Very roughly how many proteins (chains) are synthesized in the human body per hour under normal conditions? (And how many ribosomes does the human body have?)


closed as too broad by MattDMo, anongoodnurse, David, AliceD, March Ho Nov 16 '16 at 23:09

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    $\begingroup$ Welcome to SE Biology. This question has multiple issues that will see it being downvoted or closed. There are two questions rather than one. Neither question is well defined. The first question is impossible to answer. This is due to the enormous error in such an estimation. There is variation at every level of a human body; starting between tissues down to translational initiation events on individual RNA transcripts. Also, every protein/polypeptide chain has a different length so every protein will take a different length of time to produce. $\endgroup$ – Michael_A Nov 16 '16 at 21:14
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    $\begingroup$ @Michael_A It's not such a crazy question I think ... It should be possible to come up with at least a ballpark figure (say, correct within an order of magnitude) from bioenergetics. $\endgroup$ – Roland Nov 16 '16 at 22:32
  • $\begingroup$ @Roland, Changing the question to ask about the number of amino acids per hour would make a ballpark estimate feasible. Once that change is made I almost agree with you regarding the first question. The other issue is that there are two questions. Currently, there are two answers but each answer addresses a different question. $\endgroup$ – Michael_A Nov 16 '16 at 23:42
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    $\begingroup$ @Michael_A But once we know the number of amino acids, this can be converted to number of proteins, since we know the average length of a protein in human cells. (Although we must assume that protein turnover rate is independent of protein size.) I agree it would be good to split the question in two. $\endgroup$ – Roland Nov 17 '16 at 16:49
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    $\begingroup$ I'm voting to reopen this question, as I think it's fully possible to answer it with some reasonable confidence. We often overlook these quantitative questions in biology, but they are interesting and not as hard to answer as some may think --- it is not necessary to know a lot of details. I don't know the answer to the second question on number of ribosomes, but it should be possible to figure out a rough number for this as well, by estimating ribosomes as a fraction of total protein. This has been done for yeast, see book.bionumbers.org/how-many-ribosomes-are-in-a-cell $\endgroup$ – Roland Nov 28 '16 at 20:10

Regarding protein synthesis rate, here's an attempt at an estimate from bioenergetics:

  • ATP turnover in the human body is consider to be about 100 mol / day.

  • Protein synthesis is estimated to require about 1/4 of ATP consumption in a mammalian cell, and one amino acid elongation requires about 5 ATP, so about 5 mol amino acids are elongated per day, which translates to $1.2 \cdot 10^{23}$ amino acids per hour.

  • An average human protein is about 400 amino acids, which gives about $3 \cdot 10^{20}$ proteins per hour.${}^1$

I think this should be correct within an order of magnitude at least. (Unless I screwed up at some step in the calculation, it's getting late over here :) The most uncertain factor is probably the fractional ATP demand for protein synthesis.

${}^1$ The is correct with the following assumptions. Consider that the proteome has some (any) length distribution $p(n)$ where $n = 1,2,3 \dots$ is the protein length in amino acids, and $\sum_n p(n) = 1$. Let $r(n)$ be the total rate of synthesis of all proteins of length $n$. If we assume that this rate is proportional to protein abundance, $r(n) = t\ p(n)$, where $t = \sum_n r(n)$ is the total protein synthesis rate, then the total rate of amino acids per unit time is

$r_{AA} = \sum_n n\ r(n) = t \sum_n n\ p(n) = t\ \mu$

where $\mu$ is the average protein length. Hence the sought total protein synthesis rate is $t = r_{AA} / \mu$.


For your first question I think @Michael_A has explained why this question can't be answered.

And for the second one, I quote from bscb.org :

Ribosomes are found ‘free’ in the cytoplasm or bound to the endoplasmic reticulum (ER) to form rough ER. In a mammalian cell there can be as many as 10 million ribosomes (A single cell of E. Coli contains about 20,000 ribosomes and this accounts for about 25% of the total cell mass). Several ribosomes can be attached to the same mRNA strand, this structure is called a polysome. Ribosomes have only a temporary existence. When they have synthesised a polypeptide the two sub-units separate and are either re-used or broken up.

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    $\begingroup$ I don't like the last sentence in the quote. Very misleading. Almost everything in the cell turns over, but ribosomes have a long half-life compared to the time needed to translate a mRNA. $\endgroup$ – David Nov 16 '16 at 22:25

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