I'd like to know how antioxidants affect human metabolism and which ones are essential for metabolic processes.
This mainly depends on which kind of antioxidants you're talking about.
Antioxidants keep the oxidative balance of the cell and quench free radicals which are generated by the mitochondrial respiratory chain. Very simplified: Antioxidants are thought to be anti-ageing compounds because free radicals destroy cellular compounds, which can be related to the ageing process.
I would be very careful with products claiming to contain antioxidants and be therefore anti-aging. This whole anti-ageing marketing is far from being scientifically proven.
Anti-oxidants affect human metabolism by altering the redox states of the cell and redox-regulated functions and signaling mechanisms.
The following quotes are from The Redox Stress Hypothesis of Aging (Free Radic Biol Med. Feb 2012)
Many proteins contain cysteine residues which can undergo reversible modifications such as S-nitrosylation and S-glutationylation.
The redox state affects the oxidation levels of these redox-sensitive protein thiols and the consequent disruption in protein activity and the redox-regulated signaling mechanisms.
Under normal conditions the usual types of oxidation modifications of these protein thiols are reversible, and is a way to protect the thiol in transient periods of oxidative stress.
When the redox state shifts to a more oxidised point then more irreversiable protein thiol modifications occurs, i.e. damages the protein.
Protein activity and enzyme catalytic efficiency which affects metabolism, such as sirtuins, are affected by the these thiol redox modifications. (eg SIRT1 is a redox-sensitive deacetylase that is post-translationally modified by oxidants and carbonyl stress and A redox-resistant sirtuin-1 mutant protects against hepatic metabolic and oxidant stress.)
The primary arbiter of redox state is glutathione, i.e. the GSH/GSSG couple. The effect of anti-oxidants appears to vary depending on whether it affects the redox state.
The antioxidant activities of SOD and catalase neutralize the ROS species, O2− and H2O2, without having a direct impact on glutathione redox state. However this frees up glutathione from having to neutralize the ROS species, which it can do so with the glutathione peroxidase enzyme.