Download Chapter05, 06 代谢引论糖代谢

教
案
~ 2007 学年 第一 学期
2006
学 院
教
名 称
研
生命科学学院
室
课 程
名 称
授 课
对 象
授 课
教 师
陈文利
称
副教授
职
教 材
名 称
2006 年 9 月
生物化学
2005 级生物技术专业
现代生物化学
日
授课题目（教学章、节或主题）
：
教学器材
与工具
第四、五章 代谢引论 糖代谢
授课时间
多媒体设施、黑板与
笔
第 8，9 周一第 21-26
节
教学目的、要求（例如识记、理解、简单应用、综合应用等层次）
：
掌握糖 酵 解 途 径 ，三 羧 酸 循 环 ，掌握戊 糖 磷 酸 途 径 及 糖 异 生 途 径
教学内容（包括基本内容、重点、难点）
：
第四章 代谢引论
第一节 通论
中间代谢
第二节 新陈代谢研究方法
第五章 糖与糖代谢
(carbohydrate metabolism)
Overview of Glycolysis
The Embden-Meyerhof (Warburg) Pathway
Essentially all cells carry out glycolysis
Cellular location: cytosol
Ten reactions - same in all cells - but rates differ
Two phases:
–
1st phase- investing phase: glucose →F-1,6-2P →2G-3-P
–
2nd phase-harvesting phase: produces two pyruvates, ATPs and NADH
Three possible fates for pyruvate
Glycolysis
A. Energy Investment Phase:
Glycolysis
B. Energy Yielding Phase
First Phase of Glycolysis
The first reaction - phosphorylation of glucose
Hexokinase or glucokinase
This is a priming reaction - ATP is consumed here in order to get more later
ATP makes the phosphorylation of glucose spontaneous
Hexokinase
1st step in glycolysis; G large, negative
Hexokinase (and glucokinase) act to phosphorylate glucose and keep it in the cell
Km for glucose is 0.1 mM; cell has 4 mM glucose
So hexokinase is normally active!
Glucokinase (Kmglucose = 10 mM) only turns on when cell is rich in glucose
Hexokinase is regulated - allosterically inhibited by (product) glucose-6-P
Glycolysis - Second Phase
Metabolic energy produces 4 ATP
Net ATP yield for glycolysis is two ATP
Second phase involves two very high energy phosphate intermediates
– 1,3 BPG
– Phosphoenolpyruvate
Glycolysis - Second Phase
Metabolic energy produces 4 ATP
Net ATP yield for glycolysis is two ATP
Second phase involves two very high energy phosphate intermediates
– 1,3 BPG
– Phosphoenolpyruvate
Substrate-Level Phosphorylation
ATP is formed when an enzyme transfers a phosphate group from a substrate to ADP.
Rx 8: Phosphoglycerate Mutase
Phosphoryl group from C-3 to C-2
Rationale for this enzyme - repositions the phosphate to make PEP
Note the phospho-histidine intermediates!
Zelda Rose showed that a bit of 2,3-BPG is required to phosphorylate His
Rx 9: Enolase
2-P-Gly to PEP
How can such a reaction create a PEP?
"Energy content" of 2-PG and PEP are similar
Enolase just rearranges to a form from which more energy can be released in hydrolysis
Rx 10: Pyruvate Kinase
PEP to Pyruvate makes ATP
These two ATP (from one glucose) can be viewed as the "payoff" of glycolysis
Large, negative G - regulation!
The Fate of NADH and Pyruvate
Aerobic or anaerobic??
NADH is energy - two possible fates:
If O2 is available, NADH enters into Mitochondria by two ways, where it is re-oxidized in the
electron transport pathway, making ATP in oxidative phosphorylation.
