for their substrates. Enzymes are known to catalyze about 4,000 biochemical reactions.[3]
Enzymes serve a wide variety of functions inside living organisms. They are indispensable for signal transduction and cell regulation, often via kinases and phosphatases.[76] They also generate movement, with myosin hydrolyzing ATP to generate muscle contraction and also moving cargo around the cell as part of the cytoskeleton.[77] Other ATPases in the cell membrane are ion pumps involved in active transport.
Several enzymes can work together in a specific order, creating metabolic pathways. In a metabolic pathway, one enzyme takes the product of another enzyme as a substrate. After the catalytic reaction, the product is then passed on to another enzyme. Sometimes more than one enzyme can catalyze the same reaction in parallel, this can allow more complex regulation: with for example a low constant activity being provided by one enzyme but an inducible high activity from a second enzyme.
My comment
    Below, I've given links and brief discussions of two key terms; "kinase" and "ATP".       

Kinase (Wiki) 
    "In chemistry and biochemistry, a kinase,[1] alternatively known as a phosphotransferase, is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP,[2] to specific substrates. The process is referred to as phosphorylation"  


One of the largest groups of kinases are protein kinases, which act on and modify the activity of specific proteins. Kinases are used extensively to transmit signals and control complex processes in cells. Up to 518 different kinases have been identified in humans.
My comment:
    In the diagram, above, "P" stands for phosphorus, "O" stands for oxygen and "R1", "R2" and "R3" stand for three additional atoms attached to the phosphorus.  In ATP, discussed, below, the Rs are either hydrogen ("H") or another phosphate group.       

ATP (Wiki) 
Adenosinetriphosphate (ATP) is a multifunctional nucleotide used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer.

Here's what it looks like: 

    "Metabolic processes that use ATP as an energy source convert it back into its precursors. ATP is therefore continuously recycled in organisms, with the human body turning over its own weight in ATP each day.

"ATP is used as a substrate in signal transduction pathways by kinases that phosphorylate proteins and lipids, as well as by adenylate cyclase, which uses ATP to produce the second messenger molecule cyclic AMP."
My comments:   
    1.  As you can see, there are three phosphate groups.  Hence the "triphosphate" part of its name.  The remainder of the molecule is adenosine (see below). 
    2.  The third phosphate out from the adenosine is the "high energy phosphate", and the bond linking it to the second phosphate is the "high energy phosphate bond". 
ATP is extremely important to, and perhaps even the foundation of, life.  The Wiki article is quite long and should be consulted for further information. 

Adenosine (Wiki)