Analysis and prediction of the location of catalytic residues in enzymes

The catalytic residues of an enzyme are defined as the amino acids directly involved in chemical catalysis. They mainly act as a general acid--base, electrophilic or nucleophilic catalyst or they polarize and stabilize the transition state. An analysis of the structural features of 36 catalytic residues in 17 enzymes of known structure and with defined mechanism is reported. Residues that bind metal ions (Zn2+ and Cu2+) are considered separately. The features examined are: residue type, location in secondary structure, separation between the residues, accessibility to solvent, intra-protein electrostatic interactions, mobility as evaluated from crystallographic temperature factors, polarity of the environment and the sequence conservation between homologous enzymes of residues that were sequentially or spatially close to the catalytic residue. In general the environment of catalytic residues is similar to that of polar side chains that have low accessibility to solvent. Two algorithms have been developed to identify probable catalytic residues. Scanning an alignment of homologous enzyme sequences for peaks of sequence conservation identifies 13 out of the 16 catalytic residues with 50 residues overpredicted. When the conservation of the spatially close residues is used instead, a different set of 13 residues are identified with 47 residues overpredicted. A combination of the two algorithms identifies 11 residues with 36 residues overpredicted.     

Evidence of the participation of remote residues in the catalytic activity of Co-type nitrile hydratase from Pseudomonas putida

Active sites may be regarded as layers of residues, whereby the residues that interact directly with substrate also interact with residues in a second shell and these in turn interact with residues in a third shell. These residues in the second and third layers may have distinct roles in maintaining the essential chemical properties of the first-shell catalytic residues, particularly their spatial arrangement relative to the substrate binding pocket, and their electrostatic and dynamic properties. The extent to which these remote residues participate in catalysis and precisely how they affect first-shell residues remains unexplored. To improve our understanding of the roles of second- and third-shell residues in catalysis, we used THEMATICS to identify residues in the second and third shells of the Co-type nitrile hydratase from Pseudomonas putida (ppNHase) that may be important for catalysis. Five of these predicted residues, and three additional, conserved residues that were not predicted, have been conservatively mutated, and their effects have been studied both kinetically and structurally. The eight residues have no direct contact with the active site metal ion or bound substrate. These results demonstrate that three of the predicted second-shell residues (α-Asp164, β-Glu56, and β-His147) and one predicted third-shell residue (β-His71) have significant effects on the catalytic efficiency of the enzyme. One of the predicted residues (α-Glu168) and the three residues not predicted (α-Arg170, α-Tyr171, and β-Tyr215) do not have any significant effects on the catalytic efficiency of the enzyme.     

Studies on tryptophan residues of Abrus agglutinin. Stopped-flow kinetics of modification and fluorescence-quenching studies.

The presence of two essential tryptophan residues/molecule was implicated in the binding site of Abrus agglutinin [Patanjali, Swamy, Anantharam, Khan & Surolia (1984) Biochem. J. 217, 773-781]. A detailed study of the stopped-flow kinetics of the oxidation of tryptophan residues revealed three classes of tryptophan residues in the native protein. A discrete reorganization of tryptophan residues revealed three classes of tryptophan residues in the native protein. A discrete reorganization of tryptophan residues into two phases was observed upon ligand binding. The heterogeneity of tryptophan exposure was substantiated by quenching studies with acrylamide, succinimide and Cs+. Our study revealed the microenvironment of tryptophan residues to be hydrophobic, and also the presence of acidic amino acid residues in the vicinity of surface-localized tryptophan residues.     

Structure and activities of constrained analogues of human parathyroid hormone and parathyroid hormone-related peptide: implications for receptor-activating conformations of the hormones

