Calculation of the number of atoms in amino acids
Example: 1: the r group of valine is -―C3H7, and the number of atoms containing carbon and hydrogen in a valine molecule is () respectively.
A.6 10B.5 1 1
C.7 10D.6 1 1
Analysis: If the R group of valine is -―C3H7, the structural formula of valine is H2n-CH-COOH, with 5 carbon atoms, 1 1 hydrogen atoms.
Answer: b
Rule summary: In an amino acid, the number of C atoms = C atoms in R group +2, H atoms = H atoms in R group +4, O atoms = O atoms in R group +2, N atoms = N atoms in R group+1.
Variant training 1: The R group of propylamine is CH3, and the R group of glutamic acid is C3H5O2. The atomic ratio of C, H and O in the dipeptide molecule formed by their condensation is ().
A7:16: 6
C.8: 12:5
2. Calculation of the relationship among the number of amino acids, the number of peptide bonds and the number of peptide chains.
Example 2: There are 600 amino acids, of which the total number of amino groups is 665,438+00 and the total number of carboxyl groups is 608. Then the number of protein with two peptide chains synthesized from these amino acids is () respectively.
A.5982 2 B.598 12 10
599 1 1 9
Analysis: The amino acids that make up protein have at least one amino group and one carboxyl group, which are connected to the same carbon atom. There are 600 amino acids in this topic, of which the total number of amino groups is 6 10 and the total number of carboxyl groups is 608, which means that there are 10 amino acids with 2 amino groups, 8 amino acids with 8 carboxyl groups, and 600 amino acids with 2 peptide chains * * *.
Answer: b
Rule summary: If there are m amino acid molecules condensed into n peptide chains, the following rules can be formed:
Variant training 2: Every year, hundreds of people all over the world die of eating poisonous mushrooms by mistake. Amantadine is the toxin of poisonous mushrooms, and it is a cyclic octapeptide. If the average molecular weight of 20 amino acids is 128, the molecular weight of amantadine is about ().
A. 1024B.898C.880D.862
3. Calculation of the relationship between the average molecular weight of amino acids and the molecular weight of protein.
Example 3: Protein A contains two peptide chains, A chain contains 2 1 amino acid, and B chain contains 30 amino acids. The two polypeptide chains are connected by disulfide bonds. The figure below shows the plane structure of protein, so when amino acids form protein, the relative molecular weight decreases by _ _ _ _. If the average molecular weight of amino acids is 128, then the molecular weight of protein is _ _ _ _ _.
Analysis: First of all, it can be seen from the figure that protein contains two peptide chains, and the number of peptide bonds formed is (2 1+30)-2=49, and the number of water molecules produced is 49. The relative molecular weight decreased during the formation of protein also includes the relative molecular weight of H removed during the formation of disulfide bonds, that is, 18? 9+6=888, and the molecular weight of protein is 5 1? 28-888=5640。
Answer: 8885640
Rule summary: N amino acids make up M peptide chains, and the average molecular weight of each amino acid is A, so the resulting protein molecular weight is: N? a-(n-m)？ 18 (where n-m is the number of lost water molecules and 18 is the molecular weight of water); The molecular weight of protein is lower than the sum of its amino acids (n-m)? 18。 (Sometimes, it is necessary to consider the decrease of relative molecular weight caused by the formation of other chemical bonds, such as the formation of disulfide bonds and the removal of 2 H)
Variant training 3: A protein consists of two peptide chains, and * * * contains 100 amino acids. If the average molecular weight of each amino acid is 120, then the relative molecular weight of protein is about ().
a . 12000 b . 10236 c . 102 18d . 13764
Fourthly, the calculation of polypeptide types.
Example 4: Tripeptide composed of 1 lysine, 1 threonine and 1 leucine has ().
A.6 kinds of B. 5 kinds of C. 27 kinds of D. 3 kinds
Analysis: If enumeration method is used, the tripeptide composed of 1 lysine, 1 threonine and 1 leucine depends on Su Liang, Lai Liangsu, Su Liangliang, Su Lianglai, Liang Sulai and Liang Laisu. * * * There are six kinds.
A: A.
Rule summary: If there are n kinds of amino acids, the situation of peptides composed of these n kinds of amino acids can be analyzed in the following two situations: (1) If the number of each amino acid is infinite, then the type that can form m kinds of peptides is nm; (2) When the number of each amino acid is only one, there are n× (n- 1 )× (n-2 )× 1 species of M peptide.
Variant training 4: There are four kinds of amino acids: isoleucine, valine, methionine and tryptophan, and the number of each amino acid can be unlimited. Dipeptide consists of ().
A.8 kinds of B.4 kinds of C. 16 kinds of D. 12 kinds.
5. Calculation of the relationship between the number of amino acid molecules and the number of bases in 5.mRNA and gene.
Example 5: A prokaryotic mRNA can be translated into a polypeptide with 1 1 peptide bond, so the number of bases contained in this mRNA molecule and the number of trna needed to synthesize this polypeptide are () in turn.
a . 33 1 1b . 36 12c . 12 36d . 1 136
Analysis: 1 1 polypeptide with a peptide bond of 12 amino acids. Three bases on mRNA determine an amino acid, so this mRNA molecule contains at least 36 bases, one amino acid needs one tRNA to transport, and * * * needs 12 trnas.
Answer: b
Rule summary: In the process of gene-controlled protein synthesis, the number ratio of deoxynucleotide (or base), ribonucleotide (or base) and amino acid, which are the basic components of DNA, mRNA and protein, is 6:3: 1. (regardless of gene post-transcriptional processing and stop codon)
Variant training 5: Assuming that a piece of mRNA has 60 bases, in which A has 15 bases and G has 25 bases, then the number of "C+T" in the DNA molecular segment where the mRNA is transcribed and the number of amino acids required for translating the mRNA into protein are () respectively.
A.40、30B.40、20C.60、20D.80、40
(Editor Liu Hong)