Membrane channel proteins are proteins that span the phospholipid bilayer of the cellular membrane. They form a "channel" between the inside and outside of a cell or organelle that can allow particular chemicals to traverse the membrane. Enzymes are a type of protein, so membrane enzyme proteins are enzymes that are attached to a cellular membrane. They perform regular enzyme type functions including the synthesis or breaking down of biomolecules.
A g-protein is a type of protein the can be thought of as a molecular switch. An extracellular (outside the cell) stimulus sends a message through a particular type of membrane channel protein called a g-protein coupled receptor that in turn activates a g-protein within the cell. This g-protein then causes a signalling cascade that has an effect on the cell's function. Direct activation of a g-protein involves a biochemical attaching to the receptor having a direct effect on the g-protein. Indirect activation involves a series of signalling events being required to activiate the g-protein.