Parathyroid hormone (PTH) has a helix-bend-helix structure in solution. Part of the C-terminal helix, residues 21-31, is amphiphilic and forms a critical receptor-binding region. Stabilization of this alpha-helix by lactam formation between residues spaced i, i + 4 on the polar face was previously reported to increase adenylyl cyclase-stimulating (AC) activity if between residues 22 and 26 but to diminish it if between residues 26 and 30 [Barbier et al. (1997) J. Med. Chem. 40, 1373-1380]. This work reports the effects of other cyclizations on the polar face, differing in ring size or position, on alpha-helix conformation, as measured by circular dichroism, and on AC-stimulating activity. All analogues cyclized between residues 22 and 26 had at least a 1. 5-fold increase in activity, suggesting an alpha-helical structure between about residues 21 and 26. Cyclization between residues 25 and 29 or residues 26 and 30 diminished activity by 20-30%, despite stabilizing alpha-helix, suggesting that residues 25-31 bind to the receptor in a helical, but not classical alpha-helical, conformation. Analogues cyclized between residues 13 and 17 had slightly increased activity. A bicyclic analogue, with lactams between residues 13 and 17 and residues 22 and 26, had about the same activity as that cyclized only between 22 and 26. Parathyroid hormone-related peptide (PTHrP) may bind in a manner similar to the common receptor, but hydrophobic moment calculations suggest that it must bind as a tighter helix in order to optimally present its hydrophobic residues to the receptor. Both PTHrP analogues cyclized between either residues 22 and 26 or residues 26 and 30 had more stable alpha-helices but reduced AC activities, consistent with this hypothesis.     

Studies on the state of tyrosyl residues in a ribonuclease from seminal vesicles.

In order to study the state of tyrosyl residues in a ribouuclease from bovine semina vesicles [EC 3.1.4.22, RNase Vs1] several lines of experiments were carried out. Spectrophotometric titration of RNase Vs1 indicated that two out of 8 tyrosine residues were titrated very easily and their apparent pKa values were about 9.8. Next, about 4 residues were titrated at pH up to 13.5. The remaining 2 residues were titrated time-dependently at pH 13.5. In 8 M urea, about 6 tyrosine residues were titrated with apparent pK4 values of about 11.2 and about 2 residues were titrated time-dependently at pH 13.5. Acetylation of RNase Vs1 with N-acetylimidazole was studied at pH 7.5. In aqueous solution, about 1.1-3.5 tyrosine residues were acetylated, depending on the experimental conditions, and in 8 M urea, 5.3 tyrosine residues were modified. RNase Vs1 was nitrated with tetranitromethane at pH 7.5. In aqueous solution, about 2.5 tyrosine residues were nitrated very easily; the enzymatic activity of the modified enzymes was 130-200% of that of the native enzyme. In 8 M urea, the reactivity of the tyrosine residues increased and about 4-5.5 residues were modified. The results of chemical modification and spectrophotometric titration indicated that about two tyrosine residues in RNase Vs1 were exposed to the solvent and were more reactive to various reagents, and 3-4 tyrosine residues were less reactive. The final 2 residues were not accessible to the reagent even in the presence of urea, but were titraten at pH 13.5. The solvent perturbation difference spectrum using ethylene glycol as a perturbant indicated that about 4 tyrosine residues were perturbed. When the pH of the enzyme solution was changed from 7.0 to 1.0, the change in optical density of RNase Vs1 due to denaturation blue shift was about 1,600 at 287nm. The optical density change at 287 nm of native RNase Vs1 on exposure to 8 M urea and 6 M guanidine-HCl indicated that the environments of 2-3 and 4 tyrosine residues were changed by the addition of the denaturants, urea and guanidine-HCl, respectively. In RNase Vs1 having about four nitrotyrosine residues, the two most inaccessible tyrosine residues remained resistant to titration with alkali. On adding nucleotide, nitrated RNase Vs1 gave a difference spectrum in the ultraviolet region but not in 320-460 nm region, where nitrotyrosine residues absorb light. This may indicate that tyrosine residues located relatively near the surface of the molecule are not perturbed directly by nucleotide binding.     

Tolerance to the substitution of buried apolar residues by charged residues in the homologous protein structures

Occurrence and accommodation of charged amino acid residues in proteins that are structurally equivalent to buried non-polar residues in homologues have been investigated. Using a dataset of 1,852 homologous pairs of crystal structures of proteins available at 2A or better resolution, 14,024 examples of apolar residues in the structurally conserved regions replaced by charged residues in homologues have been identified. Out of 2,530 cases of buried apolar residues, 1,677 of the equivalent charged residues in homologues are exposed and the rest of the charged residues are buried. These drastic substitutions are most often observed in homologous protein pairs with low sequence identity (<30%) and in large protein domains (>300 residues). Such buried charged residues in the large proteins are often located in the interface of sub-domains or in the interface of structural repeats, Beyond 7A of residue depth of buried apolar residues, or less than 4% of solvent accessibility, almost all the substituting charged residues are buried. It is also observed that acidic sidechains have higher preference to get buried than the positively charged residues. There is a preference for buried charged residues to get accommodated in the interior by forming hydrogen bonds with another sidechain than the main chain. The sidechains interacting with a buried charged residue are most often located in the structurally conserved regions of the alignment. About 50% of the observations involving hydrogen bond between buried charged sidechain and another sidechain correspond to salt bridges. Among the buried charged residues interacting with the main chain, positively charged sidechains form hydrogen bonds commonly with main chain carbonyls while the negatively charged residues are accommodated by hydrogen bonding with the main chain amides. These carbonyls and amides are usually located in the loops that are structurally variable among homologous proteins.     

