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(未完成） 蛋白質功能Function of Globular Proteins, 蛋白質功能Function of Globular…
(未完成）
蛋白質功能Function of Globular Proteins
Methods of binding ligands and proteins
Reversible可逆 binding of ligands is essential.
specificity of ligands and binding sites
Ligand binding is often coupled to conformational changes, sometimes quite dramatically (induced fit誘導擬合會變形的鎖孔).
In multisubunit proteins, conformational changes in one subunit can affect the others (cooperativity)第一個蛋白質結合，後續更好結合
Interactions can be regulated.
Functions
Storage of ions and molecules
myoglobin肌紅蛋白/ferritin鐵蛋白
Transport of ions and molecules
hemoglobin血紅蛋白/serotonin transporter血清素轉運體
Defense against pathogens
antibodies/cytokines細胞因子
Muscle contraction
actin肌動蛋白, myosin肌球蛋白
Biologicalcatalysis
chymotrypsin糜蛋白酶/ lysozyme溶菌酶
Biding
Quantitative Description
Analysis in Terms of the Bound Fraction（結合率
we can often determine the fraction of occupied binding sites (θ).
In terms of the more commonly used equilibrium dissociation constant:
Graphical Analysis
最重要的看Kd值（Protein與Ligan的親和力
Thermodynamic（熱力學） Connections
Magnitudes幅度
strong binding： Kd < 10 nM
weak binding: Kd > 10 μM
Specificity
Lock-and-Key Model
be explained by the complementary of the binding site and the ligand.
– size / shape / charge / hydrophobic/hydrophiliccharacter
Induced Fit
特性
Conformational changes may occur upon ligand binding
Induced fit allows for tighter binding of the ligand.
Induced fit allows for high affinity for different ligands.
This adaptation is called the induced fit
Both the ligand and the protein can change their conformations.
cooperativity
How Can Affinity to Oxygen Change
Binding sites must be able to interact with each other.
This phenomenon is called "cooperativity"
positive cooperativity
first binding event increase saffinity atremaining site
:star:recognized by sigmoidal binding curves
negative cooperativity一個接上後續更差
first binding event reduces affinity are maining sites
must be a protein with multiple binding sites.
Quantitative Description
case 1
:warning::Globins Are Oxygen- Binding Proteins
Biological solution:
Capture the oxygen molecule with heme that is protein bound.
:star:Myoglobin (storage) and hemoglobin (transport) can bind oxygen via a protein-bound heme.
例：Myoglobin(儲存用）
Biological problems:
Some transition metals bind O2 well but would generate free radicals if free in solution.
Organometallic compounds such as heme are more suitable, but Fe2+(輔基 in free heme could be oxidized to Fe3+ (very reactive!).
Protein side chains lack affinity(親和力 for O2
Binding of Carbon Monoxide
CO is highly toxic, as it competes with oxygen. It blocks the function of myoglobin, hemoglobin, and mitochondrial cytochromes that are involved in oxidative phosphorylation.
CO binds heme over 20,000 times better than O2
Spectroscopic Detection(光譜檢測)
of Oxygen Binding to Globe's
The heme group is a strong chromophore(發色團） that absorbs both in ultraviolet（紫外的） and visible（可見光） range.
Binding of oxygen can be monitored by UV-Vis spectrophotometry.
Deoxyhemoglobin脫氧血紅蛋白 (in venous blood靜脈) appears purplish in color and Oxyhemoglobin (in arterial blood) is red.
蛋白質功能Function of Globular Proteins
Interaction of globins with oxygen and non-oxygen ligands
Physiological regulation of oxygen binding
Mechanism and control of antibody-antigen interaction
Mechanism of muscle contraction
• Regulation of muscle contraction
Quantitative and graphical modeling of protein-ligand interactions
A molecule that binds to a protein is called a ligand.
A region in the protein where the ligand binds is called the binding site.
Ligand binds via same noncovalent 非共價鍵interactions that dictate protein structure
。 多少結合位被結合
。與Kd有關聯性
Myoglobin Fe2+旁一隻手接Histidine，一隻手接O2
