68 votes
Accepted

What is the longest-lasting protein in a human body?

Crystallin proteins are found in the eye lens (where their main job is probably to define the refractive index of the medium); they are commonly considered to be non-regenerated. So, your crystallins ...
  • 1,883
51 votes
Accepted

Why are prions in animal diets not destroyed by the digestive system?

Proteases are enzymes in your digestive system that help break down food, acting like molecular-sized scissors that cut up proteins. Proteases have clefts, or subpockets, into which proteins fit, ...
37 votes
Accepted

What is 'protein' in food?

When we say "protein" with respect to food, what is generally meant is material that contains amino acids. Every protein is, at its heart, a long string of amino acids, which then gets ...
  • 6,967
36 votes
Accepted

Do all proteins start with methionine?

You are correct in thinking that since the translation of mRNA begins with AUG, which codes for methionine, then all proteins should contain a methionine at their N-terminus (aka start site). But, it ...
33 votes

Do all humans have an identical nucleotide sequence for certain proteins, e.g haemoglobin?

It is highly unlikely that there exist any protein that is made from completely identical nucleotide sequences across the entire human population. There will certainly be regions within a gene that ...
  • 431
32 votes
Accepted

Why don't membrane proteins move?

Proteins can move around the membrane. Most proteins do move within the membrane. The membrane is a liquid crystal and has fluid behaviour. Specifically, this is due to the membrane being in a gel-...
  • 11.3k
28 votes
Accepted

Why are sushi proteins called "sushi"? What are the origins of this name?

Because their shape reminded researchers of rolls of Sushi (Ichinose et al, 1990): These repeats were initially called GP-I structures because they were first identified in $\beta_2$-glycoprotein I....
  • 6,841
23 votes
Accepted

Do all humans have an identical nucleotide sequence for certain proteins, e.g haemoglobin?

Humans have many variants There is variation. The project I use to help understand this natural variation is gnomAD. Using VarMap and a slightly out of date gnomAD file, I counted 16007805 protein-...
  • 11.3k
22 votes

What is the longest-lasting protein in a human body?

I like Mowgli's answer, because it is a non-obvious example. However I would also point out that there are many, many protein-based structural components in the body that we know do not regenerate due ...
  • 2,919
20 votes
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Why and how does uniprot list around 150,000 proteins in the human genome?

Well you are assuming one sequenced genome/proteome per NCBI tax id. That is no longer true. So if you click on the proteome filter it decreases by half. Which gets you into the 60,000 range. Now not ...
  • 326
20 votes
Accepted

Why is protein turnover necessary or important for cells to function?

Biology is an intricate orchestration of chemical reactions and their products. Generally, this fete is accomplished by enzymatic facilitation of certain reactions that would otherwise occur too ...
  • 39.2k
17 votes

What has caused life to choose this unfathomably tiny subset of all possible proteins?

The answer is chance or, even better, contingency. About your calculations, it is true that the theoretical sequences are almost unlimited, but the basic scaffolds are not. Very different sequences ...
  • 3,058
17 votes
Accepted

What inactivates pepsin in infants?

EDIT: Thanks a lot to @abukaj for pointing out the mistake in my answer (and to @paracetamol for asking such a beautiful question). I am rewriting my answer to incorporate the (hopefully) correct ...
16 votes
Accepted

What physical evidence exists that shows motor proteins "walking" within a cell?

Since you chose kinesins as an example class of motor proteins, I will also stick to them. In general, the movement mechanisms of kinesins are well studied and the general structure of kinesin ...
  • 2,878
15 votes

What is 'protein' in food?

It's a mix of all the proteins in whatever organism the food is coming from. Some (especially vegetable/grain) sources might have fairly specific proteins present because you are eating a specific ...
  • 39.2k
14 votes
Accepted

Do proteins generally contain phosphorus and sulfur?

Of the 22 known proteinogenic amino acids, all contain hydrogen, carbon, nitrogen and oxygen. Some (methionine and cysteine) contain sulfur while one (selenocysteine) contains selenium. None contain ...
  • 17.6k
14 votes
Accepted

What is the difference between a protein and a factor?

TLDR: As far as I know, there's no specific reason some proteins are called "factors"; it's just a matter of what name was chosen. "Protein" is a specific term meaning a long chain of amino acids. ...
  • 1,515
14 votes
Accepted

Why can't H3O+ ions pass through aquaporins?

This question has been directly addressed by the paper The Mechanism of Proton Exclusion in the Aquaporin-1 Water Channel. I think it's a pretty good one too! I paste the abstract below: Aquaporins ...
  • 5,979
13 votes
Accepted

How can computer predictions of protein folding be verified computationally?

Overview Modelling has come on leaps and bounds over the last decade or so and in many cases has acted as a sometimes viable, and inexpensive substitute for experimental structures. How do you ...
  • 11.3k
12 votes

Why is glycine considered a nonpolar amino acid but a polar molecule?

The first part of your question illustrates a common confusion of beginners between the physiochemical properties of free amino acids in solution, and the properties of that part of an amino acid that ...
  • 23.1k
12 votes

What is the difference between a protein and a factor?

Short Answer There is no agreed upon naming convention for proteins - there are some rough standards because in language people usually try to convey their ideas in a way others can understand, but ...
  • 39.2k
12 votes
Accepted

Is tyrosine hydrophobic or hydrophilic?

The answer to this question emerges from an examination of the structure of tyrosine — or, more strictly, the tyrosyl residue, which is how it exists in proteins, the concern of the question: It ...
  • 23.1k
12 votes
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What is "irrational" drug/molecule design?

"Irrational" design in these papers refers to combinatorial mutagenesis, which is put forward as the alternative to "rational" protein design. Rational design involves using existing information ...
  • 8,179
12 votes
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Does the recent concern over several papers about Aβ*56 call into question the association of Alzheimers Disease with any amyloyd beta oligomer forms?

Does the recent concern over several papers about Aβ*56 call into question the association of Alzheimers Disease with any amyloyd beta oligomer forms? From my understanding, no. alzforum.org has ...
11 votes
Accepted

Did not understand a small excerpt from a research paper

The paper's description is poor, but they seem to be describing an encoding where each of 20 possible amino acids are associated with a position within a string of 20 bits, e.g. alanine with offset 0, ...
  • 8,779
11 votes
Accepted

Can the central dogma work in reverse?

Let’s first consider what the Central Dogma[1] actually says. It is precisely summarised in the following figure[2]: solid arrows represent transfer of information that has been observed directly; ...
11 votes
Accepted

Does the term 'protein expression' refer to the production of proteins only or also their regulation?

There are no formal definitions accepted in science for ambiguous phrases such “protein expression”. If you wish to know what a particular author who uses such a shorthand phrase means you need to ...
  • 23.1k
11 votes
Accepted

Arrangement of Amino Acids in the Protein alphabet

As suggested by tyersome's comment, the amino acids are grouped by their physiochemical properties. Let's add some commas: DE,KRH,NQ,ST,PGAVIL,MC,FYW aspartic ...
  • 8,179
11 votes
Accepted

Are "antibodies" and "immunoglobulins" really the same things?

See also https://en.wikipedia.org/wiki/Immunoglobulin_superfamily What you have here is a bit of imprecise terminology, and a language problem equivalent to another familiar one: cats. A lion is both ...
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