Cyclus acidi citrici
Cyclus acidi citrici vel cylcus acidorum triclyclicorum vel cyclus Krebs est iter metabolicum intracellulare et iter paenultimum in mitochondriis, ante phosphorylationem oxydativam enim, energiam cellularem (ATP) generantem.

Cyclus ex pyruvato et, non exigue, aqua generat duo substantias, electrona e- in proximum iter portantes: NADH+H+ et FAD(2H).

Hic cyclus anno 1937 ab Iohanne Adolpho Krebs primo descriptus est; nimirum Krebs anno 1953 praemium Nobelianum physiologiae et medicinae addictum est. Hac de causa cyclus non raro "cyclus Krebs" appellatur. Praeterea nomen "cycli acidorum tricyclicorum" de tribus acidis tricyclicis intra cyclum Krebs (citrato, aconitato, isocitrato) partibus cursus cycli sumitur; denique nomen "cycli acidi citrici" de molecula acidi citrici, non electrice onerati, derivatur, quamquam citratum forma electrice onerata partem cycli ipsius capit.

Cursus 
Aequatio generalis cycli Krebs:
Acetylo-CoA + 3 NAD+ + FAD + GDP + Pi+ 2 H2O → CoA + 3 NADH + 3 H+ + FAD(2H) + GTP + 2 CO2
Nota cycli Krebs ope, nec oxygenium consumatur, nec ATP generetur, sed generentur GTP et CO2. Oxygenium quidem, ut ATP, demum in itinere proximo, phosphorylatione oxydativa, momenta maxima habebit.

Condiciones 
 Substrata pyruvatum generantia
 Aqua (cyclo 2 moleculae)
 Mitochondria sana

Gradus cycli acidi citrici 
 Reactio biochemica 0/10 (Citrati synthasis): Oxaloacetatum + Acetylo-CoA + H2O → Citratum + CoA-SH
 Reactio biochemica 1 (Aconitasis): Citratum + CoA-SH → cis-Aconitatum + H2O
 Reactio biochemica 2 (Aconitasis): cis-Aconitatum + H2O → Isocitratum
 Reactio biochemica 3 (Isocitrati dehydrogenasis): Isocitratum + NAD+ → Oxalosuccinatum + NADH + H+
 Reactio biochemica 4 (Isocitrati dehydrogenasis): Oxalosuccinatum → α-Ketoglutaratum + CO2
 Reactio biochemica 5 (α-Ketoglutarate dehydrogenasis): α-Ketoglutaratum + NAD+ + CoA-SH → Succinylo-CoA + NADH + H+ + CO2
 Reactio biochemica 6 (Succinylo-CoA synthetasis): Succinylo-CoA + GDP + Pi → Succinatum + CoA-SH + GTP
 Reactio biochemica 7 (Succinati dehydrogenasis): Succinatum + Ubiquinonum (Q) → Fumaratum + Ubiquilonum (QH2)
 Reactio biochemica 8 (Fumarasis): Fumaratum + H2O → L-Malatum
 Reactio biochemica 9 (Malati dehydrogenasis) L-Malatum + NAD+ → Oxaloacetatum + NADH + H+
 Reactio biochemica 10/0 (Citrati synthasis): Oxaloacetatum + Acetylo-CoA + H2O → Citratum + CoA-SH

Eventus 
 Electrona (cyclo 3 moleculae NADH+H+, 1 molecula FAD(2H))
 GTP (cyclo 1 molecula)
 CO2 (cyclo 2 moleculae, aeris exspirati)
 Succinylo coenzymum A in parte in synthesem porphyrini defertur.

Notae 

 Acidum fumaricum
 Energia
 Mitochondrium

Nexus externi 
 Khan academy de cycli acidi citrici. 
 Alii cursus cycli Krebs. 

Biochemia
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Itinera metabolica