– In anaerobic conditions, NADH is re-oxidized by lactate dehydrogenase (LDH), providing
additional NAD+ for more glycolysis
Significance of Glycolysis
Produce ATPs
Provide biosynthetic materials
Glycolysis and Cancer
Hypoxia and hyoxia-inducible factor
The TCA Cycle
A common metabolic pathway for glucose, aa and fatty acid aka Citric Acid Cycle, Krebs Cycle
Pyruvate (actually acetate) from glycolysis is degraded to CO2
Some ATP is produced More NADH is made
NADH goes on to make more ATP in electron transport and oxidative phosphorylation
Entry into the TCA Cycle
Pyruvate Dehydrogenase Complex
Pyruvate is oxidatively decarboxylated to form acetyl-CoA
Pyruvate dehydrogenase uses TPP, CoASH, lipoic acid, FAD and NAD+
Pyruvate dehydrogenase (E1)
Dihydrolipoamide transacetylase (E2)
Dihydrolipoamide dehydrogenase (E3)
Citrate Synthase
Formation of citrate
Another example for the induced fit model
OAA, the first substrate to bind to the enzyme, induce a large conformational change,
creating a binding site for the second substrate, acetyl-CoA. When citroyl-CoA forms on the
enzyme surface, another conformational change brings the side of a crucial Asp residue into
position to cleavage the thioester.
This mechanism decreases the likelihood of premature and unproductive cleavage of the
thioester bond of acetyl-CoA
Aconitase
Isomerization of Citrate to Isocitrate
Citrate is a poor substrate for oxidation
So aconitase isomerizes citrate to yield isocitrate which has a secondary -OH, which can be
oxidized
Note the stereochemistry of the Rxn: aconitase removes the pro-R H of the pro-R arm of
citrate!
Aconitase uses an iron-sulfur cluster
Succinyl-CoA Synthetase
A substrate-level phosphorylation
A nucleoside triphosphate is made
Its synthesis is driven by hydrolysis of a CoA ester
The mechanism involves a phosphohistidine
Succinate Dehydrogenase
An oxidation involving FAD
This enzyme is actually part of the electron transport pathway in the inner mitochondrial
membrane
The electrons transferred from succinate to FAD (to form FADH2) are passed directly to
ubiquinone (UQ) in the electron transport pathway
Fumarase
Hydration across the double bond trans-addition of the elements of water across the
double bond
The actual mechanism is not known for certain
Malate Dehydrogenase
An NAD+-dependent oxidation
The carbon that gets oxidized is the one that received the -OH in the previous reaction
This reaction is energetically expensive
o' = +30 kJ/mol
This and the previous two reactions form a reaction triad that we will see over and over!
The Fate of Carbon in TCA
Carboxyl C of acetate turns to CO2 only in the second turn of the cycle (following entry of
acetate)
Methyl C of acetate survives two cycles completely, but half of what's left exits the cycle on
each turn after that.
Function of the TCA Cycle
Produces More ATPs
As a source of biosynthetic precursors
A common final metabolic pathway for glucose, aas and fatty acids
Some Immediates act as effectors to regulate other metabolic pathways
Produces CO2
The Glyoxylate Cycle
A variant of TCA for plants and bacteria
Acetate-based growth - net synthesis of carbohydrates and other intermediates from acetate is not possible with TCA
Glyoxylate cycle offers a solution for plants and some bacteria and algae
The CO2-evolving steps are bypassed and an extra acetate is utilized
Isocitrate lyase and malate synthase are the short-circuiting enzymes
Glyoxylate Cycle II
Isocitrate lyase produces glyoxylate and succinate
Malate synthase does a Claisen condensation of acetyl-CoA and the aldehyde group of
glyoxylate - classic CoA chemistry!
The glyoxylate cycle helps plants grow in the dark!
Glyoxysomes borrow three reactions from mitochondria: succinate to oxaloacetate
Pentose Phosphate Pathway
Aka:
Pentose shunt
Hexose monophosphate shunt
Phosphogluconate pathway
It occurs in the cytosol.
