Can we predict the past? Can we use probability rules to predict the occurrence of an event which has already happened in the past or already formed? 
For example, hemoglobin is a protein formed of $141$ amino acids connected like a chain with specific order, the first amino acid is Leucine (we have only $20$ types of amino acids forming any protein), is it valid to say that probability of leucine being first in this chain is $\frac1{20}$ so the probability to get the $141$ a.a hemoglobin with such order by chance is $\left(\frac{1}{20}\right)^{141}?$ or this prediction makes no sense as we already have hemoglobin formed with such order in the nature?      

If you randomly deal a deck of 52 cards, 13 cards per person for four people, the probability of that particular deal is $1$ in $53,644,737,765,488,792,839,237,440,000\approx 2^{96}$. And yet, somehow people continue to play bridge, in spite of the low probability of each of the deals they play.
If you are trying to somehow use the complexity of hemoglobin as evidence that evolution was not purely random, or even that evolution does not occur at all, then this is poor evidence. Hemoglobin did not spontaneously occur, in evolution theory. Rather, proteins like hemoglobin occurred over time, and the more effective ones were selected for.
In particular, it is much easier to "evolve" hemoglobin than it is to randomly assemble it, since various intermediate simpler proteins are useful for survival. The random process begins to resemble more like a random search, trimming away the useless parts of the tree. 
And, of course, there might well have been umpteen gazillion different proteins that ultimately could serve such a function. Evolution on another world might well evolve a different complex of proteins, or a different type of chemistry altogether.
So, it is highly unlikely that hemoglobin would spontaneously generate at any particular point in time. That almost goes without saying. But nobody is saying this is how hemoglobin was formed.