Identification and analysis of novel amino acid sequence repeats and domains in Pyrobaculum aerophilum using computational tools

We have identified four repeats and five domains that are novel in proteins encoded by the Pyrobaculum aerophilum str. IM2 proteome using automated in silico methods. A "repeat" corresponds to a region comprising less than 55 amino acid residues that occurs more than once in the protein sequence and sometimes present in tandem. A "domain" corresponds to a conserved region comprising greater than 55 amino acid residues and may be present as single or multiple copies in the protein sequence. These correspond to (1) 85 amino acid residues AAG domain, (2) 72 amino acid residues GFGN domain, (3) 43 amino acid residues KGG repeat, (4) 25 amino acid residues RWE repeat, (5) 25 amino acid residues RID repeat, (6) 108 amino acid residues NDFA domain, (7) 140 amino acid residues VxY domain, (8) 35 amino acid residues LLPN repeat and (9) 98 amino acid residues GxY domain. A repeat or domain is characterized by specific conserved sequence motifs. We discuss the presence of these repeats and domains in proteins from other genomes and their probable secondary structure.     

Three-dimensional structure of the two peptides that constitute the two-peptide bacteriocin lactococcin G

The three-dimensional structures of the two peptides, lactococcin G-alpha (LcnG-alpha; contains 39 residues) and lactococcin G-beta (LcnG-beta, contains 35 residues), that constitute the two-peptide bacteriocin lactococcin G (LcnG) have been determined by nuclear magnetic resonance (NMR) spectroscopy in the presence of DPC micelles and TFE. In DPC, LcnG-alpha has an N-terminal alpha-helix (residues 3-21) that contains a GxxxG helix-helix interaction motif (residues 7-11) and a less well defined C-terminal alpha-helix (residues 24-34), and in between (residues 18-22) there is a second somewhat flexible GxxxG-motif. Its structure in TFE was similar. In DPC, LcnG-beta has an N-terminal alpha-helix (residues 6-19). The region from residues 20 to 35, which also contains a flexible GxxxG-motif (residues 18-22), appeared to be fairly unstructured in DPC. In the presence of TFE, however, the region between and including residues 23 and 32 formed a well defined alpha-helix. The N-terminal helix between and including residues 6 and 19 seen in the presence of DPC, was broken at residues 8 and 9 in the presence of TFE. The N-terminal helices, both in LcnG-alpha and -beta, are amphiphilic. We postulate that LcnG-alpha and -beta have a parallel orientation and interact through helix-helix interactions involving the first GxxxG (residues 7-11) motif in LcnG-alpha and the one (residues 18-22) in LcnG-beta, and that they thus lie in a staggered fashion relative to each other.     

In Situ Dynamics and Spatial Heterogeneity of Soil Bacterial Communities Under Different Crop Residue Management

The effect of the location of wheat residues (soil surface vs. incorporated in soil) on their decomposition and on soil bacterial communities was investigated by the means of a field experiment. Bacterial-automated ribosomal intergenic spacer analysis of DNA extracts from residues, detritusphere (soil adjacent to residues), and bulk soil evidenced that residues constitute the zone of maximal changes in bacterial composition. However, the location of the residues influenced greatly their decomposition and the dynamics of the colonizing bacterial communities. Sequencing of 16S rRNA gene in DNA extracts from the residues at the early, middle, and late stages of degradation confirmed the difference of composition of the bacterial community according to the location. Bacteria belonging to the γ-subgroup of proteobacteria were stimulated when residues were incorporated whereas the α-subgroup was stimulated when residues were left at the soil surface. Moreover, Actinobacteria were more represented when residues were left at the soil surface. According to the ecological attributes of the populations identified, our results suggested that climatic fluctuations at the soil surface select populations harboring enhanced catabolic and/or survival capacities whereas residues characteristics likely constitute the main determinant of the composition of the bacterial community colonizing incorporated residues.     

