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Protein Chemistry(1)

The functions of proteins include acting as biological catalysts, regulating metabolic processes, transporting and storing substances, maintaining osmotic pressure and acid-base balance, etc.

Edited at 2023-11-01 15:28:02

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Protein Chemistry(1)

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Outline

protein chemistry

General Theory of Proteins

Chemical composition and classification of proteins

C,H,O,N,P,S

The nitrogen content of various proteins is similar, 16%

Kjeldahl method: protein content = protein nitrogen content * 6.25

Classification

Classification according to composition:

Simple proteins: albumin, globulin, gluten…

Binding proteins: glycoproteins, lipoproteins, nucleoproteins…

Classified by function

Enzymes, regulatory proteins, transporters…

Classification based on molecular shape

Fibrous protein (insoluble in water))

Globular protein: soluble in water

Membrane protein: insoluble in water

Protein size and relative molecular mass

The average molecular mass of amino acid residues is 110

Protein function (why protein is an important material basis in life activities)

Urgent transportation exemption (defense) transportation credit storage Homophone: Cui Gouyun Noodles Letter

As a catalyst for organism metabolism—enzymes

as a structural component of an organism

Transport function

immune defense function

Regulatory functions (functions of hormones)

Coordinated movement function

receive and deliver information

storage protein

Building blocks of proteins—amino acids

Types of amino acids in organisms

protein amino acids

definition

Common protein amino acids

There are only 20 types

Alpha amino acid definition

Except for proline and its derivatives, they are all a-amino acids

Among alpha amino acids, only glycine is a symmetrical amino acid

Amino acids in proteins are generally in the L-form

Uncommon protein amino acids

non-protein amino acids

Most of these amino acids are L-type alpha amino acid derivatives found in proteins

20 common amino acid classifications

Classification based on chemical structure of R group

Aromatic amino acids: Laoseben aromatic

Heterocyclic amino acids: histidine, proline

15 kinds of aliphatic amino acids

Classification based on polarity of R group

polar amino acids

Polar neutral amino acids: tyrosine, glycine

Polar acidic amino acids: acidic asparagus, aspartic acid is the most acidic

Polar basic amino acids: basic lysine group, arginine is the most basic

nonpolar hydrophobic amino acids

One or two fake cakes for diarrhea, phenylalanine, tryptophan

According to nutritional classification

Essential Amino Acids: Bring a light-colored book or two

Semi-essential amino acids: histidine, arginine

properties of amino acids

Acid-base properties

amino acid amphoteric dissociation

For acidic amino acids, PI＜PH＜7, the number of amino groups＜the number of carboxyl groups, so the number of negative ions (OH-)＞positive ions (H), so to reach the isoelectric point, acid H needs to be added Neutral amino acids are acidic, PI < PH < 7, carboxyl ionization > amino group, number of negative ions > number of positive ions... Acidic amino acids, PI>PH>7, amino acids>number of carboxyl groups…………

subtopic

Zwitterion

isoelectric point of amino acids

The solubility of amino acids is minimum at the isoelectric point

reason

PH>PI, amino acids are negatively charged and move toward the positive electrode in the electric field PH＜PI, amino acids are positively charged and move toward the negative electrode in the electric field. PH=PI, amino acids are uncharged and do not move

Absorption spectrum of amino acids

20 common amino acids have no light absorption in the visible light region, but have light absorption in the infrared region and far ultraviolet region

In the near-ultraviolet region, only aromatics have the ability to absorb

Phenylalanine 257nm Tyrosine 275nm Tryptophan 280nm

Since most amino acids have these amino acid residues, the maximum absorption wavelength is generally 280nm.

chemical reactions of amino acids

Ninhydrin reaction: weak base

Heat together with a amino acid to produce a purple substance

With proline and derivatives, it generates yellow substances

Amino acid quantitative and qualitative analysis

Sanger reaction (DNFB): weak base

Edman reaction (PITC): weak base

Identification of N-terminal amino acids of peptides and proteins

Reaction with dansulfonamide: weak base

N-terminal amino acid labeling of polypeptide chains and quantitative determination of trace amino acids

protein hydrolysis

acid hydrolysis

Complete protein hydrolysis

Alkaline hydrolysis

Complete protein hydrolysis

enzymatic hydrolysis

Partial hydrolysis of protein

covalent structure of protein

How amino acids are covalently linked in proteins

Peptide bond: —CO—NH—, is an amide bond

The peptide bonds in the peptide chain are all in trans configuration, and the two a carbon atoms and their substitution groups are far away from each other.

disulfide bond/disulfide bridge

Structure of peptides and peptide bonds

Peptide: Linear polymer of amino acids

Oligopeptide

Peptide: A peptide made up of more amino acids

peptide bond resonance

definition

effect

Limit free rotation around peptide bonds

Formation of amide plane (peptidyl plane)

definition

Make peptide bonds have partial double bond properties

Make the peptide bonds in the peptide chain have a trans conformation, that is, the two a carbon atoms and their substituents are far away from each other

Physical and chemical properties of peptides

Peptides and proteins containing two peptide bonds can react with copper sulfate biuret under alkaline conditions to form a purple or blue-violet complex.

active peptides present in hot weather

GlutathioneGSH

Composed of glutamic acid, cysteine, and glycine

Hormone active peptides

Oxytocin, vasopressin, bradykinin

antibiotic

Gramicidin

toxin-like active peptides

Synthetic active peptides

aspartame

primary structure of protein

Definition: The order of amino acids in a polypeptide chain, including the position of disulfide bonds, is called the primary structure of a protein.

