This contest is an attempt to promote scientific knowledge among community in a fun and interesting way with the help of molecules.
Some people like to travel around the world and tell others stories about what happens outside of our organisms/cells in other countries.
And with “Identify the molecule” contest you can try to travel inside our own organism/cells and cells of the organisms around us.

For more information about this contest read "Identify the molecule contest. Introduction. Prize: 100 STEM" post.

The prize here is

1. knowledge

facts, information, and skills acquired through experience or education; the theoretical or practical understanding of a subject
[Oxford Dictionary of English, 3rd Edition, Oxford University Press, 2010]

That understanding of our own organism and organisms around us is the real prize in the case of this contest. No matter what happens (whether or not you get 100 STEM), you can win (can acquire the knowledge by investigating different sources of information, remembering and using it to your advantage in the future).

2. 100 STEM

STEMGeeks is a community focused on STEM topics rewarding engagement with STEM tokens. STEMGeeks is powered by the Steem blockchain with it's own distribution model for STEM tokens in addition to the potential STEEM tokens.
STEM stands for Science, Technology, Engineering, Math and includes a wide range of science and technology topics.
source

Consider these tokens as a bonus.

I’ll upvote all the comments (with the guesses).

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Images and text are the clues for you to guess a riddle (the name of the molecule in this case).

Note that

a) even though this contest is called “Identify the molecule”,
-> molecule (singular)
sometimes it’s not just a molecule which is responsible for a specific function, but rather a complex of molecules.
Example:
cytochrome c oxidase (Complex IV)

is a large transmembrane protein complex… It is the last enzyme in the respiratory electron transport chain of cells located in the membrane.
source

b) some molecules/complexes have different names. For example, other names for cytochrome c oxidase (example above) are

Complex IV (mitochondrial electron transport).
Cytochrome a3.
Cytochrome aa3.
Cytochrome oxidase.
Warburg's respiratory enzyme.
source

Any of those names would be a valid entry.

c) Some almost identical molecules are grouped into classes / groups / families.
Example:
Helicases are divided into 6 groups. RecD and Dda molecules belong to helicases group (Superfamily 1 (SF1)). In this case you would need to guess only the name of the group –> Helicase/ Helicases.

d) Some proteins can have different forms – isoforms.
Example:
There’re trypsin 1 and trypsin 2. In this case you would need to guess only the general name – trypsin.

Riddle

This molecule helps us and other animals to get information from the environment, and it is a well-known member of
G protein-coupled receptors (GPCRs) family [1].
It tends to form dimers and larger oligomeric complexes [2].

(Each monomer consists of 2 components (1 small molecule (highlighted in firebrick) and the protein surrounding it (highlighted in silver)). I’ll provide the name for that small molecule in the last clue for this riddle. Cartoon representation. The image was created by me with the help of PyMol - open source tool for molecular visualization/exploration. PDB-file was obtained from PDB)

(Each monomer consists of 2 components (1 small molecule (highlighted in firebrick) and the protein surrounding it (highlighted in silver)). I’ll provide the name for that small molecule in the last clue for this riddle. Cartoon representation. The image was created by me with the help of PyMol - open source tool for molecular visualization/exploration. PDB-file was obtained from PDB)

Once this molecule was “The molecule of the month” on PDB (Protein Data Bank).

The RCSB PDB Molecule of the Month by David S. Goodsell (RCSB PDB-Rutgers and The Scripps Research Institute) presents short accounts on selected molecules from the Protein Data Bank...
This feature provides an easy introduction to the RCSB PDB for all types of users, but especially for teachers and students.
source

"Edited" (09.09.2019)

Here’s the data showing where (in what human tissues) the gene of this molecule is expressed the most and the least:

(in other words, those are the places where the molecule is present in abundance and places where the amount of this molecule is the lowest)

The data were provided with the help of Bgee and BioGPS

Bgee

is a database to retrieve and compare gene expression patterns in multiple animal species, produced from multiple data types (RNA-Seq, Affymetrix, in situ hybridization, and EST data)
source

BioGPS

A free extensible and customizable gene annotation portal, a complete resource for learning about gene and protein function.
source

"Edited" (11.09.2019)

It’s a pigment [3].
One of two components of this molecule (marked with blue question mark on that image above) belongs to Vitamin A group of compounds [4].

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Rules

This contest lasts for 6 days.

Some of the clues have been already provided in this post.

Additional clues will be given in 2 days (on Monday, September 9, 2019, 20:00 ± 2 minutes UTC)(I’ll update this post).

And the rest of the clues will be provided in 2 more days (on Wednesday, September 11, 2019, 20:00 ± 2 minutes UTC)(I’ll update this post).

The reason is that some people might need less information to make conclusions that the others.

Additional clues will be added at the end of the "Riddle" section before the "Rules" section with "Edited" label.

Leave your guess (1 guess only) in the comments section of this post.

Don’t edit your comment (otherwise your entry will be invalid).

The first user with correct answer is the winner.

The winner will be announced on Friday, September 13, 2019, 20:00 ± 2 minutes UTC.

I’ll transfer 100 STEM to the winner on that day (September 13) and add the transaction ID in this post (I’ll update it).

If there's no correct answers provided by the users, then 100 STEM will be distributed equally among all participants with guesses.

I’m planning to run this contest every week.
So, you should expect to see the 3nd task/riddle on Saturday, September 14, 2019, 20:00 ± 2 minutes UTC.

-> https://time.is/UTC

(UTC / Universal Time Coordinated / Coordinated Universal Time / Universal Time / Greenwich Mean Time)

Useful resources to participate in this contest

1/ National Center for Biotechnology Information

2/ PubMed

PubMed comprises more than 30 million citations for biomedical literature from MEDLINE, life science journals, and online books.
source

3/ Wikipedia

4/ PDB-101

PDB-101 is an online portal for teachers, students, and the general public to promote exploration in the world of proteins and nucleic acids.
source

5/ UniProt
UniProt (Universal Protein Resource) is one of the central places for us to get proteins sequences and information about them.
For more information go to https://www.uniprot.org/help/about

6/ PDB (Protein Data Bank)

a global resource for the advancement of research and education in biology and medicine.
source
It's the central resource for 3-D structures of proteins/DNAs/RNAs and associated functional information.

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I’ll discuss molecules/complexes proposed here in detail (with explanation of the riddle) in the ""Identify the molecule" contest #1 [[The name of the molecule]]. Discussion. Winner announcement" post which I'll publish after announcing the winner.

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References

I’ll add all the references for this post later, because most of them contain the name of the molecule.
That would be a spoiler.
I’ll add them in 6 days.

"Edited"

References

1. Palczewski K. G protein-coupled receptor rhodopsin. Annu Rev Biochem. 2006;75:743–767. doi:10.1146/annurev.biochem.75.103004.142743

2. Fotiadis D, Liang Y, Filipek S, Saperstein DA, Engel A, Palczewski K. The G protein-coupled receptor rhodopsin in the native membrane. FEBS Lett. 2004;564(3):281–288. doi:10.1016/S0014-5793(04)00194-2

3. Rhodopsin on Wikipedia

4. Retinal on Wikipedia

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Important
This contest is the work in progress.
This is just the beginning.
A lot can be changed in the future.
Any recommendations/suggestions are very welcome.

All images (without the license specified) are used under the doctrine known in USA as "Fair Use" (similar doctrines are used in other countries). For more information visit the US Gov website