Functional significance of conserved amino acid residues.

A systemic study of single amino acid substitutions in bacteriophage T4 lysozyme permitted a test of the concept that conserved amino acid residues are more functionally important than nonconserved residues. Substitutions of amino acid residues that are conserved among five bacteriophage-encoded lysozymes were found to lead more frequently to loss of function than substitutions of nonconserved residues. Of 163 residues tested, only 74 (45%) are sensitive to at least one substitution; however, all 14 residues that are fully conserved are sensitive to substitutions.     

Vitamin K and the biosynthesis of prothrombin. V. Gamma-carboxyglutamic acids, the vitamin K-dependent structures in prothrombin.

Tryptic peptides obtained from normal prothrombin have been compared with those obtained from prothrombin synthesized by cattle given the vitamin K antagonist dicumarol. Two peptides were found which contain vitamin K-dependent structures. These peptides contain residues 4 through 10 and residues 12 through 44, respectively. One of these (residues 4 through 10) has previously been shown to contain gamma-carboxyglutamic acid residues. Digestion of this peptide with aminopeptidase M and carboxypeptidase B yielded a tetrapeptide (residues 6 through 9). Mass spectra of this peptide showed that it has the structure Leu-Glu(CO2)-Glu(CO2)-Val. The structure of the peptide containing residues 12 through 44 was determined by automated degradation in a peptide sequenator. The modified glutamic acid residues were identified by mass spectrometric comparison with the thiohydantoin derivatives of synthetic gamma-carboxyglutamic acid. This approach unequivocally demonstrated that all of the first 10 glutamic acid residues in prothrombin are carboxylated to form gamma-carboxyglutamic acid residues. Evidence is also presented that indicates that these gamma-carboxyglutamic acid residues constitute the entire vitamin K-dependent modification of prothrombin.     

Involvement of the activation loop of ERK in the detachment from cytosolic anchoring

The Extracellular signal-regulated kinases (ERKs) are translocated into the nucleus in response to mitogenic stimulation. The mechanism of translocation and the residues in ERKs that govern this process are not clear as yet. Here we studied the involvement of residues in the activation loop of ERK2 in determining its subcellular localization. Substitution of residues in the activation loop to alanines indicated that residues 173-181 do not play a significant role in the phosphorylation and activation of ERK2. However, residues 176-181 are responsible for the detachment of ERK2 from MEK1 upon mitogenic stimulation. This dissociation can be mimicked by substitution of residues 176-178 to alanines and is prevented by deletion of these residues or by substitution of residues 179-181 to alanines. On the other hand, residues 176-181, as well as residues essential for ERK2 dimerization, do not play a role in the shuttle of ERK2 through nuclear pores. Thus, phosphorylation-induced conformational rearrangement of residues in the activation loop of ERK2 plays a major role in the control of subcellular localization of this protein.     

Evolution of nuclear- and mitochondrial-encoded subunit interaction in cytochrome c oxidase

Mitochondrial DNA (mtDNA)-encoded proteins function in eukaryotes as subunits of respiratory complexes that also contain nuclear DNA (nDNA)-encoded subunits. The importance of functional interactions between mtDNA- and nDNA-encoded proteins was previously demonstrated by testing the survivability of cybrid cells or individuals containing nDNA and mtDNA from different populations or species. This report focuses on the multisubunit respiratory complex cytochrome c oxidase (COX), made up of both mtDNA-encoded and nDNA-encoded subunits. A combination of evolutionary and crystallographic data is employed to determine whether rates of nonsynonymous substitutions have been higher, the same, or lower for residues in close proximity that are encoded by a different genome (nDNA or mtDNA). This determination is performed by simply taking the ratio, called the interaction ratio i, of the nonsynonymous substitution rate of the close-contact residues to the nonsynonymous substitution rate of the noncontact residues. We assume that the close-contact residues (which are more likely to interact) are functionally important and that, therefore, amino acid replacements among these residues cannot escape the scrutiny of natural selection. i = 1 indicates that the close-contact residues have been under neither greater purifying selection nor greater positive selection than the noncontact residues as a specific consequence of their being encoded by separate genomes. i < 1 indicates that the close-contact residues have been under greater purifying selection but less positive selection than have the noncontact residues. Conversely, i > 1 indicates that the close-contact residues have been under less purifying but greater positive selection than have the noncontact residues. i < 1 may be referred to as a constraining interaction; i.e., the close-contact residues compared with the noncontact residues appear to be under greater structural-functional constraints. On the other hand, i > 1 may be referred to as an optimizing interaction; i.e., apparently many different amino acid replacements are required to optimize this subunit's interaction with the other subunit. A major finding is that the nDNA-encoded residues in close physical proximity to mtDNA-encoded residues evolve more slowly than the other nuclear-encoded residues (and thus display a constraining interaction), whereas the mtDNA-encoded residues in close physical proximity to nDNA-encoded residues evolve more rapidly than the other mitochondrial-encoded residues (and thus display an optimizing interaction). A possible reason for this striking difference between the nuclear- and mitochondrial-encoded COX subunits in how their functional interaction evolves is discussed.     