Forces that maintain the primary structure of protein molecules: peptide bonds, disulfide bonds

Determination steps

Determine the N-terminal and C-terminal amino acids to determine the number of polypeptide chains

Resolve disulfide bonds to separate and purify different polypeptide chains

Cleavage of disulfide bonds using performic acid oxidation/mercaptoethanol

Analyze the ends of polypeptide chains

N-terminal determination: Sanger method (DNFB), Edman (PITC), dansyl chloride (DNS)

C-terminal determination: hydrazinolysis method, carboxypeptidase hydrolysis method

Use two or more methods to split the polypeptide chain into smaller peptide fragments and separate and purify them separately.

Enzymatic hydrolysis method: Trypsin specifically hydrolyzes the peptide bond formed by the carboxyl group of lysine and arginine, as well as chymotrypsin, pepsin, and thermolysin.

Chemical method: Hydrogen bromide (CNBr) specifically hydrolyzes the peptide bond formed by the carboxyl group of methionine)

Separation and purification of peptides: gel filtration, gel electrophoresis paint, high performance liquid chromatography

Use an amino acid sequencer to determine the amino acid sequence of each peptide.

Overlap and recombine peptide segments to determine the entire amino acid sequence of the peptide chain

Determine the location of disulfide bonds in a polypeptide chain

Pepsin hydrolysis combined with diagonal electrophoresis method

three-dimensional structure of protein

Methods for studying protein conformation

Configuration: refers to the unique fixed spatial arrangement of the atoms in an organic molecule. This arrangement will not change without the breaking and reforming of covalent bonds.

Conformation

Concept: In organic compounds, there are countless specific images of atoms or groups arranged in space caused by the rotation of C-C single bonds.

Research methods/Methods for studying the three-dimensional structure of proteins

Determination of protein molecular conformation in solution using NMR and circular dichroism spectroscopy...

Determining the conformation of protein molecules in crystals: X-ray crystallography

Spatial constraints on protein polypeptide chain backbone folding

Dihedral angle/conformational angle: between two adjacent amide planes, they can rotate with the common Cα as a fixed point. The angle of rotation around the Cα-NJ bond is called the φ angle, and the angle of rotation around the C-Cα bond is called the ψ angle. . φ and ψ are collectively called dihedral angle/conformational angle

Spatial constraints on polypeptide chain backbone folding

The amide plane is a rigid plane

aThe dihedral angles φ and ψ of carbon atoms restrict the conformation of the main chain

The forces that stabilize the three-dimensional conformation of proteins: hydrogen bonds, van der Waals forces, hydrophobic interactions (the main driving force for protein spatial conformation), salt bonds, and covalent disulfide bonds (mostly formed at β-turns)

protein secondary structure

Definition: Refers to the local spatial structure formed by different segments of the peptide chain main chain forming hydrogen bonds through their own interactions and spiraling and folding along a certain main axis. It is the conformational unit of the higher-order structure of proteins.

alpha helix

feature

Influencing factors

beta sheet

definition

feature

β-turn

definition

Glycine and proline often exist in β-turns, and glycine is the most suitable for β-turns.

random curl

definition

super secondary structure of protein

definition

Three common combination forms

protein domain

definition

Four common types

Tertiary structure of protein

definition

Features

Structural features of myoglobin

quaternary structure of protein

definition

subunit

definition

Subunit symmetry is one of the important properties of proteins with quaternary structure

Free subunits are inactive, and only the fully aggregated quaternary structure has complete biological activity.

Structural characteristics of hemoglobin

The structure of fiber protein

consists of a single type of secondary structure

Keratin structural support proteins, Divided into alpha keratin and beta keratin, Alpha keratin exists in mammals and is the main protein that makes up hair. It has a right-handed helix. Beta keratin is found in birds and reptiles

Collagen—a triple helix The basic unit is procollagen

Molecular structure and function of proteins

The primary structure of a protein determines its higher-order structure

Species differences and molecular evolution of homologous proteins (cytochrome c)

homologous protein

definition

Variable amino acid residues in homologous proteins provide information on phylogenetic relationships between species That is, the closer the genetic relationship is, the smaller the difference in their amino acid compositions. The more distant the relationship, the greater the difference in amino acid composition.

Structure and function of myoglobin and hemoglobin

allosteric effect

molecular disease

Immunoglobulin structure and function

Properties and separation and purification of proteins

Amphoteric dissociation properties and isoelectric point

Isoelectric point definition

Properties of protein colloids/Why is the protein aqueous solution a stable hydrophilic colloid?

molecular particle diameter

hydration layer

Like charges repel each other

Form a stable electrical double layer

protein precipitation

Salting out method

Mainly targeted at the hydration layer

Isoelectric precipitation method

Act on surface charge

organic solvent precipitation method

Act on the hydration layer

Heavy metal salt precipitation method

generate insoluble salt

Precipitation of alkaloids and certain acids

generate insoluble salt

Heating denaturation precipitation method

Denaturation and renaturation of proteins

UV absorption properties of proteins

protein color reaction

Protein separation and purification methods