In proteins, Zn ions are often coordinated to the amino acid side chains of aspartic acid, glutamic acid, cysteine and histidine.[149] The theoretical and computational description of this zinc binding in proteins (as well as that of other transition metals) is difficult.[149]
There are 2–4 grams of zinc[150] distributed throughout the human body. Most zinc is in the brain, muscle, bones, kidney, and liver, with the highest concentrations in the prostate and parts of the eye.[151] Semen is particularly rich in zinc, which is a key factor in prostate gland function and reproductive organ growth.[152]
In humans, zinc plays "ubiquitous biological roles".[1] It interacts with "a wide range of organic ligands",[1] and has roles in the metabolism of RNA and DNA, signal transduction, and gene expression. It also regulates apoptosis. A 2006 study estimated that about 10% of human proteins (2800) potentially bind zinc, in addition to hundreds which transport and traffic zinc; a similar in silico study in the plant Arabidopsis thaliana found 2367 zinc-related proteins.[84]
In the brain, zinc is stored in specific synaptic vesicles by glutamatergic neurons[153] and can "modulate brain excitability".[1] It plays a key role in synaptic plasticity and so in learning.[154] However it has been called "the brain's dark horse"[153] since it also can be a neurotoxin, suggesting zinc homeostasis plays a critical role in normal functioning of the brain and central nervous system.[153]