Half life

[Mystery_of_GodST]

True or false

 

Every biological molecule has a half life?

 

 

 

 

 

 

[SMF]

False.

[Mystery_of_GodST]

Please explain

 

 

[SMF]

Explain why you would want to know. Otherwise, check with your mom, she will be able to tell you. SM

[steevey]

A Half life is only for radioactive material. If atoms aren't radioactive or unstable, then they won't decay, so no half-life equation.

[ecoli]

Actually, SMF and steevy are wrong. All molecules can be said to have a half life. It's just that for many molecules, half life is so long, the term is only used commonly for radioactive materials. Half life of a diamond (which decays into graphite) is on the order of millions to billions of years.

 

For biological molecules, it's common to talk about mRNA decay, which is measured in half lives, as a measure of stability (also down for proteins). In this study, β-actin was determined to have a half life of 6.5-13 hours.

[Horza2002]

To make this clearer, you just need to remember that the term half-life most commonly refers to radioactive decay. But all it means is the time it takes for half you initiatial property to decrease by 50%; whether that be a radioactive atom, or compound or a protein.

 

As ecoli stated, diamond rearranges (im not sure decay is the right word in this case) to graphite VERY slowly and so its half-life is on the order of millions of years. The pharamceutical industry is very interested in the half-life of drug molecules. If they have a half-life too low, then they might be able to reach the site of action before its all been removed, or if its too long, it could result in adverse effects.

 

High temperature gas phase chemistry, in which highly unstable molecules are formed, express the stability of each of these molecules in terms of their half-life. Another example is that of sinlget oxygen...it has a half life in water of about 4milliseconds, but in organic solvents has a half-life of 16mins.

 

In terms of do molecules themsevles undergo radioactive decay...that remains to be seen. I beleive it is theorised that protons and neutrons (what make up the nucleus of atoms) are precited to have half-lifes on the order of the current age of the Universe...but that remains to be proven.

[CharonY]

For biological molecule usually the complete turnover is considered. Here usually also active inactivation of the molecule by e.g. other biological molecules counts against the stability/activity of the molecule. This is a major factor of mRNA decay, for instance.

[JorgeLobo]

As charonY inferred, the concept of half life is pretty irrelevant in context of biological molecules that effectively come and go in log terms. One prob. could estimate but in most cases it's a pretty meaningless number in context.

Edited March 23, 2011 by JorgeLobo

[CharonY]

Not precisely meaningless. In contrast to radioactive decay biological degradation is highly dependent and controlled by the circumstance (e.g. presence of nucleases, medium, etc.). It is not a material constant. But the turnover rate can be of high biological relevance.

[mississippichem]

Not precisely meaningless. In contrast to radioactive decay biological degradation is highly dependent and controlled by the circumstance (e.g. presence of nucleases, medium, etc.). It is not a material constant. But the turnover rate can be of high biological relevance.

 

I'll even go a step further and say that it is very meaningful. Turnover rate might be one of the most important factors when playing with enzyme kinetics.

[CharonY]

I had regulatory roles in mind, but yes, that is another good example.

[JorgeLobo]

Enzyme kinetics are not typically measured in half lives - it can be and sometimes is calculated the changes in that measure are usually so rapid, it becomes an imprecise and not very useful measure.

Edited March 23, 2011 by JorgeLobo

[CharonY]

I think what was meant was the turnover rate of the enzymes themselves. E.g. protease activity that, in turn, can modulate other enzyme activities.

[mississippichem]

Enzyme kinetics are not typically measured in half lives - it can be and sometimes is calculated the changes in that measure are usually so rapid, it becomes an imprecise and not very useful measure.

 

Yes, enzyme kinetics typically aren't measured in half-lives, but one of the primary goals of Michaelis-Menton kinetics is to arrive at the turnover rate which gives one a general idea of the "quickness" of said enzyme. Of course this takes the steady state assumption as a given, which has varying degrees of accuracy depending on the stability of the ES complex.

Edited March 24, 2011 by mississippichem

[John Cuthber]

For chemical processes (and in principle nuclear ones too) the rate of reaction is temperature dependent, so even if every molecule had a half life, it would depend on the environment.

 

Incidentally, by this time tomorrow there's every chance that the enzymes in my liver that are responsible for the oxidation of alcohol will be saturated and so the rate of loss of alcohol from me will be roughly zero order, rather than the first order decay that is characterised by a (single) half life.

 

On the other hand, the caffeine in the cola I had earlier is a diuretic and since the kidneys are (largely) responsible for excretion of caffeine, the more caffeine there is the faster they produce urine and get rid of the caffeine. So the loss of caffeine is a bit faster than first order and, again, that's not characterised by a simple half- life.

 

Biology is, in general, very complicated.

[Pharma Mike]

Talk of half-lives usually implies a random event that causes decay. The population of the decaying substance then follows an exponential decline that may be mathematically described by...

 

 

This leads to being able to determine the half life as....

 

 

While all molecules will eventually be changed (if only in the final death-throws of the universe), I think it could cause confusion to say that 'all molecules have a half-life' as it would imply a mathematical pattern of decay that does not usually occur (as other reactions are likely to occur first). So I might say that the statement 'all molecules have a half-life' is strictly true, but rather unhelpful at the same time.

Edited April 4, 2011 by Pharma Mike