Two oxidative processes followed by five non-oxidative steps
Operates active in the cytosol of liver and adipose cells
Pentose Phosphate Pathway
Cells need a constant supply of energy
NADH, NADPH and ATP
ATP - energy currency
NADPH - reducing power
Glucose --> NADH --> ATP
Glucose --> NADPH --> biosynthesis (reductive)
Oxidative Phase
Glucose-6-P Dehydrogenase
– Irreversible 1st step - highly regulated (inhibited by NADPH)
Gluconolactonase
– Uncatalyzed reaction happens too
6-Phosphogluconate Dehydrogenase
– An oxidative decarboxylation
The Nonoxidative Phase
Transketolase
transfer of two-carbon units
Transaldolase
transfers a three-carbon unit
The use of NADPH
Biosynthesis
– fatty acids
– photosynthesis
– DNA synthesis
Redox regulation of cellular processes
– control of cellular redox state
– transcription
– disulfide bond formation
– antioxidant defense
Gluconeogenesis
Net Synthesis of "new glucose" from non-sugar metabolites
Substrates for Gluconeogenesis
Pyruvate, lactate, glycerol, amino acids and all TCA intermediates can be utilized
Even-number Fatty acids cannot!
Most fatty acids yield only acetyl-CoA
Acetyl-CoA (through TCA cycle) cannot provide for net synthesis of sugars
Gluconeogenesis I
Occurs mainly in liver and kidneys
Not the mere reversal of glycolysis for 2 reasons:
– Energetics must change to make gluconeogenesis favorable (
G of glycolysis =
-74 kJ/mol )
– Reciprocal regulation must turn one on and the other off - this requires something
new!
Gluconeogenesis II
Something Borrowed, Something New
Seven steps of glycolysis are retained:
– Steps 2 and 4-9
Three steps are replaced:
– Steps 1, 3, and 10 (the regulated steps!)
sugar synthesis), and they provide new mechanisms of regulation
Fructose-1,6-bisphosphatase
Hydrolysis of F-1,6-P to F-6-P
Thermodynamically favorable -8.6 kJ/mol
Glucose-6-Phosphatase
Conversion of Glucose-6-P to Glucose
Presence of G-6-Pase in ER of liver and kidney cells makes gluconeogenesis possible
Muscle and brain do not do gluconeogenesis
G-6-P is hydrolyzed as it passes into the ER
Glycogen Metabolism
Glycogen Breakdown
Glycogen Synthesis
Glycogen Storage Diseases
Why use glycogen for energy storage?
Muscles cannot mobilize fat as rapidly as glycogen.
Fatty acid residues of fat cannot be metabolized anaerobically.
Animals cannot convert fatty acids to Glc, so fat metabolism alone cannot adequately maintain
essential blood Glc levels.
Glycogen Breakdown Requires Three Enzymes
A
B. Glycogen debranching enzyme
(1->6)
glucosidase activities -> Glc-1-P and Glc
C. Phosphoglucomutase to convert to usable formG
Glycogen Synthesis
Glucose must be activated into UDP-Glc
Notice: ADP-Glc for Starch; GDP-Glc or UDP-Glc for Cellulose
The primer is required
Glycogenin
Proceed from the reducing end to the non-reducing end
Glycogen synthase and Glycogen branching enzyme
重点：重点介绍糖酵解 三羧酸循环 糖异生作用途径及简单的能量计算
难点：记忆这些代谢途径及调节
教学过程设计（要求阐明对教学基本内容的展开及教学方法与手段的应用、讨论、作业布置）：
利用课件结合板书介绍糖代谢的主要代谢途径，了解糖代谢的的研究新进展，重点掌握糖酵
解 三羧酸循环 糖异生作用，引导学生记忆方法，在理解的基础上布置作业，让学生在作业中
发现问题提出问题，对于比较难理解的老师在课堂上再次强调。在教学过程中给学生介绍学习方
法及鼓励学生拓展知识。