The junction between the adjoining neurons where the axon bulb of one neuron comes

Synapses are known as the junction between the two adjoining neurons. The nerve impulses are passed through this gap with the help of the neurotransmitters. In a synapse, the axon of one neuron is present in proximity with the receptor of another neuron. The synapse between the two adjacent neurons helps them to communicate with each other.

Hence, the blank can be filled with 'synapse'.

The enzyme’s active site binds to the substrate. Since enzymes are proteins, this site is composed of a unique combination of amino acid residues (side chains or R groups). Each amino acid residue can be large or small; weakly acidic or basic; hydrophilic or hydrophobic; and positively-charged, negatively-charged, or neutral. The positions, sequences, structures, and properties of these residues create a very specific chemical environment within the active site. A specific chemical substrate matches this site like a jigsaw puzzle piece and makes the enzyme specific to its substrate.