Passage 3: ATP
Regulation of cellular ATP level is critical for diverse biological processes and may be
defective in diseases such as cancer and mitochondrial disorders. While mitochondria
play critical roles in ATP level regulation, we still lack a systematic and quantitative
picture of how individual mitochondrial-related genes contribute to cellular ATP levels
and how dysregulated ATP levels may affect downstream cellular processes. Advances
in genetically encoded ATP biosensors have provided new opportunities for addressing
these issues. ATP biosensors allow researchers to quantify the changes of ATP levels in
real-time at the single-cell level and characterize corresponding effects at the cellular,
tissue, and organismal level.
Mitochondria are best known as the powerhouses of the cell that produce ATP via
oxidative phosphorylation (OXPHOS). Mitochondrial ATP synthesis involves tricarboxylic
acid (TCA) cycle enzymes, electron transport chain complexes, and ATP synthase, in
which acetyl-coenzyme A (CoA) derived from food molecules is oxidized to produce
ATP. During bioenergetic reactions, mitochondria also produce other physiologically
important molecules such as reactive oxygen species.
Two studies presented a new generation of ATP biosensors. In the first, a circularly
permuted green fluorescent protein (cpGFP) was combined with a bacterial ATPbinding protein to generate a reporter that quantitatively reports cellular ATP to
adenosine diphosphate (ADP) ratio. Because of differential binding affinities to ATP
and ADP, the reporter exhibits different degrees of conformational change in cpGFP
and, thus, different fluorescent signal changes in response to ATP or ADP binding.
In the second study, Imamura and colleagues developed a series of highly sensitive
and selective Förster resonance energy transfer (FRET)-based reporters for cellular
ATP concentrations. FRET-based reporters contain a subunit of a bacterial ATP
synthase fused between cyan fluorescent protein (CFP) and yellow fluorescent
protein (YFP). The reversible binding of ATP leads to conformational changes of
the synthase subunit, resulting in altered spatial proximities between fluorescent
proteins and thus changes in the FRET signal.
Dissecting the regulation and function of ATP at the single-cell level. Adapted from
Zhang et al. (2018).
Correct answer: B. Flavoproteins are proteins that aid in transferring
electrons in redox reactions. First, since this is a Roman numeral question, the
Roman number that appears the most will be analyzed first. In this case, I and II
appear the most, so they will be analyzed first. In general, flavoproteins are enzymes
and proteins that have the nucleic acid derivative of riboflavin. In general, they are
often used in oxidation and reduction reactions in biological contexts. FAD is flavin
adenine dinucleotide, which is a type of flavoprotein. Flavoproteins generally have
the phrase “flavo” in their names. Likewise, FMN is a flavin mononucleotide and is
used in ox-redox reactions. Once again, the pattern of the abbreviation starting
with an “F” and the name having flavin is an indicator of a flavoprotein. This makes
statements I and II true. Now, answer choices A and C are eliminated,d and only B
and D remain. NADH itself is not a flavoprotein. However, it is the subject and the
substrate of a flavoprotein. Flavoproteins act on NADH to incorporate it into oxredox reactions. Therefore, NADH itself isn’t a flavoprotein. Therefore, statement III is
not true. Thus, the correct answer choice is B.
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