Four states of tyrosine residues in the fibrinogen molecule

The ionization of tyrosine residues in fibrinogen was studied by a spectrophotometric method. The total of 100 tyrosine residues in the fibrinogen molecule was classified into four states: (1) 28 tyrosine residues with pK 10.1 (m = 1.0). (2) tyrosine residues with pK 11.5 (m = 1.0), (3) 20 tyrosine residues with pK 12.2 (m = 3.0) and (4) 10 tyrosine residues non-ionizable. When fibrinogen was treated with 4 M guanidine . HCl, all of the tyrosine residues became ionizable with the ionization characteristics of pK 10.1 (m = 1.0). The ionization characteristics of tyrosine residues in plasmin-digested fibrinogen were similar to those of fibrinogen, while in CNBr-treated fibrinogen they were fairly different. The value, m, stands for the number of hydroxyl ions involved in the ionization of a tyrosine residue.     

Primary structure of human alpha 2-macroglobulin. I. Isolation of the 26 CNBr fragments, amino acid sequence of 13 small CNBr fragments, amino acid sequence of methionine-containing peptides, and alignment of all CNBr fragments

The isolation of the 26 CNBr fragments from the identical Mr = 180,000 subunits of human alpha 2-macroglobulin is described. The fragments have been purified by combinations of gel chromatography, ion-exchange chromatography, high voltage paper electrophoresis, paper chromatography, and high performance liquid chromatography. The complete amino acid sequences of 13 small CNBr fragments have been determined. These fragments include CB1 (residues 1-9), CB3 (residues 79-98), CB4 (residues 99-128), CB9 (residues 442-477), CB10 (residues 478-497), CB13 (residues 644-650), CB14 (residues 651-665), CB15 (residues 666-674), CB16 (residues 675-690), CB19 (residues 937-945), CB20 (residues 946-954), CB24 (residues 1356-1362), and CB25 (residues 1363-1375). The fragments determined account for 200 of the 1451 residues of the subunits of alpha 2-macroglobulin. Most likely, Cys-6 of CB9 is bound to the corresponding residue in CB9 from another subunit, thus forming an interchain disulfide bridge in alpha 2-macroglobulin. Cys-1 of CB15 is bound to Cys-35 of CB12. CB15 contains a pair of Gln residues that can react covalently with amines in a factor XIIIa-catalyzed process (Gln-5 and Gln-6). CB16 contains the primary cleavage sites for proteinases in the bait region of alpha 2-macroglobulin (-Arg7-Val-Gly-Phe-Tyr-Glu-). CB20 contains the residues which in native alpha 2-macroglobulin presumably form an internal reactive beta-cysteinyl-gamma-glutamyl thiol ester (Cys-4 and Glx-7). Partial NH2- and COOH-terminal sequence data are given for the 13 large CNBr fragments. Complete or partial sequence determination of 19 methionine-containing peptides or variants thereof allow the alignment of all the CNBr fragments.     

Architecture of beta-barrel membrane proteins: analysis of trimeric porins.

We have analyzed the known three-dimensional structures of trimeric porins from bacterial outer membranes. The distribution of surface-exposed residues in a direction perpendicular to the membrane is similar to that in helical membrane proteins, with aliphatic residues concentrated in the central 20 A of the bilayer. Outside these residues is a layer of aromatic residues, followed by polar and charged residues. Residues in the trimer interface are more conserved than residues not in the interface. By comparing the interface and noninterface residues, an interface preference scale has been derived that may be used as a basis for predicting interface surfaces in monomer models.     

