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Trinity 1

ND3 + tRNA Met + tRNA Phe

ND3:

Complex: NADH:ubiquinone oxidoreductase (Complex I)

NADH:ubiquinone oxidoreductase

NADH → electron donor

Nicotinamide Adenine Dinucleotide (reduced form)

NAD⁺ → oxidized form

NADH → reduced form (electron-rich)

NADH is composed of two linked nucleotides:

Adenine nucleotide

Nicotinamide nucleotide (the active site)

The nicotinamide ring:

Accepts 2 electrons

Accepts 1 proton

Stores them as a hydride (H⁻)

NADH is generated during catabolism:

Major Sources

Glycolysis

Pyruvate oxidation

TCA (citric acid) cycle

β-oxidation of fatty acids

Redox Signaling

Ubiquinone (CoQ) → electron acceptor

Oxidoreductase → catalyzes a redox reaction

NADH donates two electrons at once

downstream systems can only handle one at a time

Electron Mediation (Iron–Sulfur Chain)

FMN passes electrons sequentially through Fe-S clusters

Each cluster:

Accepts and donates one electron

A temporal filter that converts bulk energy into ordered flow

Ubiquinone Reduction Site

Ubiquinone binds near the junction of the matrix and membrane arms

Picks up two protons from the matrix

first and largest enzyme complex of the mitochondrial electron transport chain (ETC)

Convert chemical information (NADH) into an electrochemical gradient (proton motive force)

Core Functions

Oxidizes NADH → NAD

Transfers electrons to ubiquinone (CoQ)

the entry point of electrons into oxidative phosphorylation

Complex I is L-shaped, with two major arms:

Matrix (Peripheral) Arm

Extends into the mitochondrial matrix

Responsible for electron transfer

FMN (flavin mononucleotide)

chain of iron–sulfur (Fe-S) clusters

This arm handles information flow (electrons)

Membrane Arm

Responsible for proton pumping

Composed mostly of hydrophobic subunits

This arm handles energy transduction (proton movement)

Step-by-Step Mechanism

NADH Oxidation

NADH binds on the matrix side

Transfers 2 electrons to FMN

regenerates NAD⁺ for glycolysis and the TCA cycle

Electron Relay

Electrons move one at a time through a series of Fe-S clusters

prevents uncontrolled reactions and ROS generation

Ubiquinone Reduction

Electrons reduce CoQ (ubiquinone) → CoQH₂ (ubiquinol)

Occurs at the interface between the two arms

Proton Pumping

4 protons are pumped:

from the matrix

into the intermembrane space

it is mechanochemical, like a wave moving through a structure

marker for mitochondrial phylogeny

ND3 is a membrane-embedded protein

Contains multiple transmembrane α-helices

helps form the proton-translocation architecture of Complex I

ND3 does not directly bind NADH or transfer electrons. Instead, it:

Helps stabilize the membrane arm of Complex I

Encoded by mtDNA

Polyploid (many copies per cell)

Lacking introns

Mutations can show heteroplasmy

ND3 is constitutively expressed

Trinity 2

COX3 + tRNA-Leu(UUR) + tRNA Val

….

Trinity 3

ND4L + tRNA Leu(CUN) + tRNA Ile

….

Trinity 4

COX2 + tRNA Trp + tRNA Ala

….

Trinity 5

ND6 + tRNA Asn + tRNA Cys

….

Trinity 6

ND1 + tRNA Tyr + tRNA-Ser(UCN)

….

Trinity 7

ATP8 + tRNA Ser(AGY) + tRNA Glu

….

Trinity 8

CYB + tRNA Gln + tRNA Pro

….

Trinity 9

ND2 + tRNA His + tRNA Arg

….

Trinity 10

ND5 + tRNA Thr + tRNA Gly

….

Trinity 11

ND4 + tRNA Lys + tRNA Asp

….