Prediction of prolyl residues in cis-conformation in protein structures on the basis of the amino acid sequence

In proteins most peptide bonds are in trans-conformation: the torsion angle omega = 180 degrees. Only few show cis-conformation in known protein structures (omega = 0 degrees). Most of them are prolyl residues. About 6% of about 4000 prolyl residues are in cis-conformation. Between trans- and cis-prolyl residues significant differences are observed in the surrounding sequences. E.g. there are large amounts of aromatic residues N-terminally in case of cis-prolyl residues, but in the case of trans-prolyl residues more aromatic amino acids occur C-terminally. But in all cases there are only complex patterns which are indicative of cis- and trans-conformation, respectively. Considering the neighbours (+/- 6 residues) of prolyl residues and their physicochemical properties we find 6 different patterns which allow one to assign correctly about 75% of known cis-structured prolyl residues, whereby no false positive one is predicted.     

Primary structure of chicken pituitary prolactin deduced from the cDNA sequence. Conserved and specific amino acid residues in the domains of the prolactins

The perform of chicken prolactin (PRL) deduced from the cDNA sequence contains a signal peptide of 30 amino acid residues followed by a mature PRL of 199 residues. Chicken PRL shows 77, 68, 67, 58, and 31% identity of amino acid sequence with whale, human, ovine, rat, and salmon PRLs, respectively. Elucidation of the primary structure of avian PRL enabled extended analysis of the specific and conserved amino acid residues and domains of the PRL molecules. The mammalian, teleostean, and avian PRLs share 32 common residues, and these conserved residues are observed to cluster in four distinct domains (PD1 to PD4), corresponding to four of five conserved domains of the growth hormones. Of the 32 residues, 8 residues in the PD2 and PD4 domains, including 4 cysteines, are conserved by other members of the growth hormone family, which indicates that these 8 residues may be essential for common structural features of the gene family. On the other hand, 13 other residues distributed among all four domains are conserved almost exclusively in the PRLs, suggesting that these residues are indispensable for specific binding of the PRLs to their receptors.     

Site-directed mutagenesis of human prorenin. Substitution of three arginine residues in the propeptide with glutamine residues yields active prorenin

Human prorenin is an inactive zymogen comprising 43 amino acid residues at the amino terminus of human renin. The aim of this work was to determine why prorenin is inactive at neutral pH. Eighteen different mutant prorenins, in which positively charged residues in the propeptide were substituted with either glutamine (Gln) or lysine (Lys) residues by site-directed mutagenesis, were expressed in COS-7 cells and characterized. By replacing each of the three arginine (Arg) residues (Arg10P, Arg15P, and Arg20P) with Gln residues, partially active prorenins were produced, which exhibited significant but not full renin activity without trypsin activation. The effect of double or triple amino acid substitutions on the appearance of active prorenin was cumulative, the activity reaching about 80% in a mutant in which all the three Arg residues were replaced by Gln residues. In contrast, mutant prorenins with Lys residues substituted for the Arg residues were inactive. These results clearly indicate that the positive charges of the three Arg residues are essential for maintenance of the human prorenin in an inactive form.     

中国作物秸秆资源评估研究现状

文中分析了中国的农作物秸秆资源量,对实现秸秆资源化具有重要意义。首先明确了秸秆的概念,即收获作物主产品之后所有大田剩余的副产物及主产品初加工过程产生的副产物,根据不同产出环节将秸秆分为田间秸秆和加工副产物。其次,阐述了秸秆评估的计算方法,给出了各种作物秸秆计算方法。第三,总结了近10年发表文献和作者近期研究对1991年以来中国秸秆资源量评估结果,对1991-1999年中国作物田间秸秆资源量评估的值多数为6.0～6.8亿t,对加工副产物资源量未见报道;对2000-2007年田间秸秆产量的评估的值多数为5.9～7.3亿t,作物加工副产物量为0.9～1.1亿t。第四,分析了对中国秸秆资源量评估值差异的原因等问题,包括以往研究作物秸秆的概念不明确,草谷比取值差异大而不符合当前作物生产现状,以及对于农作物产量的统计指标和产量构成认识不清楚等。第五,报道了作者以最近5年测定的各省作物收获指数,采用完全统计方法评估了2006-2007年中国作物秸秆量及其在8个地区的分布,结果表明秸秆资源总量为7.4亿t,包括6.5亿t田间秸秆和0.9亿t作物加工副产物。